Javac

Description

Compiles a Java source tree.

The source and destination directory will be recursively scanned for Java source files to compile. Only .java files that have no corresponding .class file or where the .class file is older than the .java file will be compiled.

Note: Apache Ant uses only the names of the source and class files to find the classes that need a rebuild. It will not scan the source and therefore will have no knowledge about nested classes, classes that are named differently from the source file, and so on. See the <depend> task for dependency checking based on other than just existence/modification times.

When the source files are part of a package, the directory structure of the source tree should follow the package hierarchy.

It is possible to refine the set of files that are being compiled. This can be done with the includes, includesfile, excludes, and excludesfile attributes. With the includes or includesfile attribute, you specify the files you want to have included. The exclude or excludesfile attribute is used to specify the files you want to have excluded. In both cases, the list of files can be specified by either the filename, relative to the directory(s) specified in the srcdir attribute or nested <src> element(s), or by using wildcard patterns. See the section on directory-based tasks, for information on how the inclusion/exclusion of files works, and how to write wildcard patterns.

It is possible to use different compilers. This can be specified by either setting the global build.compiler property, which will affect all <javac> tasks throughout the build, by setting the compiler attribute, specific to the current <javac> task or by using a nested element of any typedeffed or componentdeffed type that implements org.apache.tools.ant.taskdefs.compilers.CompilerAdapter. Valid values for either the build.compiler property or the compiler attribute are:

• classic (the standard compiler of JDK 1.1/1.2) – javac1.1 and javac1.2 can be used as aliases.
• modern (the standard compiler of JDK 1.3 and later) – javac1.3, javac1.4, javac1.5 and javac1.6, javac1.7 (since Ant 1.8.2), javac1.8 (since Ant 1.8.3), javac1.9 (since Ant 1.9.5), javac9 (since Ant 1.9.8) and javac10+ (since Ant 1.10.2) can be used as aliases.
• jikes (the Jikes compiler).
• jvc (the Command-Line Compiler from Microsoft's SDK for Java / Visual J++) – microsoft can be used as an alias.
• kjc (the kopi compiler).
• gcj (the gcj compiler from GCC).
• sj (Symantec Java compiler) – symantec can be used as an alias.
• extJavac (run either modern or classic in a JVM of its own).

The default is javacX with X depending on the JDK version you use while you are running Ant. If you wish to use a different compiler interface than those supplied, you can write a class that implements the CompilerAdapter interface (package org.apache.tools.ant.taskdefs.compilers). Supply the full classname in the build.compiler property or the compiler attribute.

The fork attribute overrides the build.compiler property or compiler attribute setting and expects a JDK 1.1 or higher to be set in JAVA_HOME.

You can also use the compiler attribute to tell Ant which JDK version it shall assume when it puts together the command line switches—even if you set fork=true. This is useful if you want to run the compiler of JDK 1.1 while your current JDK is 1.2+. If you use compiler=javac1.1 and (for example) depend=true, Ant will use the command line switch -depend instead of -Xdepend.

This task will drop all entries that point to non-existent files/directories from the classpath it passes to the compiler.

The working directory for a forked executable (if any) is the project's base directory.

Windows Note: When the modern compiler is used in unforked mode on Windows, it locks up the files present in the classpath of the <javac> task, and does not release them. The side effect of this is that you will not be able to delete or move those files later on in the build. The workaround is to fork when invoking the compiler.

If your source contains native methods or fields annotated with the @Native annotation you can set the nativeheaderdir attribute in order to use the -h switch of javac to generate the native header files. Note that the logic Ant uses to determine which files to compile does not take native headers into account, i.e. if the .class is more recent than the corresponding .java file the file will not get compiled even if a native header file generated for it would be outdated.

Parameters

Parameters specified as nested elements

This task forms an implicit FileSet and supports most attributes of <fileset> (dir becomes srcdir) as well as the nested <include>, <exclude> and <patternset> elements.

srcdir, classpath, sourcepath, bootclasspath, modulepath, modulesourcepath, upgrademodulepath and extdirs

<javac>'s srcdir, classpath, sourcepath, bootclasspath, extdirs, modulepath, modulesourcepath, and upgrademodulepath attributes are path-like structures and can also be set via nested <src> (note the different name!), <classpath>, <sourcepath>, <bootclasspath>, <extdirs>, <modulepath>, <modulesourcepath> and <upgrademodulepath> elements, respectively.

compilerarg

You can specify additional command line arguments for the compiler with nested <compilerarg> elements. These elements are specified like Command-line Arguments but have an additional attribute that can be used to enable arguments only if a given compiler implementation will be used.

compilerclasspath

Since Ant 1.8.0

A path-like structure holding the classpath to use when loading the compiler implementation if a custom class has been specified. Doesn't have any effect when using one of the built-in compilers.

Any nested element of a type that implements CompilerAdapter

Since Ant 1.8.0

If a defined type implements the CompilerAdapter interface a nested element of that type can be used as an alternative to the compiler attribute.

Examples

Compile all .java files under the ${src} directory, and store the .class files in the ${build} directory. The classpath used includes xyz.jar, and compiling with debug information is on. The source level is 1.4, so you can use assert statements.

<javac srcdir="${src}"
       destdir="${build}"
       classpath="xyz.jar"
       debug="on"
       source="1.4"/>

Compile all .java files under the ${src} directory, and store the .class files in the ${build} directory. Java compiler is forked using the default javac executable. The source level is 1.2 (similar to 1.1 or 1.3) and the class files should be runnable under JDK 1.2+ as well.

<javac srcdir="${src}"
       destdir="${build}"
       fork="true"
       source="1.2"
       target="1.2"/>

Compile all .java files under the ${src} directory, and store the .class files in the ${build} directory. Java compiler is forked using the executable named java$javac.exe. Note that the $ sign needs to be escaped by a second one. The source level is 1.5, so you can use generics.

<javac srcdir="${src}"
       destdir="${build}"
       fork="java$$javac.exe"
       source="1.5"/>

Compile .java files under the ${src} directory, and store the .class files in the ${build} directory. The classpath used includes xyz.jar, and debug information is on. Only files under mypackage/p1 and mypackage/p2 are used. All files in and below the mypackage/p1/testpackage directory are excluded from compilation. You didn't specify a source or target level, so the actual values used will depend on which JDK you ran Ant with.

<javac srcdir="${src}"
       destdir="${build}"
       includes="mypackage/p1/**,mypackage/p2/**"
       excludes="mypackage/p1/testpackage/**"
       classpath="xyz.jar"
       debug="on"/>

This is the same as the previous example, with the addition of a second source path, defined by the property src2. This can also be represented using nested <src> elements as follows:

<javac destdir="${build}"
       classpath="xyz.jar"
       debug="on">
  <src path="${src}"/>
  <src path="${src2}"/>
  <include name="mypackage/p1/**"/>
  <include name="mypackage/p2/**"/>
  <exclude name="mypackage/p1/testpackage/**"/>
</javac>

If you want to run the javac compiler of a different JDK, you should tell Ant where to find the compiler and which version of JDK you will be using so it can choose the correct command line switches. The following example executes a JDK 1.1 javac in a new process and uses the correct command line switches even when Ant is running in a JVM of a different version:

<javac srcdir="${src}"
       destdir="${build}"
       fork="yes"
       executable="/opt/java/jdk1.1/bin/javac"
       compiler="javac1.1"/>

Note: If you wish to compile only source files located in certain packages below a common root, use the include/exclude attributes or <include>/<exclude> nested elements to filter for these packages. Do not include part of your package structure in the srcdir attribute (or nested <src> elements), or Ant will recompile your source files every time you run your compile target. See the Ant FAQ for additional information.

If you wish to compile only files explicitly specified and disable javac's default searching mechanism then you can unset the sourcepath attribute:

<javac sourcepath="" srcdir="${src}"
       destdir="${build}" >
  <include name="**/*.java"/>
  <exclude name="**/Example.java"/>
</javac>

That way the javac will compile all Java source files under ${src} directory but skip the examples. The compiler will even produce errors if some of the non-example files refers to them.

If you wish to compile with a special JDK (another than the one Ant is currently using), set the executable and fork attribute. Using taskname could show in the log, that these settings are fix.

<javac srcdir=""
       destdir=""
       executable="path-to-java14-home/bin/javac"
       fork="true"
       taskname="javac1.4"/>

If you want to activate other compiler options like lint you could use the <compilerarg> element:

<javac srcdir="${src.dir}"
       destdir="${classes.dir}"
       classpathref="libraries">
  <compilerarg value="-Xlint"/>
</javac>

If you want to use a custom CompilerAdapter org.example.MyAdapter you can either use the compiler attribute:

<javac srcdir="${src.dir}"
       destdir="${classes.dir}"
       compiler="org.example.MyAdapter"/>

or a define a type and nest this into the task like in:

<componentdef classname="org.example.MyAdapter"
              name="myadapter"/>
<javac srcdir="${src.dir}"
       destdir="${classes.dir}">
  <myadapter/>
</javac>

in which case your compiler adapter can support attributes and nested elements of its own.

The following examples demonstrate the use of Java 9+ modules.

Compile all .java files in a single module under the ${src} directory, and store the .class files in the ${build} directory. The compilation uses application modules located in modules folder. The source level is 9 to enable modules.

<javac srcdir="${src}"
       destdir="${build}"
       includeantruntime="false"
       modulepath="modules"
       source="9"/>

Compile all .java files in gen/classes, lin32/classes and lin64/classes in all source modules under the ${src} directory. Generate module directories in the ${build} directory. Each generated module directory under the ${build} directory contains .class files from corresponding source module. The * is a token representing the name of any of the modules in the compilation module set. The { ... , ... } express alternates for expansion. The compilation uses application modules located in modules folder. The source level is 9 to enable modules.

<javac modulesourcepath="${src}/*/{gen,lin{32,64}}/classes"
       destdir="${build}"
       includeantruntime="false"
       modulepath="modules"
       source="9"/>

Jikes notes

You need Jikes 1.15 or later.

Jikes supports some extra options, which can be set be defining the properties shown below prior to invoking the task. The setting for each property will be in effect for all <javac> tasks throughout the build. The Ant developers are aware that this is ugly and inflexible – expect a better solution in the future. All the options are boolean, and must be set to true or yes to be interpreted as anything other than false. By default, build.compiler.warnings is true, while all others are false.

Jvc notes

Jvc will enable Microsoft extensions unless you set the property build.compiler.jvc.extensions to false before invoking <javac>.

Bootstrap options

The Sun javac compiler has a -bootclasspath command line option—this corresponds to the bootclasspath attribute/element of the <javac> task. The Sun compiler also allows more control over the boot classpath using the -X and -J-X attributes. One can set these by using the <compilerarg>. Since Ant 1.6.0, there is a shortcut to convert path references to strings that can by used in an OS independent fashion (see pathshortcut). For example:

<path id="lib.path.ref">
  <fileset dir="lib" includes="*.jar"/>
</path>
<javac srcdir="src" destdir="classes">
  <compilerarg arg="-Xbootclasspath/p:${toString:lib.path.ref}"/>
</javac>

OpenJDK notes

The OpenJDK project has provided the javac compiler as an open source project. The output of this project is a javac.jar which contains the javac compiler. This compiler may be used with the <javac> task with the use of a -Xbootclasspath/p Java argument. The argument needs to be given to the runtime system of the javac executable, so it needs to be prepended with a -J, for example:

<property name="patched.javac.jar"
          location="${my.patched.compiler}/dist/lib/javac.jar"/>

<presetdef name="patched.javac">
  <javac fork="yes">
    <compilerarg value="-J-Xbootclasspath/p:${patched.javac.jar}"/>
  </javac>
</presetdef>


<patched.javac srcdir="src/java" destdir="build/classes"
               debug="yes"/>

Note on package-info.java

package-info.java files were introduced in Java 5 to allow package level annotations. On compilation, if the .java file does not contain runtime annotations, there will be no corresponding .class file. Prior to Ant 1.7.1, when the <javac> task is run again, the task will try to compile the package-info.java files again.

With Ant 1.7.1, a different kind of logic was introduced that involved the timestamp of the directory that would normally contain the .class file. This logic turned out to lead to Ant not recompiling package-info.java in certain setup.

Since Ant 1.8.0, an "empty" package-info.class file is created if Ant compiles a package-info.java and no package-info.class file has been created by the compiler itself.