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 -Q scons -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 -Q scons -Q -c classes To ensure correct handling of .class dependencies in all cases, you need to tell &SCons; which Java version is being used. This is needed because Java 1.5 changed the .class file names for nested anonymous inner classes. Use the JAVAVERSION construction variable to specify the version in use. With Java 1.6, the one-liner example can then be defined like this: Java('classes', 'src', JAVAVERSION='1.6') See JAVAVERSION in the man page for more information.
Building 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 -Q
Building 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 -Q
Building 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.