So far, we've been using examples of
building C and C++ programs
to demonstrate the features of &SCons;.
&SCons; also supports building Java programs,
but Java builds are handled slightly differently,
which reflects the ways in which
the Java compiler and tools
build programs differently than
other languages' tool chains.
Building Java Class Files: the &b-Java; Builder
The basic activity when programming in Java,
of course, is to take one or more .java files
containing Java source code
and to call the Java compiler
to turn them into one or more
.class files.
In &SCons;, you do this
by giving the &b-link-Java; Builder
a target directory in which
to put the .class files,
and a source directory that contains
the .java files:
Java('classes', 'src')
public class Example1
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class Example2
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class Example3
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
If the src directory contains
three .java source files,
then running &SCons; might look like this:
scons -Q
&SCons; will actually search the src
directory tree for all of the .java files.
The Java compiler will then create the
necessary class files in the classes subdirectory,
based on the class names found in the .java files.
How &SCons; Handles Java Dependencies
In addition to searching the source directory for
.java files,
&SCons; actually runs the .java files
through a stripped-down Java parser that figures out
what classes are defined.
In other words, &SCons; knows,
without you having to tell it,
what .class files
will be produced by the &javac; call.
So our one-liner example from the preceding section:
Java('classes', 'src')
public class Example1
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class AdditionalClass1
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class Example2
{
class Inner2 {
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
}
public class Example3
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class AdditionalClass3
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
Will not only tell you reliably
that the .class files
in the classes subdirectory
are up-to-date:
scons -Qscons -Q classes
But it will also remove all of the generated
.class files,
even for inner classes,
without you having to specify them manually.
For example, if our
Example1.java
and
Example3.java
files both define additional classes,
and the class defined in Example2.java
has an inner class,
running scons -c
will clean up all of those .class files
as well:
scons -Qscons -Q -c classesBuilding Java Archive (<filename>.jar</filename>) Files: the &b-Jar; Builder
After building the class files,
it's common to collect them into
a Java archive (.jar) file,
which you do by calling the &b-link-Jar; Builder method.
If you want to just collect all of the
class files within a subdirectory,
you can just specify that subdirectory
as the &b-Jar; source:
Java(target = 'classes', source = 'src')
Jar(target = 'test.jar', source = 'classes')
public class Example1
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class Example2
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class Example3
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
&SCons; will then pass that directory
to the &jar; command,
which will collect all of the underlying
.class files:
scons -Q
If you want to keep all of the
.class files
for multiple programs in one location,
and only archive some of them in
each .jar file,
you can pass the &b-Jar; builder a
list of files as its source.
It's extremely simple to create multiple
.jar files this way,
using the lists of target class files created
by calls to the &b-link-Java; builder
as sources to the various &b-Jar; calls:
prog1_class_files = Java(target = 'classes', source = 'prog1')
prog2_class_files = Java(target = 'classes', source = 'prog2')
Jar(target = 'prog1.jar', source = prog1_class_files)
Jar(target = 'prog2.jar', source = prog2_class_files)
public class Example1
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class Example2
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class Example3
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
public class Example4
{
public static void main(String[] args)
{
System.out.println("Hello Java world!\n");
}
}
This will then create
prog1.jar
and prog2.jar
next to the subdirectories
that contain their .java files:
scons -QBuilding C Header and Stub Files: the &b-JavaH; Builder
You can generate C header and source files
for implementing native methods,
by using the &b-link-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)
package pkg.sub;
public class Example1
{
public static void main(String[] args)
{
}
}
package pkg.sub;
public class Example2
{
public static void main(String[] args)
{
}
}
package pkg.sub;
public class Example3
{
public static void main(String[] args)
{
}
}
The source is a list of class files generated by the
call to the &b-link-Java; Builder,
and the target is the output directory in
which we want the C header files placed.
The target
gets converted into the
when &SCons; runs &javah;:
scons -Q
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 option
to the call to &javah;.
Although it's more convenient to use
the list of class files returned by
the &b-Java; Builder
as the source of a call to the &b-JavaH; Builder,
you can
specify the list of class files
by hand, if you prefer.
If you do,
you need to set the
&cv-link-JAVACLASSDIR; construction variable
when calling &b-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')
package pkg.sub;
public class Example1
{
public static void main(String[] args)
{
}
}
package pkg.sub;
public class Example2
{
public static void main(String[] args)
{
}
}
package pkg.sub;
public class Example3
{
public static void main(String[] args)
{
}
}
The &cv-JAVACLASSDIR; value then
gets converted into the
when &SCons; runs &javah;:
scons -Q
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 &b-JavaH; Builder:
classes = Java(target = 'classes', source = 'src/pkg/sub')
JavaH(target = File('native.h'), source = classes)
package pkg.sub;
public class Example1
{
public static void main(String[] args)
{
}
}
package pkg.sub;
public class Example2
{
public static void main(String[] args)
{
}
}
package pkg.sub;
public class Example3
{
public static void main(String[] args)
{
}
}
Because &SCons; assumes by default
that the target of the &b-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; option:
scons -QBuilding RMI Stub and Skeleton Class Files: the &b-RMIC; Builder
You can generate Remote Method Invocation stubs
by using the &b-link-RMIC; Builder.
The source is a list of directories,
typically returned by a call to the &b-link-Java; Builder,
and the target is an output directory
where the _Stub.class
and _Skel.class files will
be placed:
classes = Java(target = 'classes', source = 'src/pkg/sub')
RMIC(target = 'outdir', source = classes)
package pkg.sub;
public class Example1
{
public static void main(String[] args)
{
}
}
package pkg.sub;
public class Example2
{
public static void main(String[] args)
{
}
}
As it did with the &b-link-JavaH; Builder,
&SCons; remembers the class directory
and passes it as the option
to &rmic:
scons -Q
This example would generate the files
outdir/pkg/sub/Example1_Skel.class,
outdir/pkg/sub/Example1_Stub.class,
outdir/pkg/sub/Example2_Skel.class and
outdir/pkg/sub/Example2_Stub.class.