You can generate C header and source files
for implementing native methods,
by using the JavaH
Builder.
There are several ways of using the JavaH
Builder.
One typical invocation might look like:
classes = Java(target='classes', source='src/pkg/sub') JavaH(target='native', source=classes)
The source is a list of class files generated by the
call to the Java
Builder,
and the target is the output directory in
which we want the C header files placed.
The target
gets converted into the -d
when SCons runs javah:
% scons -Q
javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java src/pkg/sub/Example3.java
javah -d native -classpath classes pkg.sub.Example1 pkg.sub.Example2 pkg.sub.Example3
In this case,
the call to javah
will generate the header files
native/pkg_sub_Example1.h
,
native/pkg_sub_Example2.h
and
native/pkg_sub_Example3.h
.
Notice that SCons remembered that the class
files were generated with a target directory of
classes
,
and that it then specified that target directory
as the -classpath
option
to the call to javah.
Although it's more convenient to use
the list of class files returned by
the Java
Builder
as the source of a call to the JavaH
Builder,
you can
specify the list of class files
by hand, if you prefer.
If you do,
you need to set the
$JAVACLASSDIR
construction variable
when calling JavaH
:
Java(target='classes', source='src/pkg/sub') class_file_list = [ 'classes/pkg/sub/Example1.class', 'classes/pkg/sub/Example2.class', 'classes/pkg/sub/Example3.class', ] JavaH(target='native', source=class_file_list, JAVACLASSDIR='classes')
The $JAVACLASSDIR
value then
gets converted into the -classpath
when SCons runs javah:
% scons -Q
javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java src/pkg/sub/Example3.java
javah -d native -classpath classes pkg.sub.Example1 pkg.sub.Example2 pkg.sub.Example3
Lastly, if you don't want a separate header file
generated for each source file,
you can specify an explicit File Node
as the target of the JavaH
Builder:
classes = Java(target='classes', source='src/pkg/sub') JavaH(target=File('native.h'), source=classes)
Because SCons assumes by default
that the target of the JavaH
builder is a directory,
you need to use the File
function
to make sure that SCons doesn't
create a directory named native.h
.
When a file is used, though,
SCons correctly converts the file name
into the javah -o
option:
% scons -Q
javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java src/pkg/sub/Example3.java
javah -o native.h -classpath classes pkg.sub.Example1 pkg.sub.Example2 pkg.sub.Example3
Note that the the javah command was
removed from the JDK as of JDK 10, and the approved method
(available since JDK 8) is to use javac
to generate native headers at the same time as the Java source
code is compiled.. As such the JavaH
builder
is of limited utility in later Java versions.