Javah
Description
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.
If you are building with Java 8 or above consider using
javac
'snativeheaderdir
attribute instead which allows you to compile the classes and generate the native header files with a single step.Note the
javah
has been deprecated as of Java 9 and removed as of Java 10. Trying to use it with Java10 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:
- default - the default compiler for the platform.
- sun (the standard compiler of the JDK) - default when not running on Kaffee or gcj/gij or Java9.
- kaffeh (the native standard compiler of Kaffe)
- gcjh (the native standard compiler of gcj and gij) since Apache Ant 1.8.2
- forking - runs the javah executable via its command line interface in a separate process. Default when running on Java9. since Ant 1.9.8
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.
Parameters
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
arg
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. line file path 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 theimplementation
attribute.Examples
<javah destdir="c" class="org.foo.bar.Wibble"/>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="org.foo.bar.Wibble,org.foo.bar.Bobble"/> </javah>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="org.foo.bar.Wibble"/> <class name="org.foo.bar.Bobble"/> <class name="org.foo.bar.Tribble"/> </javah>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="org.foo.bar.Wibble"/> <class name="org.foo.bar.Bobble"/> <class name="org.foo.bar.Tribble"/> </javah> <javah destdir="c" verbose="yes" stubs="yes" old="yes" force="yes"> <class name="org.foo.bar.Wibble"/> <class name="org.foo.bar.Bobble"/> <class name="org.foo.bar.Tribble"/> </javah>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="org.foo.bar.Wibble" implementation="org.example.MyAdapter"/>or a define a type and nest this into the task like in:
<componentdef classname="org.example.MyAdapter" name="myadapter"/> <javah destdir="c" class="org.foo.bar.Wibble"> <myadapter/> </javah>in which case your javah adapter can support attributes and nested elements of its own.