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. JNI operates differently depending on whether JDK1.2+ or pre-JDK1.2 systems are used.

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 Yes
destdir sets the directory where javah saves the header files or the stub files.
force specifies that output files should always be written (JDK1.2 only) No
old specifies that old JDK1.0-style header files should be generated (otherwise output file contain JNI-style native method function prototypes) (JDK1.2 only) 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. If this attribute is not set, the default compiler for the current VM will be used. (See the above list of valid compilers.) No

Either outputFile or destdir must be supplied, but not both. 

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

implementationclasspath 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.


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

makes a JNI header of the named class, using the JDK1.2 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 outputFile="wibble.h">
    <class name=","/>

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

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

writes 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" 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=""/>

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

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.