Generates JNI headers from a Java class.

When this task executes, it will generate the C header and source files that are needed to implement native methods.

If you are building with Java 8 or above consider using javac's nativeheaderdir attribute instead which allows you to compile the classes and generate the native header files in a single step.

Note: javah has been deprecated in Java 9 and removed in Java 10. Attempts to use it with Java 10 will fail.

It is possible to use different compilers. This can be selected with the implementation attribute or a nested element. Here are the choices of the attribute:

Note: if you are using this task to work on multiple files the command line may become too long on some operating systems. Unfortunately the javah command doesn't support command argument files the way javac (for example) does, so all that can be done is breaking the amount of classes to compile into smaller chunks.


Attribute Description Required
class the fully-qualified name of the class (or classes, separated by commas) Yes
outputFile concatenates the resulting header or source files for all the classes listed into this file Exactly one of the two
destdir sets the directory where javah saves the header files or the stub files.
force specifies that output files should always be written (only when using an external javah of JDK 1.2) No
old specifies that old JDK 1.0-style header files should be generated (otherwise output file contain JNI-style native method function prototypes) (only when using an external javah of JDK 1.2) No
stubs generate C declarations from the Java object file (used with old) No
verbose causes javah to print a message concerning the status of the generated files No
classpath the classpath to use No
bootclasspath location of bootstrap class files No
extdirs location of installed extensions No
implementation The compiler implementation to use. (See the above list of valid compilers.) No; defaults to default compiler for the current JDK

Parameters specified as nested elements


You can specify additional command line arguments for the compiler with nested <arg> 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.

Attribute Description Required
value See Command-line Arguments. Exactly one of these
prefix See Command-line Arguments. Since Ant 1.8. No
suffix No
implementation Only pass the specified argument if the chosen compiler implementation matches the value of this attribute. Legal values are the same as those in the above list of valid compilers.) No


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 JavahAdapter

Since Ant 1.8.0

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


Make a JNI header of the named class, using the JDK 1.2 JNI model. Assuming the directory c already exists, the file org_foo_bar_Wibble.h is created there. If this file already exists, it is left unchanged.

<javah destdir="c" class=""/>

This is similar to the previous example, except the output is written to a file called wibble.h in the current directory.

<javah outputFile="wibble.h">
  <class name=","/>

Write three header files, one for each of the classes named. Because the force option is set, these header files are always written when the Javah task is invoked, even if they already exist.

<javah destdir="c" force="yes">
  <class name=""/>
  <class name=""/>
  <class name=""/>

Write the headers for the three classes using the 'old' JNI format, then write the corresponding .c stubs. The verbose option will cause Javah to describe its progress.

<javah destdir="c" verbose="yes" old="yes" force="yes">
  <class name=""/>
  <class name=""/>
  <class name=""/>
<javah destdir="c" verbose="yes" stubs="yes" old="yes" force="yes">
  <class name=""/>
  <class name=""/>
  <class name=""/>

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

<javah destdir="c" class=""

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

<componentdef classname="org.example.MyAdapter"
<javah destdir="c" class="">

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