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Interface Message

All Known Subinterfaces:
BytesMessage,MapMessage,ObjectMessage,StreamMessage,TextMessage
public interfaceMessage

TheMessage interface is the root interface of all JMS messages. It defines the message header and theacknowledge method used for all messages.

Most message-oriented middleware (MOM) products treat messages as lightweight entities that consist of a header and a payload. The header contains fields used for message routing and identification; the payload contains the application data being sent.

Within this general form, the definition of a message varies significantly across products. It would be quite difficult for the JMS API to support all of these message models.

With this in mind, the JMS message model has the following goals:

JMS messages are composed of the following parts:

Message Bodies

The JMS API defines five types of message body:

Message Headers

TheJMSCorrelationID header field is used for linking one message with another. It typically links a reply message with its requesting message.

JMSCorrelationID can hold a provider-specific message ID, an application-specificString object, or a provider-nativebyte[] value.

Message Properties

AMessage object contains a built-in facility for supporting application-defined property values. In effect, this provides a mechanism for adding application-specific header fields to a message.

Properties allow an application, via message selectors, to have a JMS provider select, or filter, messages on its behalf using application-specific criteria.

Property names must obey the rules for a message selector identifier.

Property values can beboolean,byte,short,int,long,float,double, andString.

Property values are set prior to sending a message. When a client receives a message, its properties are in read-only mode. If a client attempts to set properties at this point, aMessageNotWriteableException is thrown. IfclearProperties is called, the properties can now be both read from and written to. Note that header fields are distinct from properties. Header fields are never in read-only mode.

A property value may duplicate a value in a message's body, or it may not. Although JMS does not define a policy for what should or should not be made a property, application developers should note that JMS providers will likely handle data in a message's body more efficiently than data in a message's properties. For best performance, applications should use message properties only when they need to customize a message's header. The primary reason for doing this is to support customized message selection.

Message properties support the following conversion table. The marked cases must be supported. The unmarked cases must throw aJMSException. TheString-to-primitive conversions may throw a runtime exception if the primitive'svalueOf method does not accept theString as a valid representation of the primitive.

A value written as the row type can be read as the column type.

 |        | boolean byte short int long float double String  |---------------------------------------------------------- |boolean |    X                                       X |byte    |          X     X    X   X                  X  |short   |                X    X   X                  X  |int     |                     X   X                  X  |long    |                         X                  X  |float   |                               X     X      X  |double  |                                     X      X  |String  |    X     X     X    X   X     X     X      X  |----------------------------------------------------------

In addition to the type-specific set/get methods for properties, JMS provides thesetObjectProperty andgetObjectProperty methods. These support the same set of property types using the objectified primitive values. Their purpose is to allow the decision of property type to made at execution time rather than at compile time. They support the same property value conversions.

ThesetObjectProperty method accepts values of classBoolean,Byte,Short,Integer,Long,Float,Double, andString. An attempt to use any other class must throw aJMSException.

ThegetObjectProperty method only returns values of classBoolean,Byte,Short,Integer,Long,Float,Double, andString.

The order of property values is not defined. To iterate through a message's property values, usegetPropertyNames to retrieve a property name enumeration and then use the various property get methods to retrieve their values.

A message's properties are deleted by theclearProperties method. This leaves the message with an empty set of properties.

Getting a property value for a name which has not been set returns a null value. Only thegetStringProperty andgetObjectProperty methods can return a null value. Attempting to read a null value as a primitive type must be treated as calling the primitive's correspondingvalueOf(String) conversion method with a null value.

The JMS API reserves theJMSX property name prefix for JMS defined properties. The full set of these properties is defined in the Java Message Service specification. New JMS defined properties may be added in later versions of the JMS API. Support for these properties is optional. TheString[] ConnectionMetaData.getJMSXPropertyNames method returns the names of the JMSX properties supported by a connection.

JMSX properties may be referenced in message selectors whether or not they are supported by a connection. If they are not present in a message, they are treated like any other absent property.

JMSX properties defined in the specification as "set by provider on send" are available to both the producer and the consumers of the message. JMSX properties defined in the specification as "set by provider on receive" are available only to the consumers.

JMSXGroupID andJMSXGroupSeq are standard properties that clients should use if they want to group messages. All providers must support them. Unless specifically noted, the values and semantics of the JMSX properties are undefined.

The JMS API reserves theJMS_vendor_name property name prefix for provider-specific properties. Each provider defines its own value forvendor_name. This is the mechanism a JMS provider uses to make its special per-message services available to a JMS client.

The purpose of provider-specific properties is to provide special features needed to integrate JMS clients with provider-native clients in a single JMS application. They should not be used for messaging between JMS clients.

Provider Implementations of JMS Message Interfaces

The JMS API provides a set of message interfaces that define the JMS message model. It does not provide implementations of these interfaces.

Each JMS provider supplies a set of message factories with itsSession object for creating instances of messages. This allows a provider to use message implementations tailored to its specific needs.

A provider must be prepared to accept message implementations that are not its own. They may not be handled as efficiently as its own implementation; however, they must be handled.

Note the following exception case when a provider is handling a foreign message implementation. If the foreign message implementation contains aJMSReplyTo header field that is set to a foreign destination implementation, the provider is not required to handle or preserve the value of this header field.

Message Selectors

A JMS message selector allows a client to specify, by header field references and property references, the messages it is interested in. Only messages whose header and property values match the selector are delivered. What it means for a message not to be delivered depends on theMessageConsumer being used (seeQueueReceiver andTopicSubscriber).

Message selectors cannot reference message body values.

A message selector matches a message if the selector evaluates to true when the message's header field values and property values are substituted for their corresponding identifiers in the selector.

A message selector is aString whose syntax is based on a subset of the SQL92 conditional expression syntax. If the value of a message selector is an empty string, the value is treated as a null and indicates that there is no message selector for the message consumer.

The order of evaluation of a message selector is from left to right within precedence level. Parentheses can be used to change this order.

Predefined selector literals and operator names are shown here in uppercase; however, they are case insensitive.

A selector can contain: