javac

Synopsis

javac [options ] [sourcefiles ] [classes] [@argfiles ]

Arguments can be in any order:

options

Command-line options. SeeOptions.

sourcefiles

One or more source files to be compiled (such asMyClass.java).

classes

One or more classes to be processed for annotations (such asMyPackage.MyClass).

@argfiles

One or more files that list options and source files. The-J options are not allowed in these files. SeeCommand-Line Argument Files.

Description

Thejavac command reads class and interface definitions, written in the Java programming language, and compiles them into bytecode class files. Thejavac command can also process annotations in Java source files and classes.

There are two ways to pass source code file names tojavac.

• For a small number of source files, list the file names on the command line.

• For a large number of source files, list the file names in a file that is separated by blanks or line breaks. Use the list file name preceded by an at sign (@) with thejavac command.

Source code file names must have .java suffixes, class file names must have .class suffixes, and both source and class files must have root names that identify the class. For example, a class calledMyClass would be written in a source file calledMyClass.java and compiled into a bytecode class file calledMyClass.class.

Inner class definitions produce additional class files. These class files have names that combine the inner and outer class names, such asMyClass$MyInnerClass.class.

Arrange source files in a directory tree that reflects their package tree. For example, if all of your source files are in\workspace, then put the source code forcom.mysoft.mypack.MyClass in/workspace in \workspace\com\mysoft\mypack\MyClass.java.

By default, the compiler puts each class file in the same directory as its source file. You can specify a separate destination directory with the-d option.

Options

The compiler has a set of standard options that are supported on the current development environment. An additional set of nonstandard options are specific to the current virtual machine and compiler implementations and are subject to change in the future. Nonstandard options begin with the-X option.

• See alsoCross-Compilation Options

• See alsoNonstandard Options

Enable or Disable Warnings with the -Xlint Option

Enable warningname with the-Xlint:name option, wherename is one of the following warning names. Note that you can disable a warning with the-Xlint:-name: option.

cast

Warns about unnecessary and redundant casts, for example:

String s = (String) "Hello!"

classfile

Warns about issues related to class file contents.

deprecation

Warns about the use of deprecated items, for example:

java.util.Date myDate = new java.util.Date();int currentDay = myDate.getDay();

The methodjava.util.Date.getDay has been deprecated since JDK 1.1

dep-ann

Warns about items that are documented with an@deprecated Javadoc comment, but do not have a@Deprecated annotation, for example:

/**  * @deprecated As of Java SE 7, replaced by {@link #newMethod()}  */public static void deprecatedMethood() { }public static void newMethod() { }

divzero

Warns about division by the constant integer 0, for example:

int divideByZero = 42 / 0;

empty

Warns about empty statements afterif statements, for example:

class E {    void m() {         if (true) ;    }}

fallthrough

Checks the switch blocks for fall-through cases and provides a warning message for any that are found. Fall-through cases are cases in a switch block, other than the last case in the block, whose code does not include a break statement, allowing code execution to fall through from that case to the next case. For example, the code following the case 1 label in this switch block does not end with a break statement:

switch (x) {case 1:  System.out.println("1");  // No break statement here.case 2:  System.out.println("2");}

If the-Xlint:fallthrough option was used when compiling this code, then the compiler emits a warning about possible fall-through into case, with the line number of the case in question.

finally

Warns aboutfinally clauses that cannot complete normally, for example:

public static int m() {  try {     throw new NullPointerException();  }  catch (NullPointerException(); {     System.err.println("Caught NullPointerException.");     return 1;   } finally {     return 0;   }  }

The compiler generates a warning for thefinally block in this example. When theint method is called, it returns a value of 0. Afinally block executes when thetry block exits. In this example, when control is transferred to thecatch block, theint method exits. However, thefinally block must execute, so it is executed, even though control was transferred outside the method.

options

Warns about issues that related to the use of command-line options. SeeCross-Compilation Options.

overrides

Warns about issues regarding method overrides. For example, consider the following two classes:

public class ClassWithVarargsMethod {  void varargsMethod(String... s) { }}public class ClassWithOverridingMethod extends ClassWithVarargsMethod {   @Override   void varargsMethod(String[] s) { }}

The compiler generates a warning similar to the following:.

warning: [override] varargsMethod(String[]) in ClassWithOverridingMethod overrides varargsMethod(String...) in ClassWithVarargsMethod; overridingmethod is missing '...'

When the compiler encounters avarargs method, it translates thevarargs formal parameter into an array. In the methodClassWithVarargsMethod.varargsMethod, the compiler translates thevarargs formal parameterString... s to the formal parameterString[] s, an array, which matches the formal parameter of the methodClassWithOverridingMethod.varargsMethod. Consequently, this example compiles.

path

Warns about invalid path elements and nonexistent path directories on the command line (with regard to the class path, the source path, and other paths). Such warnings cannot be suppressed with the@SuppressWarnings annotation, for example:

javac -Xlint:path -classpath C:\nonexistentpath Example.java

processing

Warn about issues regarding annotation processing. The compiler generates this warning when you have a class that has an annotation, and you use an annotation processor that cannot handle that type of exception. For example, the following is a simple annotation processor:

Source file AnnocProc.java:

import java.util.*;import javax.annotation.processing.*;import javax.lang.model.*;import.javaz.lang.model.element.*;@SupportedAnnotationTypes("NotAnno")public class AnnoProc extends AbstractProcessor {  public boolean process(Set<? extends TypeElement> elems, RoundEnvironment renv){     return true;  }  public SourceVersion getSupportedSourceVersion() {     return SourceVersion.latest();   }}

Source file AnnosWithoutProcessors.java:

@interface Anno { } @Annoclass AnnosWithoutProcessors { }

The following commands compile the annotation processorAnnoProc, then run this annotation processor against the source fileAnnosWithoutProcessors.java:

javac AnnoProc.javajavac -cp . -Xlint:processing -processor AnnoProc -proc:only AnnosWithoutProcessors.java

When the compiler runs the annotation processor against the source fileAnnosWithoutProcessors.java, it generates the following warning:

warning: [processing] No processor claimed any of these annotations: Anno

To resolve this issue, you can rename the annotation defined and used in the classAnnosWithoutProcessors fromAnno toNotAnno.

rawtypes

Warns about unchecked operations on raw types. The following statement generates arawtypes warning:

void countElements(List l) { ... }

The following example does not generate arawtypes warning

void countElements(List<?> l) { ... }

List is a raw type. However,List<?> is an unbounded wildcard parameterized type. BecauseList is a parameterized interface, always specify its type argument. In this example, theList formal argument is specified with an unbounded wildcard (?) as its formal type parameter, which means that thecountElements method can accept any instantiation of theList interface.

Serial

Warns about missingserialVersionUID definitions on serializable classes, for example:

public class PersistentTime implements Serializable{  private Date time;    public PersistentTime() {     time = Calendar.getInstance().getTime();   }    public Date getTime() {     return time;   }}

The compiler generates the following warning:

warning: [serial] serializable class PersistentTime has no definition ofserialVersionUID

If a serializable class does not explicitly declare a field namedserialVersionUID, then the serialization runtime environment calculates a defaultserialVersionUID value for that class based on various aspects of the class, as described in the Java Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declareserialVersionUID values because the default process of computingserialVersionUID vales is highly sensitive to class details that can vary depending on compiler implementations, and as a result, might cause an unexpectedInvalidClassExceptions during deserialization. To guarantee a consistentserialVersionUID value across different Java compiler implementations, a serializable class must declare an explicitserialVersionUID value.

static

Warns about issues relating to the use of statics, for example:

class XLintStatic {    static void m1() { }    void m2() { this.m1(); }}

The compiler generates the following warning:

warning: [static] static method should be qualified by type name, XLintStatic, instead of by an expression

To resolve this issue, you can call thestatic methodm1 as follows:

XLintStatic.m1();

Alternately, you can remove thestatic keyword from the declaration of the methodm1.

try

Warns about issues relating to use oftry blocks, including try-with-resources statements. For example, a warning is generated for the following statement because the resourceac declared in thetry block is not used:

try ( AutoCloseable ac = getResource() ) {    // do nothing}

unchecked

Gives more detail for unchecked conversion warnings that are mandated by the Java Language Specification, for example:

List l = new ArrayList<Number>();List<String> ls = l;       // unchecked warning

During type erasure, the typesArrayList<Number> andList<String> becomeArrayList andList, respectively.

Thels command has the parameterized typeList<String>. When theList referenced byl is assigned tols, the compiler generates an unchecked warning. At compile time, the compiler and JVM cannot determine whetherl refers to aList<String> type. In this case,l does not refer to aList<String> type. As a result, heap pollution occurs.

A heap pollution situation occurs when theList objectl, whose static type isList<Number>, is assigned to anotherList object,ls, that has a different static type,List<String>. However, the compiler still allows this assignment. It must allow this assignment to preserve backward compatibility with releases of Java SE that do not support generics. Because of type erasure,List<Number> andList<String> both becomeList. Consequently, the compiler allows the assignment of the objectl, which has a raw type ofList, to the objectls.

varargs

Warns about unsafe usages of variable arguments (varargs) methods, in particular, those that contain non-reifiable arguments, for example:

public class ArrayBuilder {  public static <T> void addToList (List<T> listArg, T... elements) {    for (T x : elements) {      listArg.add(x);    }  }}

Note: A non-reifiable type is a type whose type information is not fully available at runtime.

The compiler generates the following warning for the definition of the methodArrayBuilder.addToList

warning: [varargs] Possible heap pollution from parameterized vararg type T

When the compiler encounters a varargs method, it translates thevarargs formal parameter into an array. However, the Java programming language does not permit the creation of arrays of parameterized types. In the methodArrayBuilder.addToList, the compiler translates thevarargs formal parameterT... elements to the formal parameterT[] elements, an array. However, because of type erasure, the compiler converts thevarargs formal parameter toObject[] elements. Consequently, there is a possibility of heap pollution.

Command-Line Argument Files

To shorten or simplify thejavac command, you can specify one or more files that contain arguments to thejavac command (except-J options). This enables you to createjavac commands of any length on any operating system.

An argument file can includejavac options and source file names in any combination. The arguments within a file can be separated by spaces or new line characters. If a file name contains embedded spaces, then put the whole file name in double quotation marks.

File Names within an argument file are relative to the current directory, not the location of the argument file. Wild cards (*) are not allowed in these lists (such as for specifying*.java). Use of the at sign (@) to recursively interpret files is not supported. The-J options are not supported because they are passed to the launcher, which does not support argument files.

When executing thejavac command, pass in the path and name of each argument file with the at sign (@) leading character. When thejavac command encounters an argument beginning with the at sign (@), it expands the contents of that file into the argument list.

Example 1 - Single Argument File

You could use a single argument file namedargfile to hold alljavac arguments:

javac @argfile

This argument file could contain the contents of both files shown in Example 2

Example 2 - Two Argument Files

You can create two argument files: one for thejavac options and the other for the source file names. Note that the following lists have no line-continuation characters.

Create a file named options that contains the following:

-d classes-g-sourcepath C:\java\pubs\ws\1.3\src\share\classes

Create a file named classes that contains the following:

MyClass1.javaMyClass2.javaMyClass3.java

Then, run thejavac command as follows:

javac @options @classes

Example 3 - Argument Files with Paths

The argument files can have paths, but any file names inside the files are relative to the current working directory (notpath1 orpath2):

javac @path1/options @path2/classes

Annotation Processing

Thejavac command provides direct support for annotation processing, superseding the need for the separate annotation processing command,apt.

The API for annotation processors is defined in thejavax.annotation.processing and javax.lang.model packages and subpackages.

Searching for Types

To compile a source file, the compiler often needs information about a type, but the type definition is not in the source files specified on the command line. The compiler needs type information for every class or interface used, extended, or implemented in the source file. This includes classes and interfaces not explicitly mentioned in the source file, but that provide information through inheritance.

For example, when you create a subclassjava.applet.Applet, you are also using the ancestor classes ofApplet:java.awt.Panel,java.awt.Container,java.awt.Component, andjava.lang.Object.

When the compiler needs type information, it searches for a source file or class file that defines the type. The compiler searches for class files first in the bootstrap and extension classes, then in the user class path (which by default is the current directory). The user class path is defined by setting theCLASSPATH environment variable or by using the-classpath option.

If you set the-sourcepath option, then the compiler searches the indicated path for source files. Otherwise, the compiler searches the user class path for both class files and source files.

You can specify different bootstrap or extension classes with the-bootclasspath and the-extdirs options. SeeCross-Compilation Options.

A successful type search may produce a class file, a source file, or both. If both are found, then you can use the-Xprefer option to instruct the compiler which to use. Ifnewer is specified, then the compiler uses the newer of the two files. Ifsource is specified, the compiler uses the source file. The default isnewer.

If a type search finds a source file for a required type, either by itself, or as a result of the setting for the-Xprefer option, then the compiler reads the source file to get the information it needs. By default the compiler also compiles the source file. You can use the-implicit option to specify the behavior. Ifnone is specified, then no class files are generated for the source file. Ifclass is specified, then class files are generated for the source file.

The compiler might not discover the need for some type information until after annotation processing completes. When the type information is found in a source file and no-implicit option is specified, the compiler gives a warning that the file is being compiled without being subject to annotation processing. To disable the warning, either specify the file on the command line (so that it will be subject to annotation processing) or use the-implicit option to specify whether or not class files should be generated for such source files.

Programmatic Interface

Thejavac command supports the new Java Compiler API defined by the classes and interfaces in thejavax.tools package.

JavaCompiler javac = ToolProvider.getSystemJavaCompiler();

public static int compile(String[] args);public static int compile(String[] args, PrintWriter out);

Examples

Example 1 - Compile a Simple Program

This example shows how to compile theHello.java source file in the greetings directory. The class defined inHello.java is calledgreetings.Hello. The greetings directory is the package directory both for the source file and the class file and is underneath the current directory. This makes it possible to use the default user class path. It also makes it unnecessary to specify a separate destination directory with the-d option.

The source code inHello.java:

package greetings; public class Hello {    public static void main(String[] args) {        for (int i=0; i < args.length; i++) {            System.out.println("Hello " + args[i]);        }    }}

Compile greetings.Hello:

javac greetings/Hello.java

Rungreetings.Hello:

java greetings.Hello World Universe EveryoneHello WorldHello UniverseHello Everyone

Example 2 - Compile Multiple Source Files

This example compiles theAloha.java,GutenTag.java,Hello.java, andHi.java source files in thegreetings package.

C:\>javac greetings\*.javaC:\>dir greetingsAloha.class         GutenTag.class      Hello.class         Hi.classAloha.java          GutenTag.java       Hello.java          Hi.java

Example 3 - Specify a User Class Path

After changing one of the source files in the previous example, recompile it:

C:\>cd\examplesC:\>javac greetings\Hi.java

Becausegreetings.Hi refers to other classes in thegreetings package, the compiler needs to find these other classes. The previous example works because the default user class path is the directory that contains the package directory. If you want to recompile this file without concern for which directory you are in, then add the examples directory to the user class path by settingCLASSPATH. This example uses the-classpath option.

C:\>javac -classpath \examples \examples\greetings\Hi.java

If you changegreetings.Hi to use a banner utility, then that utility also needs to be accessible through the user class path.

C:\>javac -classpath \examples;\lib\Banners.jar ^            \examples\greetings\Hi.java

To execute a class in thegreetings package, the program needs access to thegreetings package, and to the classes that thegreetings classes use.

C:\>java -classpath \examples;\lib\Banners.jar greetings.Hi

Example 4 - Separate Source Files and Class Files

The following example usesjavac to compile code that runs on JVM 1.7.

javac -source 1.7 -target 1.7 -bootclasspath jdk1.7.0/lib/rt.jar \ -extdirs "" OldCode.java

The-source 1.7 option specifies that release 1.7 (or 7) of the Java programming language be used to compileOldCode.java. The option-target 1.7 option ensures that the generated class files are compatible with JVM 1.7. Note that in most cases, the value of the-target option is the value of the-source option; in this example, you can omit the-target option.

You must specify the-bootclasspath option to specify the correct version of the bootstrap classes (thert.jar library). If not, then the compiler generates a warning:

javac -source 1.7 OldCode.javawarning: [options] bootstrap class path not set in conjunction with -source 1.7

If you do not specify the correct version of bootstrap classes, then the compiler uses the old language rules (in this example, it uses version 1.7 of the Java programming language) combined with the new bootstrap classes, which can result in class files that do not work on the older platform (in this case, Java SE 7) because reference to nonexistent methods can get included.

Example 5 - Cross Compile

This example usesjavac to compile code that runs on JVM 1.7.

javac -source 1.7 -target 1.7 -bootclasspath C:\jdk1.7.0\lib\rt.jar ^            -extdirs "" OldCode.java

The-source 1.7 option specifies that release 1.7 (or 7) of the Java programming language to be used to compile OldCode.java. The-target 1.7 option ensures that the generated class files are compatible with JVM 1.7.

You must specify the-bootclasspath option to specify the correct version of the bootstrap classes (thert.jar library). If not, then the compiler generates a warning:

javac -source 1.7 OldCode.javawarning: [options] bootstrap class path not set in conjunction with -source 1.7

If you do not specify the correct version of bootstrap classes, then the compiler uses the old language rules combined with the new bootstrap classes. This combination can result in class files that do not work on the older platform (in this case, Java SE 7) because reference to nonexistent methods can get included. In this example, the compiler uses release 1.7 of the Java programming language.

See Also

• java(1)

• jdb(1)

• javah(1)

• javadoc(1)

• jar(1)

• jdb(1)