Table of Contents
Object/relational mappings can be defined in three approaches:
using Java 5 annotations (via the Java Persistence 2 annotations)
using JPA 2 XML deployment descriptors (described in chapter XXX)
using the Hibernate legacy XML files approach known as hbm.xml
Annotations are split in two categories, the logical mapping annotations (describing the object model, the association between two entities etc.) and the physical mapping annotations (describing the physical schema, tables, columns, indexes, etc). We will mix annotations from both categories in the following code examples.
JPA annotations are in thejavax.persistence.* package. Hibernate specific extensions are inorg.hibernate.annotations.*. You favorite IDE can auto-complete annotations and their attributes for you (even without a specific "JPA" plugin, since JPA annotations are plain Java 5 annotations).
Here is an example of mapping
package eg;@Entity @Table(name="cats") @Inheritance(strategy=SINGLE_TABLE)@DiscriminatorValue("C") @DiscriminatorColumn(name="subclass", discriminatorType=CHAR)public class Cat { @Id @GeneratedValue public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } private Integer id; public BigDecimal getWeight() { return weight; } public void setWeight(BigDecimal weight) { this.weight = weight; } private BigDecimal weight; @Temporal(DATE) @NotNull @Column(updatable=false) public Date getBirthdate() { return birthdate; } public void setBirthdate(Date birthdate) { this.birthdate = birthdate; } private Date birthdate; @org.hibernate.annotations.Type(type="eg.types.ColorUserType") @NotNull @Column(updatable=false) public ColorType getColor() { return color; } public void setColor(ColorType color) { this.color = color; } private ColorType color; @NotNull @Column(updatable=false) public String getSex() { return sex; } public void setSex(String sex) { this.sex = sex; } private String sex; @NotNull @Column(updatable=false) public Integer getLitterId() { return litterId; } public void setLitterId(Integer litterId) { this.litterId = litterId; } private Integer litterId; @ManyToOne @JoinColumn(name="mother_id", updatable=false) public Cat getMother() { return mother; } public void setMother(Cat mother) { this.mother = mother; } private Cat mother; @OneToMany(mappedBy="mother") @OrderBy("litterId") public Set<Cat> getKittens() { return kittens; } public void setKittens(Set<Cat> kittens) { this.kittens = kittens; } private Set<Cat> kittens = new HashSet<Cat>();}@Entity @DiscriminatorValue("D")public class DomesticCat extends Cat { public String getName() { return name; } public void setName(String name) { this.name = name } private String name;}@Entitypublic class Dog { ... }The legacy hbm.xml approach uses an XML schema designed to be readable and hand-editable. The mapping language is Java-centric, meaning that mappings are constructed around persistent class declarations and not table declarations.
Please note that even though many Hibernate users choose to write the XML by hand, a number of tools exist to generate the mapping document. These include XDoclet, Middlegen and AndroMDA.
Here is an example mapping:
<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="eg"> <class name="Cat" table="cats" discriminator-value="C"> <id name="id"> <generator/> </id> <discriminator column="subclass" type="character"/> <property name="weight"/> <property name="birthdate" type="date" not-null="true" update="false"/> <property name="color" type="eg.types.ColorUserType" not-null="true" update="false"/> <property name="sex" not-null="true" update="false"/> <property name="litterId" column="litterId" update="false"/> <many-to-one name="mother" column="mother_id" update="false"/> <set name="kittens" inverse="true" order-by="litter_id"> <key column="mother_id"/> <one-to-many/> </set> <subclass name="DomesticCat" discriminator-value="D"> <property name="name" type="string"/> </subclass> </class> <class name="Dog"> <!-- mapping for Dog could go here --> </class></hibernate-mapping>
We will now discuss the concepts of the mapping documents (both annotations and XML). We will only describe, however, the document elements and attributes that are used by Hibernate at runtime. The mapping document also contains some extra optional attributes and elements that affect the database schemas exported by the schema export tool (for example, the not-null attribute).
An entity is a regular Java object (aka POJO) which will be persisted by Hibernate.
To mark an object as an entity in annotations, use the@Entity annotation.
@Entity
public class Flight implements Serializable {
Long id;
@Id
public Long getId() { return id; }
public void setId(Long id) { this.id = id; }
}
That's pretty much it, the rest is optional. There are however any options to tweak your entity mapping, let's explore them.
@Table lets you define the table the entity will be persisted into. If undefined, the table name is the unqualified class name of the entity. You can also optionally define the catalog, the schema as well as unique constraints on the table.
@Entity@Table(name="TBL_FLIGHT", schema="AIR_COMMAND", uniqueConstraints= @UniqueConstraint( name="flight_number", columnNames={"comp_prefix", "flight_number"} ) )public class Flight implements Serializable { @Column(name="comp_prefix") public String getCompagnyPrefix() { return companyPrefix; } @Column(name="flight_number") public String getNumber() { return number; }}The constraint name is optional (generated if left undefined). The column names composing the constraint correspond to the column names as defined before the HibernateNamingStrategy is applied.
Tip
Be sure to use the database-level column names for thecolumnNames property of a@UniqueConstraint. For example, whilst for simple types the database-level column name may be the same as the entity-level property name, this is often not the case for relational properties.
@Entity.name lets you define the shortcut name of the entity you can used in JP-QL and HQL queries. It defaults to the unqualified class name of the class.
Hibernate goes beyond the JPA specification and provide additional configurations. Some of them are hosted on@org.hibernate.annotations.Entity:
dynamicInsert/dynamicUpdate(defaults to false): specifies thatINSERT/UPDATESQL should be generated at runtime and contain only the columns whose values are not null. Thedynamic-updateanddynamic-insertsettings are not inherited by subclasses. Although these settings can increase performance in some cases, they can actually decrease performance in others.selectBeforeUpdate(defaults to false): specifies that Hibernate shouldnever perform an SQLUPDATEunless it is certain that an object is actually modified. Only when a transient object has been associated with a new session usingupdate(), will Hibernate perform an extra SQLSELECTto determine if anUPDATEis actually required. Use ofselect-before-updatewill usually decrease performance. It is useful to prevent a database update trigger being called unnecessarily if you reattach a graph of detached instances to aSession.polymorphisms(defaults toIMPLICIT): determines whether implicit or explicit query polymorphisms is used.Implicit polymorphisms means that instances of the class will be returned by a query that names any superclass or implemented interface or class, and that instances of any subclass of the class will be returned by a query that names the class itself.Explicit polymorphisms means that class instances will be returned only by queries that explicitly name that class. Queries that name the class will return only instances of subclasses mapped. For most purposes, the defaultpolymorphisms=IMPLICITis appropriate. Explicit polymorphisms is useful when two different classes are mapped to the same table This allows a "lightweight" class that contains a subset of the table columns.persister: specifies a customClassPersister. Thepersisterattribute lets you customize the persistence strategy used for the class. You can, for example, specify your own subclass oforg.hibernate.persister.EntityPersister, or you can even provide a completely new implementation of the interfaceorg.hibernate.persister.ClassPersisterthat implements, for example, persistence via stored procedure calls, serialization to flat files or LDAP. Seeorg.hibernate.test.CustomPersisterfor a simple example of "persistence" to aHashtable.optimisticLock(defaults toVERSION): determines the optimistic locking strategy. If you enabledynamicUpdate, you will have a choice of optimistic locking strategies:version: check the version/timestamp columnsall: check all columnsdirty: check the changed columns, allowing some concurrent updatesnone: do not use optimistic locking
It isstrongly recommended that you use version/timestamp columns for optimistic locking with Hibernate. This strategy optimizes performance and correctly handles modifications made to detached instances (i.e. when
Session.merge()is used).
Tip
Be sure to import@javax.persistence.Entity to mark a class as an entity. It's a common mistake to import@org.hibernate.annotations.Entity by accident.
Some entities are not mutable. They cannot be updated by the application. This allows Hibernate to make some minor performance optimizations.. Use the@Immutable annotation.
You can also alter how Hibernate deals with lazy initialization for this class. On@Proxy, uselazy=false to disable lazy fetching (not recommended). You can also specify an interface to use for lazy initializing proxies (defaults to the class itself): useproxyClass on@Proxy. Hibernate will initially return proxies (Javassist or CGLIB) that implement the named interface. The persistent object will load when a method of the proxy is invoked. See "Initializing collections and proxies" below.
@BatchSize specifies a "batch size" for fetching instances of this class by identifier. Not yet loaded instances are loaded batch-size at a time (default 1).
You can specific an arbitrary SQL WHERE condition to be used when retrieving objects of this class. Use@Where for that.
In the same vein,@Check lets you define an SQL expression used to generate a multi-rowcheck constraint for automatic schema generation.
There is no difference between a view and a base table for a Hibernate mapping. This is transparent at the database level, although some DBMS do not support views properly, especially with updates. Sometimes you want to use a view, but you cannot create one in the database (i.e. with a legacy schema). In this case, you can map an immutable and read-only entity to a given SQL subselect expression using@org.hibernate.annotations.Subselect:
@Entity@Subselect("select item.name, max(bid.amount), count(*) " + "from item " + "join bid on bid.item_id = item.id " + "group by item.name")@Synchronize( {"item", "bid"} ) //tables impactedpublic class Summary { @Id public String getId() { return id; } ...}Declare the tables to synchronize this entity with, ensuring that auto-flush happens correctly and that queries against the derived entity do not return stale data. The<subselect> is available both as an attribute and a nested mapping element.
We will now explore the same options using the hbm.xml structure. You can declare a persistent class using theclass element. For example:
<class name="ClassName"table="tableName"
discriminator-value="discriminator_value"
mutable="true|false"
schema="owner"
catalog="catalog"
proxy="ProxyInterface"
dynamic-update="true|false"
dynamic-insert="true|false"
select-before-update="true|false"
polymorphism="implicit|explicit"
where="arbitrary sql where condition"
persister="PersisterClass"
batch-size="N"
optimistic-lock="none|version|dirty|all"
lazy="true|false"(16) entity-name="EntityName"(17) check="arbitrary sql check condition"(18) rowxml:id="rowid"(19) subselect="SQL expression"(20) abstract="true|false"(21) node="element-name"/>
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It is acceptable for the named persistent class to be an interface. You can declare implementing classes of that interface using the<subclass> element. You can persist anystatic inner class. Specify the class name using the standard form i.e.e.g.Foo$Bar.
Here is how to do a virtual view (subselect) in XML:
<class name="Summary"> <subselect> select item.name, max(bid.amount), count(*) from item join bid on bid.item_id = item.id group by item.name </subselect> <synchronize table="item"/> <synchronize table="bid"/> <id name="name"/> ...</class>
The<subselect> is available both as an attribute and a nested mapping element.
Mapped classesmust declare the primary key column of the database table. Most classes will also have a JavaBeans-style property holding the unique identifier of an instance.
Mark the identifier property with@Id.
@Entitypublic class Person { @Id Integer getId() { ... } ...}In hbm.xml, use the<id> element which defines the mapping from that property to the primary key column.
<id name="propertyName"
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If thename attribute is missing, it is assumed that the class has no identifier property.
Theunsaved-value attribute is almost never needed in Hibernate3 and indeed has no corresponding element in annotations.
You can also declare the identifier as a composite identifier. This allows access to legacy data with composite keys. Its use is strongly discouraged for anything else.
You can define a composite primary key through several syntaxes:
use a component type to represent the identifier and map it as a property in the entity: you then annotated the property as
@EmbeddedId. The component type has to beSerializable.map multiple properties as
@Idproperties: the identifier type is then the entity class itself and needs to beSerializable. This approach is unfortunately not standard and only supported by Hibernate.map multiple properties as
@Idproperties and declare an external class to be the identifier type. This class, which needs to beSerializable, is declared on the entity via the@IdClassannotation. The identifier type must contain the same properties as the identifier properties of the entity: each property name must be the same, its type must be the same as well if the entity property is of a basic type, its type must be the type of the primary key of the associated entity if the entity property is an association (either a@OneToOneor a@ManyToOne).
As you can see the last case is far from obvious. It has been inherited from the dark ages of EJB 2 for backward compatibilities and we recommend you not to use it (for simplicity sake).
Let's explore all three cases using examples.
Here is a simple example of@EmbeddedId.
@Entity
class User {
@EmbeddedId
@AttributeOverride(name="firstName", column=@Column(name="fld_firstname")
UserId id;
Integer age;
}
@Embeddable
class UserId implements Serializable {
String firstName;
String lastName;
}
You can notice that theUserId class is serializable. To override the column mapping, use@AttributeOverride.
An embedded id can itself contains the primary key of an associated entity.
@Entity
class Customer {
@EmbeddedId CustomerId id;
boolean preferredCustomer;
@MapsId("userId")
@JoinColumns({
@JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
@JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
})
@OneToOne User user;
}
@Embeddable
class CustomerId implements Serializable {
UserId userId;
String customerNumber;
//implements equals and hashCode
}
@Entity
class User {
@EmbeddedId UserId id;
Integer age;
}
@Embeddable
class UserId implements Serializable {
String firstName;
String lastName;
//implements equals and hashCode
}
In the embedded id object, the association is represented as the identifier of the associated entity. But you can link its value to a regular association in the entity via the@MapsId annotation. The@MapsId value correspond to the property name of the embedded id object containing the associated entity's identifier. In the database, it means that theCustomer.user and theCustomerId.userId properties share the same underlying column (user_fk in this case).
Tip
The component type used as identifier must implementequals() andhashCode().
In practice, your code only sets theCustomer.user property and the user id value is copied by Hibernate into theCustomerId.userId property.
Warning
The id value can be copied as late as flush time, don't rely on it until after flush time.
While not supported in JPA, Hibernate lets you place your association directly in the embedded id component (instead of having to use the@MapsId annotation).
@Entity
class Customer {
@EmbeddedId CustomerId id;
boolean preferredCustomer;
}
@Embeddable
class CustomerId implements Serializable {
@OneToOne
@JoinColumns({
@JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
@JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
})
User user;
String customerNumber;
//implements equals and hashCode
}
@Entity
class User {
@EmbeddedId UserId id;
Integer age;
}
@Embeddable
class UserId implements Serializable {
String firstName;
String lastName;
//implements equals and hashCode
}
Let's now rewrite these examples using the hbm.xml syntax.
<composite-id name="propertyName" mapped="true|false" access="field|property|ClassName" node="element-name|."> <key-property name="propertyName" type="typename" column="column_name"/> <key-many-to-one name="propertyName" column="column_name"/> ......</composite-id>
First a simple example:
<class name="User"> <composite-id name="id"> <key-property name="firstName" column="fld_firstname"/> <key-property name="lastName"/> </composite-id></class>
Then an example showing how an association can be mapped.
<class name="Customer"> <composite-id name="id"> <key-property name="firstName" column="userfirstname_fk"/> <key-property name="lastName" column="userfirstname_fk"/> <key-property name="customerNumber"/> </composite-id> <property name="preferredCustomer"/> <many-to-one name="user"> <column name="userfirstname_fk" updatable="false" insertable="false"/> <column name="userlastname_fk" updatable="false" insertable="false"/> </many-to-one></class><class name="User"> <composite-id name="id"> <key-property name="firstName"/> <key-property name="lastName"/> </composite-id> <property name="age"/></class>
Notice a few things in the previous example:
the order of the properties (and column) matters. It must be the same between the association and the primary key of the associated entity
the many to one uses the same columns as the primary key and thus must be marked as read only (
insertableandupdatableto false).unlike with
@MapsId, the id value of the associated entity is not transparently copied, check theforeignid generator for more information.
The last example shows how to map association directly in the embedded id component.
<class name="Customer"> <composite-id name="id"> <key-many-to-one name="user"> <column name="userfirstname_fk"/> <column name="userlastname_fk"/> </key-many-to-one> <key-property name="customerNumber"/> </composite-id> <property name="preferredCustomer"/></class><class name="User"> <composite-id name="id"> <key-property name="firstName"/> <key-property name="lastName"/> </composite-id> <property name="age"/></class>
This is the recommended approach to map composite identifier. The following options should not be considered unless some constraint are present.
Another, arguably more natural, approach is to place@Id on multiple properties of your entity. This approach is only supported by Hibernate (not JPA compliant) but does not require an extra embeddable component.
@Entity
class Customer implements Serializable {
@Id @OneToOne
@JoinColumns({
@JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
@JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
})
User user;
@Id String customerNumber;
boolean preferredCustomer;
//implements equals and hashCode
}
@Entity
class User {
@EmbeddedId UserId id;
Integer age;
}
@Embeddable
class UserId implements Serializable {
String firstName;
String lastName;
//implements equals and hashCode
}
In this caseCustomer is its own identifier representation: it must implementSerializable and must implementequals() andhashCode().
In hbm.xml, the same mapping is:
<class name="Customer"> <composite-id> <key-many-to-one name="user"> <column name="userfirstname_fk"/> <column name="userlastname_fk"/> </key-many-to-one> <key-property name="customerNumber"/> </composite-id> <property name="preferredCustomer"/></class><class name="User"> <composite-id name="id"> <key-property name="firstName"/> <key-property name="lastName"/> </composite-id> <property name="age"/></class>
@IdClass on an entity points to the class (component) representing the identifier of the class. The properties marked@Id on the entity must have their corresponding property on the@IdClass. The return type of search twin property must be either identical for basic properties or must correspond to the identifier class of the associated entity for an association.
Warning
This approach is inherited from the EJB 2 days and we recommend against its use. But, after all it's your application and Hibernate supports it.
@Entity
@IdClass(CustomerId.class)
class Customer implements Serializable {
@Id @OneToOne
@JoinColumns({
@JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
@JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
})
User user;
@Id String customerNumber;
boolean preferredCustomer;
}
class CustomerId implements Serializable {
UserId user;
String customerNumber;
//implements equals and hashCode
}
@Entity
class User {
@EmbeddedId UserId id;
Integer age;
//implements equals and hashCode
}
@Embeddable
class UserId implements Serializable {
String firstName;
String lastName;
//implements equals and hashCode
}
Customer andCustomerId do have the same propertiescustomerNumber as well asuser.CustomerId must beSerializable and implementequals() andhashCode().
While not JPA standard, Hibernate let's you declare the vanilla associated property in the@IdClass.
@Entity
@IdClass(CustomerId.class)
class Customer implements Serializable {
@Id @OneToOne
@JoinColumns({
@JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
@JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
})
User user;
@Id String customerNumber;
boolean preferredCustomer;
}
class CustomerId implements Serializable {
@OneToOne User user;
String customerNumber;
//implements equals and hashCode
}
@Entity
class User {
@EmbeddedId UserId id;
Integer age;
//implements equals and hashCode
}
@Embeddable
class UserId implements Serializable {
String firstName;
String lastName;
}
This feature is of limited interest though as you are likely to have chosen the@IdClass approach to stay JPA compliant or you have a quite twisted mind.
Here are the equivalent on hbm.xml files:
<class name="Customer"> <composite-id mapped="true"> <key-many-to-one name="user"> <column name="userfirstname_fk"/> <column name="userlastname_fk"/> </key-many-to-one> <key-property name="customerNumber"/> </composite-id> <property name="preferredCustomer"/></class><class name="User"> <composite-id name="id"> <key-property name="firstName"/> <key-property name="lastName"/> </composite-id> <property name="age"/></class>
Hibernate can generate and populate identifier values for you automatically. This is the recommended approach over "business" or "natural" id (especially composite ids).
Hibernate offers various generation strategies, let's explore the most common ones first that happens to be standardized by JPA:
IDENTITY: supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type
long,shortorint.SEQUENCE (called
seqhiloin Hibernate): uses a hi/lo algorithm to efficiently generate identifiers of typelong,shortorint, given a named database sequence.TABLE (called
MultipleHiLoPerTableGeneratorin Hibernate) : uses a hi/lo algorithm to efficiently generate identifiers of typelong,shortorint, given a table and column as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.AUTO: selects
IDENTITY,SEQUENCEorTABLEdepending upon the capabilities of the underlying database.
Important
We recommend all new projects to use the new enhanced identifier generators. They are deactivated by default for entities using annotations but can be activated usinghibernate.id.new_generator_mappings=true. These new generators are more efficient and closer to the JPA 2 specification semantic.
However they are not backward compatible with existing Hibernate based application (if a sequence or a table is used for id generation). See XXXXXXX??? for more information on how to activate them.
To mark an id property as generated, use the@GeneratedValue annotation. You can specify the strategy used (default toAUTO) by settingstrategy.
@Entitypublic class Customer { @Id @GeneratedValue Integer getId() { ... };}@Entity public class Invoice { @Id @GeneratedValue(strategy=GenerationType.IDENTITY) Integer getId() { ... };}SEQUENCE andTABLE require additional configurations that you can set using@SequenceGenerator and@TableGenerator:
name: name of the generatortable/sequenceName: name of the table or the sequence (defaulting respectively tohibernate_sequencesandhibernate_sequence)catalog/schema:initialValue: the value from which the id is to start generatingallocationSize: the amount to increment by when allocating id numbers from the generator
In addition, theTABLE strategy also let you customize:
pkColumnName: the column name containing the entity identifiervalueColumnName: the column name containing the identifier valuepkColumnValue: the entity identifieruniqueConstraints: any potential column constraint on the table containing the ids
To link a table or sequence generator definition with an actual generated property, use the same name in both the definitionname and the generator valuegenerator as shown below.
@Id
@GeneratedValue(
strategy=GenerationType.SEQUENCE,
generator="SEQ_GEN")
@javax.persistence.SequenceGenerator(
name="SEQ_GEN",
sequenceName="my_sequence",
allocationSize=20
)
public Integer getId() { ... }
The scope of a generator definition can be the application or the class. Class-defined generators are not visible outside the class and can override application level generators. Application level generators are defined in JPA's XML deployment descriptors (see XXXXXX???):
<table-generator name="EMP_GEN"
table="GENERATOR_TABLE"
pk-column-name="key"
value-column-name="hi"
pk-column-value="EMP"
allocation-size="20"/>
//and the annotation equivalent
@javax.persistence.TableGenerator(
name="EMP_GEN",
table="GENERATOR_TABLE",
pkColumnName = "key",
valueColumnName = "hi"
pkColumnValue="EMP",
allocationSize=20
)
<sequence-generator name="SEQ_GEN"
sequence-name="my_sequence"
allocation-size="20"/>
//and the annotation equivalent
@javax.persistence.SequenceGenerator(
name="SEQ_GEN",
sequenceName="my_sequence",
allocationSize=20
)
If a JPA XML descriptor (likeMETA-INF/orm.xml) is used to define the generators,EMP_GEN andSEQ_GEN are application level generators.
Note
Package level definition is not supported by the JPA specification. However, you can use the@GenericGenerator at the package level (see???).
These are the four standard JPA generators. Hibernate goes beyond that and provide additional generators or additional options as we will see below. You can also write your own custom identifier generator by implementingorg.hibernate.id.IdentifierGenerator.
To define a custom generator, use the@GenericGenerator annotation (and its plural counter part@GenericGenerators) that describes the class of the identifier generator or its short cut name (as described below) and a list of key/value parameters. When using@GenericGenerator and assigning it via@GeneratedValue.generator, the@GeneratedValue.strategy is ignored: leave it blank.
@Id @GeneratedValue(generator="system-uuid")
@GenericGenerator(name="system-uuid", strategy = "uuid")
public String getId() {
@Id @GeneratedValue(generator="trigger-generated")
@GenericGenerator(
name="trigger-generated",
strategy = "select",
parameters = @Parameter(name="key", value = "socialSecurityNumber")
)
public String getId() {
The hbm.xml approach uses the optional<generator> child element inside<id>. If any parameters are required to configure or initialize the generator instance, they are passed using the<param> element.
<id name="id" type="long" column="cat_id"> <generator> <param name="table">uid_table</param> <param name="column">next_hi_value_column</param> </generator></id>
All generators implement the interfaceorg.hibernate.id.IdentifierGenerator. This is a very simple interface. Some applications can choose to provide their own specialized implementations, however, Hibernate provides a range of built-in implementations. The shortcut names for the built-in generators are as follows:
incrementgenerates identifiers of type
long,shortorintthat are unique only when no other process is inserting data into the same table.Do not use in a cluster.identitysupports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type
long,shortorint.sequenceuses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is of type
long,shortorinthilouses a hi/lo algorithm to efficiently generate identifiers of type
long,shortorint, given a table and column (by defaulthibernate_unique_keyandnext_hirespectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.seqhilouses a hi/lo algorithm to efficiently generate identifiers of type
long,shortorint, given a named database sequence.uuidGenerates a 128-bit UUID based on a custom algorithm. The value generated is represented as a string of 32 hexidecimal digits. Users can also configure it to use a separator (config parameter "separator") which separates the hexidecimal digits into 8{sep}8{sep}4{sep}8{sep}4. Note specifically that this is different than the IETF RFC 4122 representation of 8-4-4-4-12. If you need RFC 4122 compliant UUIDs, consider using "uuid2" generator discussed below.
uuid2Generates a IETF RFC 4122 compliant (variant 2) 128-bit UUID. The exact "version" (the RFC term) generated depends on the pluggable "generation strategy" used (see below). Capable of generating values as
java.util.UUID,java.lang.Stringor as a byte array of length 16 (byte[16]). The "generation strategy" is defined by the interfaceorg.hibernate.id.UUIDGenerationStrategy. The generator defines 2 configuration parameters for defining which generation strategy to use:uuid_gen_strategy_classNames the UUIDGenerationStrategy class to use
uuid_gen_strategyNames the UUIDGenerationStrategy instance to use
Out of the box, comes with the following strategies:
org.hibernate.id.uuid.StandardRandomStrategy(the default) - generates "version 3" (aka, "random") UUID values via therandomUUIDmethod ofjava.util.UUIDorg.hibernate.id.uuid.CustomVersionOneStrategy- generates "version 1" UUID values, using IP address since mac address not available. If you need mac address to be used, consider leveraging one of the existing third party UUID generators which sniff out mac address and integrating it via theorg.hibernate.id.UUIDGenerationStrategycontract. Two such libraries known at time of this writing includehttp://johannburkard.de/software/uuid/ andhttp://commons.apache.org/sandbox/id/uuid.html
guiduses a database-generated GUID string on MS SQL Server and MySQL.
nativeselects
identity,sequenceorhilodepending upon the capabilities of the underlying database.assignedlets the application assign an identifier to the object before
save()is called. This is the default strategy if no<generator>element is specified.selectretrieves a primary key, assigned by a database trigger, by selecting the row by some unique key and retrieving the primary key value.
foreignuses the identifier of another associated object. It is usually used in conjunction with a
<one-to-one>primary key association.sequence-identitya specialized sequence generation strategy that utilizes a database sequence for the actual value generation, but combines this with JDBC3 getGeneratedKeys to return the generated identifier value as part of the insert statement execution. This strategy is only supported on Oracle 10g drivers targeted for JDK 1.4. Comments on these insert statements are disabled due to a bug in the Oracle drivers.
Thehilo andseqhilo generators provide two alternate implementations of the hi/lo algorithm. The first implementation requires a "special" database table to hold the next available "hi" value. Where supported, the second uses an Oracle-style sequence.
<id name="id" type="long" column="cat_id"> <generator> <param name="table">hi_value</param> <param name="column">next_value</param> <param name="max_lo">100</param> </generator></id>
<id name="id" type="long" column="cat_id"> <generator> <param name="sequence">hi_value</param> <param name="max_lo">100</param> </generator></id>
Unfortunately, you cannot usehilo when supplying your ownConnection to Hibernate. When Hibernate uses an application server datasource to obtain connections enlisted with JTA, you must configure thehibernate.transaction.manager_lookup_class.
The UUID contains: IP address, startup time of the JVM that is accurate to a quarter second, system time and a counter value that is unique within the JVM. It is not possible to obtain a MAC address or memory address from Java code, so this is the best option without using JNI.
For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you can useidentity key generation. For databases that support sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you can usesequence style key generation. Both of these strategies require two SQL queries to insert a new object. For example:
<id name="id" type="long" column="person_id"> <generator> <param name="sequence">person_id_sequence</param> </generator></id>
<id name="id" type="long" column="person_id" unsaved-value="0"> <generator/></id>
For cross-platform development, thenative strategy will, depending on the capabilities of the underlying database, choose from theidentity,sequence andhilo strategies.
If you want the application to assign identifiers, as opposed to having Hibernate generate them, you can use theassigned generator. This special generator uses the identifier value already assigned to the object's identifier property. The generator is used when the primary key is a natural key instead of a surrogate key. This is the default behavior if you do not specify@GeneratedValue nor<generator> elements.
Theassigned generator makes Hibernate useunsaved-value="undefined". This forces Hibernate to go to the database to determine if an instance is transient or detached, unless there is a version or timestamp property, or you defineInterceptor.isUnsaved().
Hibernate does not generate DDL with triggers. It is for legacy schemas only.
<id name="id" type="long" column="person_id"> <generator> <param name="key">socialSecurityNumber</param> </generator></id>
In the above example, there is a unique valued property namedsocialSecurityNumber. It is defined by the class, as a natural key and a surrogate key namedperson_id, whose value is generated by a trigger.
Finally, you can ask Hibernate to copy the identifier from another associated entity. In the Hibernate jargon, it is known as a foreign generator but the JPA mapping reads better and is encouraged.
@Entity
class MedicalHistory implements Serializable {
@Id @OneToOne
@JoinColumn(name = "person_id")
Person patient;
}
@Entity
public class Person implements Serializable {
@Id @GeneratedValue Integer id;
}
Or alternatively
@Entity
class MedicalHistory implements Serializable {
@Id Integer id;
@MapsId @OneToOne
@JoinColumn(name = "patient_id")
Person patient;
}
@Entity
class Person {
@Id @GeneratedValue Integer id;
}
In hbm.xml use the following approach:
<class name="MedicalHistory"> <id name="id"> <generator> <param name="property">patient</param> </generator> </id> <one-to-one name="patient" constrained="true"/></class>
Starting with release 3.2.3, there are 2 new generators which represent a re-thinking of 2 different aspects of identifier generation. The first aspect is database portability; the second is optimization Optimization means that you do not have to query the database for every request for a new identifier value. These two new generators are intended to take the place of some of the named generators described above, starting in 3.3.x. However, they are included in the current releases and can be referenced by FQN.
The first of these new generators isorg.hibernate.id.enhanced.SequenceStyleGenerator which is intended, firstly, as a replacement for thesequence generator and, secondly, as a better portability generator thannative. This is becausenative generally chooses betweenidentity andsequence which have largely different semantics that can cause subtle issues in applications eyeing portability.org.hibernate.id.enhanced.SequenceStyleGenerator, however, achieves portability in a different manner. It chooses between a table or a sequence in the database to store its incrementing values, depending on the capabilities of the dialect being used. The difference between this andnative is that table-based and sequence-based storage have the same exact semantic. In fact, sequences are exactly what Hibernate tries to emulate with its table-based generators. This generator has a number of configuration parameters:
sequence_name(optional, defaults tohibernate_sequence): the name of the sequence or table to be used.initial_value(optional, defaults to1): the initial value to be retrieved from the sequence/table. In sequence creation terms, this is analogous to the clause typically named "STARTS WITH".increment_size(optional - defaults to1): the value by which subsequent calls to the sequence/table should differ. In sequence creation terms, this is analogous to the clause typically named "INCREMENT BY".force_table_use(optional - defaults tofalse): should we force the use of a table as the backing structure even though the dialect might support sequence?value_column(optional - defaults tonext_val): only relevant for table structures, it is the name of the column on the table which is used to hold the value.prefer_sequence_per_entity(optional - defaults tofalse): should we create separate sequence for each entity that share current generator based on its name?sequence_per_entity_suffix(optional - defaults to_SEQ): suffix added to the name of a dedicated sequence.optimizer(optional - defaults tonone): SeeSection 5.1.2.3.1, “Identifier generator optimization”
The second of these new generators isorg.hibernate.id.enhanced.TableGenerator, which is intended, firstly, as a replacement for thetable generator, even though it actually functions much more likeorg.hibernate.id.MultipleHiLoPerTableGenerator, and secondly, as a re-implementation oforg.hibernate.id.MultipleHiLoPerTableGenerator that utilizes the notion of pluggable optimizers. Essentially this generator defines a table capable of holding a number of different increment values simultaneously by using multiple distinctly keyed rows. This generator has a number of configuration parameters:
table_name(optional - defaults tohibernate_sequences): the name of the table to be used.value_column_name(optional - defaults tonext_val): the name of the column on the table that is used to hold the value.segment_column_name(optional - defaults tosequence_name): the name of the column on the table that is used to hold the "segment key". This is the value which identifies which increment value to use.segment_value(optional - defaults todefault): The "segment key" value for the segment from which we want to pull increment values for this generator.segment_value_length(optional - defaults to255): Used for schema generation; the column size to create this segment key column.initial_value(optional - defaults to1): The initial value to be retrieved from the table.increment_size(optional - defaults to1): The value by which subsequent calls to the table should differ.optimizer(optional - defaults to??): SeeSection 5.1.2.3.1, “Identifier generator optimization”.
For identifier generators that store values in the database, it is inefficient for them to hit the database on each and every call to generate a new identifier value. Instead, you can group a bunch of them in memory and only hit the database when you have exhausted your in-memory value group. This is the role of the pluggable optimizers. Currently only the two enhanced generators (Section 5.1.2.3, “Enhanced identifier generators” support this operation.
none(generally this is the default if no optimizer was specified): this will not perform any optimizations and hit the database for each and every request.hilo: applies a hi/lo algorithm around the database retrieved values. The values from the database for this optimizer are expected to be sequential. The values retrieved from the database structure for this optimizer indicates the "group number". Theincrement_sizeis multiplied by that value in memory to define a group "hi value".pooled: as with the case ofhilo, this optimizer attempts to minimize the number of hits to the database. Here, however, we simply store the starting value for the "next group" into the database structure rather than a sequential value in combination with an in-memory grouping algorithm. Here,increment_sizerefers to the values coming from the database.
Hibernate supports the automatic generation of some of the identifier properties. Simply use the@GeneratedValue annotation on one or several id properties.
Warning
The Hibernate team has always felt such a construct as fundamentally wrong. Try hard to fix your data model before using this feature.
@Entity
public class CustomerInventory implements Serializable {
@Id
@TableGenerator(name = "inventory",
table = "U_SEQUENCES",
pkColumnName = "S_ID",
valueColumnName = "S_NEXTNUM",
pkColumnValue = "inventory",
allocationSize = 1000)
@GeneratedValue(strategy = GenerationType.TABLE, generator = "inventory")
Integer id;
@Id @ManyToOne(cascade = CascadeType.MERGE)
Customer customer;
}
@Entity
public class Customer implements Serializable {
@Id
private int id;
}
You can also generate properties inside an@EmbeddedId class.
When using long transactions or conversations that span several database transactions, it is useful to store versioning data to ensure that if the same entity is updated by two conversations, the last to commit changes will be informed and not override the other conversation's work. It guarantees some isolation while still allowing for good scalability and works particularly well in read-often write-sometimes situations.
You can use two approaches: a dedicated version number or a timestamp.
A version or timestamp property should never be null for a detached instance. Hibernate will detect any instance with a null version or timestamp as transient, irrespective of what otherunsaved-value strategies are specified.Declaring a nullable version or timestamp property is an easy way to avoid problems with transitive reattachment in Hibernate. It is especially useful for people using assigned identifiers or composite keys.
You can add optimistic locking capability to an entity using the@Version annotation:
@Entity
public class Flight implements Serializable {
...
@Version
@Column(name="OPTLOCK")
public Integer getVersion() { ... }
}
The version property will be mapped to theOPTLOCK column, and the entity manager will use it to detect conflicting updates (preventing lost updates you might otherwise see with the last-commit-wins strategy).
The version column may be a numeric. Hibernate supports any kind of type provided that you define and implement the appropriateUserVersionType.
The application must not alter the version number set up by Hibernate in any way. To artificially increase the version number, check in Hibernate Entity Manager's reference documentationLockModeType.OPTIMISTIC_FORCE_INCREMENT orLockModeType.PESSIMISTIC_FORCE_INCREMENT.
If the version number is generated by the database (via a trigger for example), make sure to use@org.hibernate.annotations.Generated(GenerationTime.ALWAYS).
To declare a version property in hbm.xml, use:
<version column="version_column"
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Alternatively, you can use a timestamp. Timestamps are a less safe implementation of optimistic locking. However, sometimes an application might use the timestamps in other ways as well.
Simply mark a property of typeDate orCalendar as@Version.
@Entity
public class Flight implements Serializable {
...
@Version
public Date getLastUpdate() { ... }
}
When using timestamp versioning you can tell Hibernate where to retrieve the timestamp value from - database or JVM - by optionally adding the@org.hibernate.annotations.Source annotation to the property. Possible values for the value attribute of the annotation areorg.hibernate.annotations.SourceType.VM andorg.hibernate.annotations.SourceType.DB. The default isSourceType.DB which is also used in case there is no@Source annotation at all.
Like in the case of version numbers, the timestamp can also be generated by the database instead of Hibernate. To do that, use@org.hibernate.annotations.Generated(GenerationTime.ALWAYS).
In hbm.xml, use the<timestamp> element:
<timestamp column="timestamp_column"
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Note
<Timestamp> is equivalent to<version type="timestamp">. And<timestamp source="db"> is equivalent to<version type="dbtimestamp">
You need to decide which property needs to be made persistent in a given entity. This differs slightly between the annotation driven metadata and the hbm.xml files.
In the annotations world, every non static non transient property (field or method depending on the access type) of an entity is considered persistent, unless you annotate it as@Transient. Not having an annotation for your property is equivalent to the appropriate@Basic annotation.
The@Basic annotation allows you to declare the fetching strategy for a property. If set toLAZY, specifies that this property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation, if your classes are not instrumented, property level lazy loading is silently ignored. The default isEAGER. You can also mark a property as not optional thanks to the@Basic.optional attribute. This will ensure that the underlying column are not nullable (if possible). Note that a better approach is to use the@NotNull annotation of the Bean Validation specification.
Let's look at a few examples:
public transient int counter; //transient property
private String firstname; //persistent property
@Transient
String getLengthInMeter() { ... } //transient property
String getName() {... } // persistent property
@Basic
int getLength() { ... } // persistent property
@Basic(fetch = FetchType.LAZY)
String getDetailedComment() { ... } // persistent property
@Temporal(TemporalType.TIME)
java.util.Date getDepartureTime() { ... } // persistent property
@Enumerated(EnumType.STRING)
Starred getNote() { ... } //enum persisted as String in database
counter, a transient field, andlengthInMeter, a method annotated as@Transient, and will be ignored by the Hibernate.name,length, andfirstname properties are mapped persistent and eagerly fetched (the default for simple properties). ThedetailedComment property value will be lazily fetched from the database once a lazy property of the entity is accessed for the first time. Usually you don't need to lazy simple properties (not to be confused with lazy association fetching). The recommended alternative is to use the projection capability of JP-QL (Java Persistence Query Language) or Criteria queries.
JPA support property mapping of all basic types supported by Hibernate (all basic Java types , their respective wrappers and serializable classes). Hibernate Annotations supports out of the box enum type mapping either into a ordinal column (saving the enum ordinal) or a string based column (saving the enum string representation): the persistence representation, defaulted to ordinal, can be overridden through the@Enumerated annotation as shown in thenote property example.
In plain Java APIs, the temporal precision of time is not defined. When dealing with temporal data you might want to describe the expected precision in database. Temporal data can haveDATE,TIME, orTIMESTAMP precision (ie the actual date, only the time, or both). Use the@Temporal annotation to fine tune that.
@Lob indicates that the property should be persisted in a Blob or a Clob depending on the property type:java.sql.Clob,Character[],char[] and java.lang.String will be persisted in a Clob.java.sql.Blob,Byte[],byte[]andSerializable type will be persisted in a Blob.
@Lob
public String getFullText() {
return fullText;
}
@Lob
public byte[] getFullCode() {
return fullCode;
}
If the property type implementsjava.io.Serializable and is not a basic type, and if the property is not annotated with@Lob, then the Hibernateserializable type is used.
You can also manually specify a type using the@org.hibernate.annotations.Type and some parameters if needed.@Type.type could be:
The name of a Hibernate basic type:
integer, string, character, date, timestamp, float, binary, serializable, object, blobetc.The name of a Java class with a default basic type:
int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clobetc.The name of a serializable Java class.
The class name of a custom type:
com.illflow.type.MyCustomTypeetc.
If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct Hibernate type. Hibernate will attempt to interpret the name of the return class of the property getter using, in order, rules 2, 3, and 4.
@org.hibernate.annotations.TypeDef and@org.hibernate.annotations.TypeDefs allows you to declare type definitions. These annotations can be placed at the class or package level. Note that these definitions are global for the session factory (even when defined at the class level). If the type is used on a single entity, you can place the definition on the entity itself. Otherwise, it is recommended to place the definition at the package level. In the example below, when Hibernate encounters a property of classPhoneNumer, it delegates the persistence strategy to the custom mapping typePhoneNumberType. However, properties belonging to other classes, too, can delegate their persistence strategy toPhoneNumberType, by explicitly using the@Type annotation.
Note
Package level annotations are placed in a file namedpackage-info.java in the appropriate package. Place your annotations before the package declaration.
@TypeDef(
name = "phoneNumber",
defaultForType = PhoneNumber.class,
typeClass = PhoneNumberType.class
)
@Entity
public class ContactDetails {
[...]
private PhoneNumber localPhoneNumber;
@Type(type="phoneNumber")
private OverseasPhoneNumber overseasPhoneNumber;
[...]
}
The following example shows the usage of theparameters attribute to customize the TypeDef.
//in org/hibernate/test/annotations/entity/package-info.java
@TypeDefs(
{
@TypeDef(
name="caster",
typeClass = CasterStringType.class,
parameters = {
@Parameter(name="cast", value="lower")
}
)
}
)
package org.hibernate.test.annotations.entity;
//in org/hibernate/test/annotations/entity/Forest.java
public class Forest {
@Type(type="caster")
public String getSmallText() {
...
}
When using composite user type, you will have to express column definitions. The@Columns has been introduced for that purpose.
@Type(type="org.hibernate.test.annotations.entity.MonetaryAmountUserType")
@Columns(columns = {
@Column(name="r_amount"),
@Column(name="r_currency")
})
public MonetaryAmount getAmount() {
return amount;
}
public class MonetaryAmount implements Serializable {
private BigDecimal amount;
private Currency currency;
...
}
By default the access type of a class hierarchy is defined by the position of the@Id or@EmbeddedId annotations. If these annotations are on a field, then only fields are considered for persistence and the state is accessed via the field. If there annotations are on a getter, then only the getters are considered for persistence and the state is accessed via the getter/setter. That works well in practice and is the recommended approach.
Note
The placement of annotations within a class hierarchy has to be consistent (either field or on property) to be able to determine the default access type. It is recommended to stick to one single annotation placement strategy throughout your whole application.
However in some situations, you need to:
force the access type of the entity hierarchy
override the access type of a specific entity in the class hierarchy
override the access type of an embeddable type
The best use case is an embeddable class used by several entities that might not use the same access type. In this case it is better to force the access type at the embeddable class level.
To force the access type on a given class, use the@Access annotation as showed below:
@Entity
public class Order {
@Id private Long id;
public Long getId() { return id; }
public void setId(Long id) { this.id = id; }
@Embedded private Address address;
public Address getAddress() { return address; }
public void setAddress() { this.address = address; }
}
@Entity
public class User {
private Long id;
@Id public Long getId() { return id; }
public void setId(Long id) { this.id = id; }
private Address address;
@Embedded public Address getAddress() { return address; }
public void setAddress() { this.address = address; }
}
@Embeddable
@Access(AcessType.PROPERTY)
public class Address {
private String street1;
public String getStreet1() { return street1; }
public void setStreet1() { this.street1 = street1; }
private hashCode; //not persistent
}
You can also override the access type of a single property while keeping the other properties standard.
@Entity
public class Order {
@Id private Long id;
public Long getId() { return id; }
public void setId(Long id) { this.id = id; }
@Transient private String userId;
@Transient private String orderId;
@Access(AccessType.PROPERTY)
public String getOrderNumber() { return userId + ":" + orderId; }
public void setOrderNumber() { this.userId = ...; this.orderId = ...; }
}
In this example, the default access type isFIELD except for theorderNumber property. Note that the corresponding field, if any must be marked as@Transient ortransient.
@org.hibernate.annotations.AccessType
The annotation@org.hibernate.annotations.AccessType should be considered deprecated for FIELD and PROPERTY access. It is still useful however if you need to use a custom access type.
It is sometimes useful to avoid increasing the version number even if a given property is dirty (particularly collections). You can do that by annotating the property (or collection) with@OptimisticLock(excluded=true).
More formally, specifies that updates to this property do not require acquisition of the optimistic lock.
The column(s) used for a property mapping can be defined using the@Column annotation. Use it to override default values (see the JPA specification for more information on the defaults). You can use this annotation at the property level for properties that are:
not annotated at all
annotated with
@Basicannotated with
@Versionannotated with
@Lobannotated with
@Temporal
@Entity
public class Flight implements Serializable {
...
@Column(updatable = false, name = "flight_name", nullable = false, length=50)
public String getName() { ... }
Thename property is mapped to theflight_name column, which is not nullable, has a length of 50 and is not updatable (making the property immutable).
This annotation can be applied to regular properties as well as@Id or@Version properties.
@Column( name="columnName";
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Sometimes, you want the Database to do some computation for you rather than in the JVM, you might also create some kind of virtual column. You can use a SQL fragment (aka formula) instead of mapping a property into a column. This kind of property is read only (its value is calculated by your formula fragment).
@Formula("obj_length * obj_height * obj_width")
public long getObjectVolume()
The SQL fragment can be as complex as you want and even include subselects.
If a property is not annotated, the following rules apply:
If the property is of a single type, it is mapped as @Basic
Otherwise, if the type of the property is annotated as @Embeddable, it is mapped as @Embedded
Otherwise, if the type of the property is
Serializable, it is mapped as@Basicin a column holding the object in its serialized versionOtherwise, if the type of the property is
java.sql.Cloborjava.sql.Blob, it is mapped as@Lobwith the appropriateLobType
The<property> element declares a persistent JavaBean style property of the class.
<property name="propertyName"
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typename could be:
The name of a Hibernate basic type:
integer, string, character, date, timestamp, float, binary, serializable, object, blobetc.The name of a Java class with a default basic type:
int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clobetc.The name of a serializable Java class.
The class name of a custom type:
com.illflow.type.MyCustomTypeetc.
If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct Hibernate type. Hibernate will attempt to interpret the name of the return class of the property getter using, in order, rules 2, 3, and 4. In certain cases you will need thetype attribute. For example, to distinguish betweenHibernate.DATE andHibernate.TIMESTAMP, or to specify a custom type.
Theaccess attribute allows you to control how Hibernate accesses the property at runtime. By default, Hibernate will call the property get/set pair. If you specifyaccess="field", Hibernate will bypass the get/set pair and access the field directly using reflection. You can specify your own strategy for property access by naming a class that implements the interfaceorg.hibernate.property.PropertyAccessor.
A powerful feature is derived properties. These properties are by definition read-only. The property value is computed at load time. You declare the computation as an SQL expression. This then translates to aSELECT clause subquery in the SQL query that loads an instance:
<property name="totalPrice" formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li, Product p WHERE li.productId = p.productId AND li.customerId = customerId AND li.orderNumber = orderNumber )"/>
You can reference the entity table by not declaring an alias on a particular column. This would becustomerId in the given example. You can also use the nested<formula> mapping element if you do not want to use the attribute.
Embeddable objects (or components) are objects whose properties are mapped to the same table as the owning entity's table. Components can, in turn, declare their own properties, components or collections
It is possible to declare an embedded component inside an entity and even override its column mapping. Component classes have to be annotated at the class level with the@Embeddable annotation. It is possible to override the column mapping of an embedded object for a particular entity using the@Embedded and@AttributeOverride annotation in the associated property:
@Entity
public class Person implements Serializable {
// Persistent component using defaults
Address homeAddress;
@Embedded
@AttributeOverrides( {
@AttributeOverride(name="iso2", column = @Column(name="bornIso2") ),
@AttributeOverride(name="name", column = @Column(name="bornCountryName") )
} )
Country bornIn;
...
}
@Embeddable
public class Address implements Serializable {
String city;
Country nationality; //no overriding here
}
@Embeddable
public class Country implements Serializable {
private String iso2;
@Column(name="countryName") private String name;
public String getIso2() { return iso2; }
public void setIso2(String iso2) { this.iso2 = iso2; }
public String getName() { return name; }
public void setName(String name) { this.name = name; }
...
}
An embeddable object inherits the access type of its owning entity (note that you can override that using the@Access annotation).
ThePerson entity has two component properties,homeAddress andbornIn.homeAddress property has not been annotated, but Hibernate will guess that it is a persistent component by looking for the@Embeddable annotation in the Address class. We also override the mapping of a column name (tobornCountryName) with the@Embedded and@AttributeOverrideannotations for each mapped attribute ofCountry. As you can see,Countryis also a nested component ofAddress, again using auto-detection by Hibernate and JPA defaults. Overriding columns of embedded objects of embedded objects is through dotted expressions.
@Embedded
@AttributeOverrides( {
@AttributeOverride(name="city", column = @Column(name="fld_city") ),
@AttributeOverride(name="nationality.iso2", column = @Column(name="nat_Iso2") ),
@AttributeOverride(name="nationality.name", column = @Column(name="nat_CountryName") )
//nationality columns in homeAddress are overridden
} )
Address homeAddress;
Hibernate Annotations supports something that is not explicitly supported by the JPA specification. You can annotate a embedded object with the@MappedSuperclass annotation to make the superclass properties persistent (see@MappedSuperclass for more informations).
You can also use association annotations in an embeddable object (ie@OneToOne,@ManyToOne,@OneToMany or@ManyToMany). To override the association columns you can use@AssociationOverride.
If you want to have the same embeddable object type twice in the same entity, the column name defaulting will not work as several embedded objects would share the same set of columns. In plain JPA, you need to override at least one set of columns. Hibernate, however, allows you to enhance the default naming mechanism through theNamingStrategy interface. You can write a strategy that prevent name clashing in such a situation.DefaultComponentSafeNamingStrategy is an example of this.
If a property of the embedded object points back to the owning entity, annotate it with the@Parent annotation. Hibernate will make sure this property is properly loaded with the entity reference.
In XML, use the<component> element.
<component name="propertyName"
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The child<property> tags map properties of the child class to table columns.
The<component> element allows a<parent> subelement that maps a property of the component class as a reference back to the containing entity.
The<dynamic-component> element allows aMap to be mapped as a component, where the property names refer to keys of the map. SeeSection 9.5, “Dynamic components” for more information. This feature is not supported in annotations.
Java is a language supporting polymorphism: a class can inherit from another. Several strategies are possible to persist a class hierarchy:
Single table per class hierarchy strategy: a single table hosts all the instances of a class hierarchy
Joined subclass strategy: one table per class and subclass is present and each table persist the properties specific to a given subclass. The state of the entity is then stored in its corresponding class table and all its superclasses
Table per class strategy: one table per concrete class and subclass is present and each table persist the properties of the class and its superclasses. The state of the entity is then stored entirely in the dedicated table for its class.
With this approach the properties of all the subclasses in a given mapped class hierarchy are stored in a single table.
Each subclass declares its own persistent properties and subclasses. Version and id properties are assumed to be inherited from the root class. Each subclass in a hierarchy must define a unique discriminator value. If this is not specified, the fully qualified Java class name is used.
@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
name="planetype",
discriminatorType=DiscriminatorType.STRING
)
@DiscriminatorValue("Plane")
public class Plane { ... }
@Entity
@DiscriminatorValue("A320")
public class A320 extends Plane { ... }
In hbm.xml, for the table-per-class-hierarchy mapping strategy, the<subclass> declaration is used. For example:
<subclass name="ClassName"
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For information about inheritance mappings seeChapter 10,Inheritance mapping.
Discriminators are required for polymorphic persistence using the table-per-class-hierarchy mapping strategy. It declares a discriminator column of the table. The discriminator column contains marker values that tell the persistence layer what subclass to instantiate for a particular row. Hibernate Core supports the follwoing restricted set of types as discriminator column:string,character,integer,byte,short,boolean,yes_no,true_false.
Use the@DiscriminatorColumn to define the discriminator column as well as the discriminator type.
Note
The enum DiscriminatorType used injavax.persitence.DiscriminatorColumn only contains the valuesSTRING,CHAR andINTEGER which means that not all Hibernate supported types are available via the@DiscriminatorColumn annotation.
You can also use@DiscriminatorFormula to express in SQL a virtual discriminator column. This is particularly useful when the discriminator value can be extracted from one or more columns of the table. Both@DiscriminatorColumn and@DiscriminatorFormula are to be set on the root entity (once per persisted hierarchy).
@org.hibernate.annotations.DiscriminatorOptions allows to optionally specify Hibernate specific discriminator options which are not standardized in JPA. The available options areforce andinsert. Theforce attribute is useful if the table contains rows with "extra" discriminator values that are not mapped to a persistent class. This could for example occur when working with a legacy database. Ifforce is set totrue Hibernate will specify the allowed discriminator values in theSELECT query, even when retrieving all instances of the root class. The second option -insert - tells Hibernate whether or not to include the discriminator column in SQLINSERTs. Usually the column should be part of theINSERT statement, but if your discriminator column is also part of a mapped composite identifier you have to set this option tofalse.
Tip
There is also a@org.hibernate.annotations.ForceDiscriminator annotation which is deprecated since version 3.6. Use@DiscriminatorOptions instead.
Finally, use@DiscriminatorValue on each class of the hierarchy to specify the value stored in the discriminator column for a given entity. If you do not set@DiscriminatorValue on a class, the fully qualified class name is used.
@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
name="planetype",
discriminatorType=DiscriminatorType.STRING
)
@DiscriminatorValue("Plane")
public class Plane { ... }
@Entity
@DiscriminatorValue("A320")
public class A320 extends Plane { ... }
In hbm.xml, the<discriminator> element is used to define the discriminator column or formula:
<discriminator column="discriminator_column"
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Actual values of the discriminator column are specified by thediscriminator-value attribute of the<class> and<subclass> elements.
Theformula attribute allows you to declare an arbitrary SQL expression that will be used to evaluate the type of a row. For example:
<discriminator formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end" type="integer"/>Each subclass can also be mapped to its own table. This is called the table-per-subclass mapping strategy. An inherited state is retrieved by joining with the table of the superclass. A discriminator column is not required for this mapping strategy. Each subclass must, however, declare a table column holding the object identifier. The primary key of this table is also a foreign key to the superclass table and described by the@PrimaryKeyJoinColumns or the<key> element.
@Entity @Table(name="CATS")
@Inheritance(strategy=InheritanceType.JOINED)
public class Cat implements Serializable {
@Id @GeneratedValue(generator="cat-uuid")
@GenericGenerator(name="cat-uuid", strategy="uuid")
String getId() { return id; }
...
}
@Entity @Table(name="DOMESTIC_CATS")
@PrimaryKeyJoinColumn(name="CAT")
public class DomesticCat extends Cat {
public String getName() { return name; }
}
Note
The table name still defaults to the non qualified class name. Also if@PrimaryKeyJoinColumn is not set, the primary key / foreign key columns are assumed to have the same names as the primary key columns of the primary table of the superclass.
In hbm.xml, use the<joined-subclass> element. For example:
<joined-subclass name="ClassName"
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Use the<key> element to declare the primary key / foreign key column. The mapping at the start of the chapter would then be re-written as:
<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="eg"> <class name="Cat" table="CATS"> <id name="id" column="uid" type="long"> <generator/> </id> <property name="birthdate" type="date"/> <property name="color" not-null="true"/> <property name="sex" not-null="true"/> <property name="weight"/> <many-to-one name="mate"/> <set name="kittens"> <key column="MOTHER"/> <one-to-many/> </set> <joined-subclass name="DomesticCat" table="DOMESTIC_CATS"> <key column="CAT"/> <property name="name" type="string"/> </joined-subclass> </class> <class name="eg.Dog"> <!-- mapping for Dog could go here --> </class></hibernate-mapping>
For information about inheritance mappings seeChapter 10,Inheritance mapping.
A third option is to map only the concrete classes of an inheritance hierarchy to tables. This is called the table-per-concrete-class strategy. Each table defines all persistent states of the class, including the inherited state. In Hibernate, it is not necessary to explicitly map such inheritance hierarchies. You can map each class as a separate entity root. However, if you wish use polymorphic associations (e.g. an association to the superclass of your hierarchy), you need to use the union subclass mapping.
@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Flight implements Serializable { ... }
Or in hbm.xml:
<union-subclass name="ClassName"
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No discriminator column or key column is required for this mapping strategy.
For information about inheritance mappings seeChapter 10,Inheritance mapping.
This is sometimes useful to share common properties through a technical or a business superclass without including it as a regular mapped entity (ie no specific table for this entity). For that purpose you can map them as@MappedSuperclass.
@MappedSuperclass
public class BaseEntity {
@Basic
@Temporal(TemporalType.TIMESTAMP)
public Date getLastUpdate() { ... }
public String getLastUpdater() { ... }
...
}
@Entity class Order extends BaseEntity {
@Id public Integer getId() { ... }
...
}
In database, this hierarchy will be represented as anOrder table having theid,lastUpdate andlastUpdater columns. The embedded superclass property mappings are copied into their entity subclasses. Remember that the embeddable superclass is not the root of the hierarchy though.
Note
Properties from superclasses not mapped as@MappedSuperclass are ignored.
Note
The default access type (field or methods) is used, unless you use the@Access annotation.
Note
The same notion can be applied to@Embeddable objects to persist properties from their superclasses. You also need to use@MappedSuperclass to do that (this should not be considered as a standard EJB3 feature though)
Note
It is allowed to mark a class as@MappedSuperclass in the middle of the mapped inheritance hierarchy.
Note
Any class in the hierarchy non annotated with@MappedSuperclass nor@Entity will be ignored.
You can override columns defined in entity superclasses at the root entity level using the@AttributeOverride annotation.
@MappedSuperclass
public class FlyingObject implements Serializable {
public int getAltitude() {
return altitude;
}
@Transient
public int getMetricAltitude() {
return metricAltitude;
}
@ManyToOne
public PropulsionType getPropulsion() {
return metricAltitude;
}
...
}
@Entity
@AttributeOverride( name="altitude", column = @Column(name="fld_altitude") )
@AssociationOverride(
name="propulsion",
joinColumns = @JoinColumn(name="fld_propulsion_fk")
)
public class Plane extends FlyingObject {
...
}
Thealtitude property will be persisted in anfld_altitude column of tablePlane and the propulsion association will be materialized in afld_propulsion_fk foreign key column.
You can define@AttributeOverride(s) and@AssociationOverride(s) on@Entity classes,@MappedSuperclass classes and properties pointing to an@Embeddable object.
In hbm.xml, simply map the properties of the superclass in the<class> element of the entity that needs to inherit them.
While not recommended for a fresh schema, some legacy databases force your to map a single entity on several tables.
Using the@SecondaryTable or@SecondaryTables class level annotations. To express that a column is in a particular table, use thetable parameter of@Column or@JoinColumn.
@Entity
@Table(name="MainCat")
@SecondaryTables({
@SecondaryTable(name="Cat1", pkJoinColumns={
@PrimaryKeyJoinColumn(name="cat_id", referencedColumnName="id")
),
@SecondaryTable(name="Cat2", uniqueConstraints={@UniqueConstraint(columnNames={"storyPart2"})})
})
public class Cat implements Serializable {
private Integer id;
private String name;
private String storyPart1;
private String storyPart2;
@Id @GeneratedValue
public Integer getId() {
return id;
}
public String getName() {
return name;
}
@Column(table="Cat1")
public String getStoryPart1() {
return storyPart1;
}
@Column(table="Cat2")
public String getStoryPart2() {
return storyPart2;
}
}
In this example,name will be inMainCat.storyPart1 will be inCat1 andstoryPart2 will be inCat2.Cat1 will be joined toMainCat using thecat_id as a foreign key, andCat2 usingid (ie the same column name, theMainCat id column has). Plus a unique constraint onstoryPart2 has been set.
There is also additional tuning accessible via the@org.hibernate.annotations.Table annotation:
fetch: If set to JOIN, the default, Hibernate will use an inner join to retrieve a secondary table defined by a class or its superclasses and an outer join for a secondary table defined by a subclass. If set toSELECTthen Hibernate will use a sequential select for a secondary table defined on a subclass, which will be issued only if a row turns out to represent an instance of the subclass. Inner joins will still be used to retrieve a secondary defined by the class and its superclasses.inverse: If true, Hibernate will not try to insert or update the properties defined by this join. Default to false.optional: If enabled (the default), Hibernate will insert a row only if the properties defined by this join are non-null and will always use an outer join to retrieve the properties.foreignKey: defines the Foreign Key name of a secondary table pointing back to the primary table.
Make sure to use the secondary table name in theappliesto property
@Entity
@Table(name="MainCat")
@SecondaryTable(name="Cat1")
@org.hibernate.annotations.Table(
appliesTo="Cat1",
fetch=FetchMode.SELECT,
optional=true)
public class Cat implements Serializable {
private Integer id;
private String name;
private String storyPart1;
private String storyPart2;
@Id @GeneratedValue
public Integer getId() {
return id;
}
public String getName() {
return name;
}
@Column(table="Cat1")
public String getStoryPart1() {
return storyPart1;
}
@Column(table="Cat2")
public String getStoryPart2() {
return storyPart2;
}
}
In hbm.xml, use the<join> element.
<join table="tablename"
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For example, address information for a person can be mapped to a separate table while preserving value type semantics for all properties:
<class name="Person" table="PERSON"> <id name="id" column="PERSON_ID">...</id> <join table="ADDRESS"> <key column="ADDRESS_ID"/> <property name="address"/> <property name="zip"/> <property name="country"/> </join> ...
This feature is often only useful for legacy data models. We recommend fewer tables than classes and a fine-grained domain model. However, it is useful for switching between inheritance mapping strategies in a single hierarchy, as explained later.
To link one entity to an other, you need to map the association property as a to one association. In the relational model, you can either use a foreign key or an association table, or (a bit less common) share the same primary key value between the two entities.
To mark an association, use either@ManyToOne or@OnetoOne.
@ManyToOne and@OneToOne have a parameter namedtargetEntity which describes the target entity name. You usually don't need this parameter since the default value (the type of the property that stores the association) is good in almost all cases. However this is useful when you want to use interfaces as the return type instead of the regular entity.
Setting a value of thecascade attribute to any meaningful value other than nothing will propagate certain operations to the associated object. The meaningful values are divided into three categories.
basic operations, which include:
persist, merge, delete, save-update, evict, replicate, lock and refresh;special values:
delete-orphanorall;comma-separated combinations of operation names:
cascade="persist,merge,evict"orcascade="all,delete-orphan". SeeSection 11.11, “Transitive persistence” for a full explanation. Note that single valued many-to-one associations do not support orphan delete.
By default, single point associations are eagerly fetched in JPA 2. You can mark it as lazily fetched by using@ManyToOne(fetch=FetchType.LAZY)in which case Hibernate will proxy the association and load it when the state of the associated entity is reached. You can force Hibernate not to use a proxy by using@LazyToOne(NO_PROXY). In this case, the property is fetched lazily when the instance variable is first accessed. This requires build-time bytecode instrumentation. lazy="false" specifies that the association will always be eagerly fetched.
With the default JPA options, single-ended associations are loaded with a subsequent select if set toLAZY, or a SQL JOIN is used forEAGER associations. You can however adjust the fetching strategy, ie how data is fetched by using@Fetch.FetchMode can beSELECT (a select is triggered when the association needs to be loaded) orJOIN (use a SQL JOIN to load the association while loading the owner entity).JOIN overrides any lazy attribute (an association loaded through aJOIN strategy cannot be lazy).
An ordinary association to another persistent class is declared using a
@ManyToOneif several entities can point to the the target entity@OneToOneif only a single entity can point to the the target entity
and a foreign key in one table is referencing the primary key column(s) of the target table.
@Entity
public class Flight implements Serializable {
@ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} )
@JoinColumn(name="COMP_ID")
public Company getCompany() {
return company;
}
...
}
The@JoinColumn attribute is optional, the default value(s) is the concatenation of the name of the relationship in the owner side,_ (underscore), and the name of the primary key column in the owned side. In this examplecompany_id because the property name iscompany and the column id of Company isid.
@Entity
public class Flight implements Serializable {
@ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE}, targetEntity=CompanyImpl.class )
@JoinColumn(name="COMP_ID")
public Company getCompany() {
return company;
}
...
}
public interface Company {
...
}
You can also map a to one association through an association table. This association table described by the@JoinTable annotation will contains a foreign key referencing back the entity table (through@JoinTable.joinColumns) and a a foreign key referencing the target entity table (through@JoinTable.inverseJoinColumns).
@Entity
public class Flight implements Serializable {
@ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} )
@JoinTable(name="Flight_Company",
joinColumns = @JoinColumn(name="FLIGHT_ID"),
inverseJoinColumns = @JoinColumn(name="COMP_ID")
)
public Company getCompany() {
return company;
}
...
}
Note
You can use a SQL fragment to simulate a physical join column using the@JoinColumnOrFormula /@JoinColumnOrformulas annotations (just like you can use a SQL fragment to simulate a property column via the@Formula annotation).
@Entity
public class Ticket implements Serializable {
@ManyToOne
@JoinColumnOrFormula(formula="(firstname + ' ' + lastname)")
public Person getOwner() {
return person;
}
...
}
You can mark an association as mandatory by using theoptional=false attribute. We recommend to use Bean Validation's@NotNull annotation as a better alternative however. As a consequence, the foreign key column(s) will be marked as not nullable (if possible).
When Hibernate cannot resolve the association because the expected associated element is not in database (wrong id on the association column), an exception is raised. This might be inconvenient for legacy and badly maintained schemas. You can ask Hibernate to ignore such elements instead of raising an exception using the@NotFound annotation.
Example 5.1. @NotFound annotation
@Entity
public class Child {
...
@ManyToOne
@NotFound(action=NotFoundAction.IGNORE)
public Parent getParent() { ... }
...
}
Sometimes you want to delegate to your database the deletion of cascade when a given entity is deleted. In this case Hibernate generates a cascade delete constraint at the database level.
Example 5.2. @OnDelete annotation
@Entity
public class Child {
...
@ManyToOne
@OnDelete(action=OnDeleteAction.CASCADE)
public Parent getParent() { ... }
...
}
Foreign key constraints, while generated by Hibernate, have a fairly unreadable name. You can override the constraint name using@ForeignKey.
Example 5.3. @ForeignKey annotation
@Entity
public class Child {
...
@ManyToOne
@ForeignKey(name="FK_PARENT")
public Parent getParent() { ... }
...
}
alter table Child add constraint FK_PARENT foreign key (parent_id) references Parent
Sometimes, you want to link one entity to an other not by the target entity primary key but by a different unique key. You can achieve that by referencing the unique key column(s) in@JoinColumn.referenceColumnName.
@Entityclass Person { @Id Integer personNumber; String firstName; @Column(name="I") String initial; String lastName;}@Entityclass Home { @ManyToOne @JoinColumns({ @JoinColumn(name="first_name", referencedColumnName="firstName"), @JoinColumn(name="init", referencedColumnName="I"), @JoinColumn(name="last_name", referencedColumnName="lastName"), }) Person owner}This is not encouraged however and should be reserved to legacy mappings.
In hbm.xml, mapping an association is similar. The main difference is that a@OneToOne is mapped as<many-to-one unique="true"/>, let's dive into the subject.
<many-to-one name="propertyName"
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Setting a value of thecascade attribute to any meaningful value other thannone will propagate certain operations to the associated object. The meaningful values are divided into three categories. First, basic operations, which include:persist, merge, delete, save-update, evict, replicate, lock and refresh; second, special values:delete-orphan; and third,all comma-separated combinations of operation names:cascade="persist,merge,evict" orcascade="all,delete-orphan". SeeSection 11.11, “Transitive persistence” for a full explanation. Note that single valued, many-to-one and one-to-one, associations do not support orphan delete.
Here is an example of a typicalmany-to-one declaration:
<many-to-one name="product" column="PRODUCT_ID"/>
Theproperty-ref attribute should only be used for mapping legacy data where a foreign key refers to a unique key of the associated table other than the primary key. This is a complicated and confusing relational model. For example, if theProduct class had a unique serial number that is not the primary key. Theunique attribute controls Hibernate's DDL generation with the SchemaExport tool.
<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>
Then the mapping forOrderItem might use:
<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>
This is not encouraged, however.
If the referenced unique key comprises multiple properties of the associated entity, you should map the referenced properties inside a named<properties> element.
If the referenced unique key is the property of a component, you can specify a property path:
<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>
The second approach is to ensure an entity and its associated entity share the same primary key. In this case the primary key column is also a foreign key and there is no extra column. These associations are always one to one.
Example 5.4. One to One association
@Entity
public class Body {
@Id
public Long getId() { return id; }
@OneToOne(cascade = CascadeType.ALL)
@MapsId
public Heart getHeart() {
return heart;
}
...
}
@Entity
public class Heart {
@Id
public Long getId() { ...}
}
Note
Many people got confused by these primary key based one to one associations. They can only be lazily loaded if Hibernate knows that the other side of the association is always present. To indicate to Hibernate that it is the case, use@OneToOne(optional=false).
In hbm.xml, use the following mapping.
<one-to-one name="propertyName"
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Primary key associations do not need an extra table column. If two rows are related by the association, then the two table rows share the same primary key value. To relate two objects by a primary key association, ensure that they are assigned the same identifier value.
For a primary key association, add the following mappings toEmployee andPerson respectively:
<one-to-one name="person"/>
<one-to-one name="employee" constrained="true"/>
Ensure that the primary keys of the related rows in the PERSON and EMPLOYEE tables are equal. You use a special Hibernate identifier generation strategy calledforeign:
<class name="person" table="PERSON"> <id name="id" column="PERSON_ID"> <generator> <param name="property">employee</param> </generator> </id> ... <one-to-one name="employee" constrained="true"/></class>
A newly saved instance ofPerson is assigned the same primary key value as theEmployee instance referred with theemployee property of thatPerson.
Although we recommend the use of surrogate keys as primary keys, you should try to identify natural keys for all entities. A natural key is a property or combination of properties that is unique and non-null. It is also immutable. Map the properties of the natural key as@NaturalId or map them inside the<natural-id> element. Hibernate will generate the necessary unique key and nullability constraints and, as a result, your mapping will be more self-documenting.
@Entity
public class Citizen {
@Id
@GeneratedValue
private Integer id;
private String firstname;
private String lastname;
@NaturalId
@ManyToOne
private State state;
@NaturalId
private String ssn;
...
}
//and later on query
List results = s.createCriteria( Citizen.class )
.add( Restrictions.naturalId().set( "ssn", "1234" ).set( "state", ste ) )
.list();
Or in XML,
<natural-id mutable="true|false"/> <property ... /> <many-to-one ... /> ......</natural-id>
It is recommended that you implementequals() andhashCode() to compare the natural key properties of the entity.
This mapping is not intended for use with entities that have natural primary keys.
mutable(optional - defaults tofalse): by default, natural identifier properties are assumed to be immutable (constant).
There is one more type of property mapping. The@Any mapping defines a polymorphic association to classes from multiple tables. This type of mapping requires more than one column. The first column contains the type of the associated entity. The remaining columns contain the identifier. It is impossible to specify a foreign key constraint for this kind of association. This is not the usual way of mapping polymorphic associations and you should use this only in special cases. For example, for audit logs, user session data, etc.
The@Any annotation describes the column holding the metadata information. To link the value of the metadata information and an actual entity type, The@AnyDef and@AnyDefs annotations are used. ThemetaType attribute allows the application to specify a custom type that maps database column values to persistent classes that have identifier properties of the type specified byidType. You must specify the mapping from values of themetaType to class names.
@Any( metaColumn = @Column( name = "property_type" ), fetch=FetchType.EAGER )
@AnyMetaDef(
idType = "integer",
metaType = "string",
metaValues = {
@MetaValue( value = "S", targetEntity = StringProperty.class ),
@MetaValue( value = "I", targetEntity = IntegerProperty.class )
} )
@JoinColumn( name = "property_id" )
public Property getMainProperty() {
return mainProperty;
}
Note that@AnyDef can be mutualized and reused. It is recommended to place it as a package metadata in this case.
//on a package
@AnyMetaDef( name="property"
idType = "integer",
metaType = "string",
metaValues = {
@MetaValue( value = "S", targetEntity = StringProperty.class ),
@MetaValue( value = "I", targetEntity = IntegerProperty.class )
} )
package org.hibernate.test.annotations.any;
//in a class
@Any( metaDef="property", metaColumn = @Column( name = "property_type" ), fetch=FetchType.EAGER )
@JoinColumn( name = "property_id" )
public Property getMainProperty() {
return mainProperty;
}
The hbm.xml equivalent is:
<any name="being" id-type="long" meta-type="string"> <meta-value value="TBL_ANIMAL"/> <meta-value value="TBL_HUMAN"/> <meta-value value="TBL_ALIEN"/> <column name="table_name"/> <column name="id"/></any>
Note
You cannot mutualize the metadata in hbm.xml as you can in annotations.
<any name="propertyName"
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The<properties> element allows the definition of a named, logical grouping of the properties of a class. The most important use of the construct is that it allows a combination of properties to be the target of aproperty-ref. It is also a convenient way to define a multi-column unique constraint. For example:
<properties name="logicalName"
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For example, if we have the following<properties> mapping:
<class name="Person"> <id name="personNumber"/> ... <properties name="name" unique="true" update="false"> <property name="firstName"/> <property name="initial"/> <property name="lastName"/> </properties></class>
You might have some legacy data association that refers to this unique key of thePerson table, instead of to the primary key:
<many-to-one name="owner" property-ref="name"> <column name="firstName"/> <column name="initial"/> <column name="lastName"/></many-to-one>
Note
When using annotations as a mapping strategy, such construct is not necessary as the binding between a column and its related column on the associated table is done directly
@Entityclass Person { @Id Integer personNumber; String firstName; @Column(name="I") String initial; String lastName;}@Entityclass Home { @ManyToOne @JoinColumns({ @JoinColumn(name="first_name", referencedColumnName="firstName"), @JoinColumn(name="init", referencedColumnName="I"), @JoinColumn(name="last_name", referencedColumnName="lastName"), }) Person owner}The use of this outside the context of mapping legacy data is not recommended.
The hbm.xml structure has some specificities naturally not present when using annotations, let's describe them briefly.
All XML mappings should declare the doctype shown. The actual DTD can be found at the URL above, in the directoryhibernate-x.x.x/src/org/hibernate, or inhibernate3.jar. Hibernate will always look for the DTD in its classpath first. If you experience lookups of the DTD using an Internet connection, check the DTD declaration against the contents of your classpath.
Hibernate will first attempt to resolve DTDs in its classpath. It does this is by registering a customorg.xml.sax.EntityResolver implementation with the SAXReader it uses to read in the xml files. This customEntityResolver recognizes two different systemId namespaces:
a
hibernate namespaceis recognized whenever the resolver encounters a systemId starting withhttp://www.hibernate.org/dtd/. The resolver attempts to resolve these entities via the classloader which loaded the Hibernate classes.a
user namespaceis recognized whenever the resolver encounters a systemId using aclasspath://URL protocol. The resolver will attempt to resolve these entities via (1) the current thread context classloader and (2) the classloader which loaded the Hibernate classes.
The following is an example of utilizing user namespacing:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" [
<!ENTITY types SYSTEM "classpath://your/domain/types.xml">
]>
<hibernate-mapping package="your.domain">
<class name="MyEntity">
<id name="id" type="my-custom-id-type">
...
</id>
<class>
&types;
</hibernate-mapping>
Wheretypes.xml is a resource in theyour.domain package and contains a customtypedef.
This element has several optional attributes. Theschema andcatalog attributes specify that tables referred to in this mapping belong to the named schema and/or catalog. If they are specified, tablenames will be qualified by the given schema and catalog names. If they are missing, tablenames will be unqualified. Thedefault-cascade attribute specifies what cascade style should be assumed for properties and collections that do not specify acascade attribute. By default, theauto-import attribute allows you to use unqualified class names in the query language.
<hibernate-mapping schema="schemaName"
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If you have two persistent classes with the same unqualified name, you should setauto-import="false". An exception will result if you attempt to assign two classes to the same "imported" name.
Thehibernate-mapping element allows you to nest several persistent<class> mappings, as shown above. It is, however, good practice (and expected by some tools) to map only a single persistent class, or a single class hierarchy, in one mapping file and name it after the persistent superclass. For example,Cat.hbm.xml,Dog.hbm.xml, or if using inheritance,Animal.hbm.xml.
The<key> element is featured a few times within this guide. It appears anywhere the parent mapping element defines a join to a new table that references the primary key of the original table. It also defines the foreign key in the joined table:
<key column="columnname"
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For systems where delete performance is important, we recommend that all keys should be definedon-delete="cascade". Hibernate uses a database-levelON CASCADE DELETE constraint, instead of many individualDELETE statements. Be aware that this feature bypasses Hibernate's usual optimistic locking strategy for versioned data.
Thenot-null andupdate attributes are useful when mapping a unidirectional one-to-many association. If you map a unidirectional one-to-many association to a non-nullable foreign key, youmust declare the key column using<key not-null="true">.
If your application has two persistent classes with the same name, and you do not want to specify the fully qualified package name in Hibernate queries, classes can be "imported" explicitly, rather than relying uponauto-import="true". You can also import classes and interfaces that are not explicitly mapped:
<import rename="Universe"/>
<import
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Note
This feature is unique to hbm.xml and is not supported in annotations.
Mapping elements which accept acolumn attribute will alternatively accept a<column> subelement. Likewise,<formula> is an alternative to theformula attribute. For example:
<column name="column_name" length="N" precision="N" scale="N" not-null="true|false" unique="true|false" unique-key="multicolumn_unique_key_name" index="index_name" sql-type="sql_type_name" check="SQL expression" default="SQL expression" read="SQL expression" write="SQL expression"/>
<formula>SQL expression</formula>
Most of the attributes oncolumn provide a means of tailoring the DDL during automatic schema generation. Theread andwrite attributes allow you to specify custom SQL that Hibernate will use to access the column's value. For more on this, see the discussion ofcolumn read and write expressions.
Thecolumn andformula elements can even be combined within the same property or association mapping to express, for example, exotic join conditions.
<many-to-one name="homeAddress" insert="false" update="false"> <column name="person_id" not-null="true" length="10"/> <formula>'MAILING'</formula></many-to-one>
In relation to the persistence service, Java language-level objects are classified into two groups:
Anentity exists independently of any other objects holding references to the entity. Contrast this with the usual Java model, where an unreferenced object is garbage collected. Entities must be explicitly saved and deleted. Saves and deletions, however, can becascaded from a parent entity to its children. This is different from the ODMG model of object persistence by reachability and corresponds more closely to how application objects are usually used in large systems. Entities support circular and shared references. They can also be versioned.
An entity's persistent state consists of references to other entities and instances ofvalue types. Values are primitives: collections (not what is inside a collection), components and certain immutable objects. Unlike entities, values in particular collections and components,are persisted and deleted by reachability. Since value objects and primitives are persisted and deleted along with their containing entity, they cannot be independently versioned. Values have no independent identity, so they cannot be shared by two entities or collections.
Until now, we have been using the term "persistent class" to refer to entities. We will continue to do that. Not all user-defined classes with a persistent state, however, are entities. Acomponent is a user-defined class with value semantics. A Java property of typejava.lang.String also has value semantics. Given this definition, all types (classes) provided by the JDK have value type semantics in Java, while user-defined types can be mapped with entity or value type semantics. This decision is up to the application developer. An entity class in a domain model will normally have shared references to a single instance of that class, while composition or aggregation usually translates to a value type.
We will revisit both concepts throughout this reference guide.
The challenge is to map the Java type system, and the developers' definition of entities and value types, to the SQL/database type system. The bridge between both systems is provided by Hibernate. For entities,<class>,<subclass> and so on are used. For value types we use<property>,<component>etc., that usually have atype attribute. The value of this attribute is the name of a Hibernatemapping type. Hibernate provides a range of mappings for standard JDK value types out of the box. You can write your own mapping types and implement your own custom conversion strategies.
With the exception of collections, all built-in Hibernate types support null semantics.
The built-inbasic mapping types can be roughly categorized into the following:
integer, long, short, float, double, character, byte, boolean, yes_no, true_falseType mappings from Java primitives or wrapper classes to appropriate (vendor-specific) SQL column types.
boolean, yes_noandtrue_falseare all alternative encodings for a Javabooleanorjava.lang.Boolean.stringA type mapping from
java.lang.StringtoVARCHAR(or OracleVARCHAR2).date, time, timestampType mappings from
java.util.Dateand its subclasses to SQL typesDATE,TIMEandTIMESTAMP(or equivalent).calendar, calendar_dateType mappings from
java.util.Calendarto SQL typesTIMESTAMPandDATE(or equivalent).big_decimal, big_integerType mappings from
java.math.BigDecimalandjava.math.BigIntegertoNUMERIC(or OracleNUMBER).locale, timezone, currencyType mappings from
java.util.Locale,java.util.TimeZoneandjava.util.CurrencytoVARCHAR(or OracleVARCHAR2). Instances ofLocaleandCurrencyare mapped to their ISO codes. Instances ofTimeZoneare mapped to theirID.classA type mapping from
java.lang.ClasstoVARCHAR(or OracleVARCHAR2). AClassis mapped to its fully qualified name.binaryMaps byte arrays to an appropriate SQL binary type.
textMaps long Java strings to a SQL
LONGVARCHARorTEXTtype.imageMaps long byte arrays to a SQL
LONGVARBINARY.serializableMaps serializable Java types to an appropriate SQL binary type. You can also indicate the Hibernate type
serializablewith the name of a serializable Java class or interface that does not default to a basic type.clob, blobType mappings for the JDBC classes
java.sql.Clobandjava.sql.Blob. These types can be inconvenient for some applications, since the blob or clob object cannot be reused outside of a transaction. Driver support is patchy and inconsistent.materialized_clobMaps long Java strings to a SQL
CLOBtype. When read, theCLOBvalue is immediately materialized into a Java string. Some drivers require theCLOBvalue to be read within a transaction. Once materialized, the Java string is available outside of the transaction.materialized_blobMaps long Java byte arrays to a SQL
BLOBtype. When read, theBLOBvalue is immediately materialized into a byte array. Some drivers require theBLOBvalue to be read within a transaction. Once materialized, the byte array is available outside of the transaction.imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date, imm_serializable, imm_binaryType mappings for what are considered mutable Java types. This is where Hibernate makes certain optimizations appropriate only for immutable Java types, and the application treats the object as immutable. For example, you should not call
Date.setTime()for an instance mapped asimm_timestamp. To change the value of the property, and have that change made persistent, the application must assign a new, nonidentical, object to the property.
Unique identifiers of entities and collections can be of any basic type exceptbinary,blob andclob. Composite identifiers are also allowed. See below for more information.
The basic value types have correspondingType constants defined onorg.hibernate.Hibernate. For example,Hibernate.STRING represents thestring type.
It is relatively easy for developers to create their own value types. For example, you might want to persist properties of typejava.lang.BigInteger toVARCHAR columns. Hibernate does not provide a built-in type for this. Custom types are not limited to mapping a property, or collection element, to a single table column. So, for example, you might have a Java propertygetName()/setName() of typejava.lang.String that is persisted to the columnsFIRST_NAME,INITIAL,SURNAME.
To implement a custom type, implement eitherorg.hibernate.UserType ororg.hibernate.CompositeUserType and declare properties using the fully qualified classname of the type. Vieworg.hibernate.test.DoubleStringType to see the kind of things that are possible.
<property name="twoStrings" type="org.hibernate.test.DoubleStringType"> <column name="first_string"/> <column name="second_string"/></property>
Notice the use of<column> tags to map a property to multiple columns.
TheCompositeUserType,EnhancedUserType,UserCollectionType, andUserVersionType interfaces provide support for more specialized uses.
You can even supply parameters to aUserType in the mapping file. To do this, yourUserType must implement theorg.hibernate.usertype.ParameterizedType interface. To supply parameters to your custom type, you can use the<type> element in your mapping files.
<property name="priority"> <type name="com.mycompany.usertypes.DefaultValueIntegerType"> <param name="default">0</param> </type></property>
TheUserType can now retrieve the value for the parameter nameddefault from theProperties object passed to it.
If you regularly use a certainUserType, it is useful to define a shorter name for it. You can do this using the<typedef> element. Typedefs assign a name to a custom type, and can also contain a list of default parameter values if the type is parameterized.
<typedef name="default_zero"> <param name="default">0</param></typedef>
<property name="priority" type="default_zero"/>
It is also possible to override the parameters supplied in a typedef on a case-by-case basis by using type parameters on the property mapping.
Even though Hibernate's rich range of built-in types and support for components means you will rarely need to use a custom type, it is considered good practice to use custom types for non-entity classes that occur frequently in your application. For example, aMonetaryAmount class is a good candidate for aCompositeUserType, even though it could be mapped as a component. One reason for this is abstraction. With a custom type, your mapping documents would be protected against changes to the way monetary values are represented.
It is possible to provide more than one mapping for a particular persistent class. In this case, you must specify anentity name to disambiguate between instances of the two mapped entities. By default, the entity name is the same as the class name. Hibernate lets you specify the entity name when working with persistent objects, when writing queries, or when mapping associations to the named entity.
<class name="Contract" table="Contracts" entity-name="CurrentContract"> ... <set name="history" inverse="true" order-by="effectiveEndDate desc"> <key column="currentContractId"/> <one-to-many entity-name="HistoricalContract"/> </set></class><class name="Contract" table="ContractHistory" entity-name="HistoricalContract"> ... <many-to-one name="currentContract" column="currentContractId" entity-name="CurrentContract"/></class>
Associations are now specified usingentity-name instead ofclass.
Note
This feature is not supported in Annotations
You can force Hibernate to quote an identifier in the generated SQL by enclosing the table or column name in backticks in the mapping document. Hibernate will use the correct quotation style for the SQLDialect. This is usually double quotes, but the SQL Server uses brackets and MySQL uses backticks.
@Entity @Table(name="`Line Item`")class LineItem { @id @Column(name="`Item Id`") Integer id; @Column(name="`Item #`") int itemNumber}<class name="LineItem" table="`Line Item`"> <id name="id" column="`Item Id`"/><generator/></id> <property name="itemNumber" column="`Item #`"/> ...</class>Generated properties are properties that have their values generated by the database. Typically, Hibernate applications needed torefresh objects that contain any properties for which the database was generating values. Marking properties as generated, however, lets the application delegate this responsibility to Hibernate. When Hibernate issues an SQL INSERT or UPDATE for an entity that has defined generated properties, it immediately issues a select afterwards to retrieve the generated values.
Properties marked as generated must additionally be non-insertable and non-updateable. Onlyversions,timestamps, andsimple properties, can be marked as generated.
never (the default): the given property value is not generated within the database.
insert: the given property value is generated on insert, but is not regenerated on subsequent updates. Properties like created-date fall into this category. Even thoughversion andtimestamp properties can be marked as generated, this option is not available.
always: the property value is generated both on insert and on update.
To mark a property as generated, use@Generated.
Hibernate allows you to customize the SQL it uses to read and write the values of columns mapped tosimple properties. For example, if your database provides a set of data encryption functions, you can invoke them for individual columns like this:
@Entityclass CreditCard { @Column(name="credit_card_num") @ColumnTransformer( read="decrypt(credit_card_num)", write="encrypt(?)") public String getCreditCardNumber() { return creditCardNumber; } public void setCreditCardNumber(String number) { this.creditCardNumber = number; } private String creditCardNumber;}or in XML
<property name="creditCardNumber"> <column name="credit_card_num" read="decrypt(credit_card_num)" write="encrypt(?)"/></property>
Note
You can use the plural form@ColumnTransformers if more than one columns need to define either of these rules.
If a property uses more that one column, you must use theforColumn attribute to specify which column, the expressions are targeting.
@Entityclass User { @Type(type="com.acme.type.CreditCardType") @Columns( { @Column(name="credit_card_num"), @Column(name="exp_date") } ) @ColumnTransformer( forColumn="credit_card_num", read="decrypt(credit_card_num)", write="encrypt(?)") public CreditCard getCreditCard() { return creditCard; } public void setCreditCard(CreditCard card) { this.creditCard = card; } private CreditCard creditCard;}Hibernate applies the custom expressions automatically whenever the property is referenced in a query. This functionality is similar to a derived-propertyformula with two differences:
The property is backed by one or more columns that are exported as part of automatic schema generation.
The property is read-write, not read-only.
Thewrite expression, if specified, must contain exactly one '?' placeholder for the value.
Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects. In conjunction with Hibernate's schema evolution tools, they have the ability to fully define a user schema within the Hibernate mapping files. Although designed specifically for creating and dropping things like triggers or stored procedures, any SQL command that can be run via ajava.sql.Statement.execute() method is valid (for example, ALTERs, INSERTS, etc.). There are essentially two modes for defining auxiliary database objects:
The first mode is to explicitly list the CREATE and DROP commands in the mapping file:
<hibernate-mapping> ... <database-object> <create>CREATE TRIGGER my_trigger ...</create> <drop>DROP TRIGGER my_trigger</drop> </database-object></hibernate-mapping>
The second mode is to supply a custom class that constructs the CREATE and DROP commands. This custom class must implement theorg.hibernate.mapping.AuxiliaryDatabaseObject interface.
<hibernate-mapping> ... <database-object> <definition/> </database-object></hibernate-mapping>
Additionally, these database objects can be optionally scoped so that they only apply when certain dialects are used.
<hibernate-mapping> ... <database-object> <definition/> <dialect-scope name="org.hibernate.dialect.Oracle9iDialect"/> <dialect-scope name="org.hibernate.dialect.Oracle10gDialect"/> </database-object></hibernate-mapping>
Note
This feature is not supported in Annotations