path: root/doc/user/java.sgml

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  <para>

  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.

  </para>

  <section>
  <title>Building Java Class Files:  the &b-Java; Builder</title>

    <para>

    The basic activity when programming in Java,
    of course, is to take one or more <filename>.java</filename> files
    containing Java source code
    and to call the Java compiler
    to turn them into one or more
    <filename>.class</filename> files.
    In &SCons;, you do this
    by giving the &b-link-Java; Builder
    a target directory in which
    to put the <filename>.class</filename> files,
    and a source directory that contains
    the <filename>.java</filename> files:

    </para>

    <programlisting>
      Java('classes', 'src')
    </programlisting>

    <para>

    If the <filename>src</filename> directory contains
    three <filename>.java</filename> source files,
    then running &SCons; might look like this:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
    </screen>

    <para>

    &SCons; will actually search the <filename>src</filename>
    directory tree for all of the <filename>.java</filename> files.
    The Java compiler will then create the
    necessary class files in the <filename>classes</filename> subdirectory,
    based on the class names found in the <filename>.java</filename> files.

    </para>

  </section>

  <section>
  <title>How &SCons; Handles Java Dependencies</title>

    <para>

    In addition to searching the source directory for
    <filename>.java</filename> files,
    &SCons; actually runs the <filename>.java</filename> 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 <filename>.class</filename> files
    will be produced by the &javac; call.
    So our one-liner example from the preceding section:

    </para>

    <programlisting>
      Java('classes', 'src')
    </programlisting>

    <para>

    Will not only tell you reliably
    that the <filename>.class</filename> files
    in the <filename>classes</filename> subdirectory 
    are up-to-date:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
      % <userinput>scons -Q classes</userinput>
      scons: `classes' is up to date.
    </screen>

    <para>

    But it will also remove all of the generated
    <filename>.class</filename> files,
    even for inner classes,
    without you having to specify them manually.
    For example, if our
    <filename>Example1.java</filename>
    and
    <filename>Example3.java</filename>
    files both define additional classes,
    and the class defined in <filename>Example2.java</filename>
    has an inner class,
    running <userinput>scons -c</userinput>
    will clean up all of those <filename>.class</filename> files
    as well:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
      % <userinput>scons -Q -c classes</userinput>
      Removed classes/Example1.class
      Removed classes/AdditionalClass1.class
      Removed classes/Example2$Inner2.class
      Removed classes/Example2.class
      Removed classes/Example3.class
      Removed classes/AdditionalClass3.class
    </screen>

  </section>

  <section>
  <title>Building Java Archive (<filename>.jar</filename>) Files:  the &b-Jar; Builder</title>

    <para>

    After building the class files,
    it's common to collect them into
    a Java archive (<filename>.jar</filename>) 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:

    </para>

    <programlisting>
      Java(target = 'classes', source = 'src')
      Jar(target = 'test.jar', source = 'classes')
    </programlisting>

    <para>

    &SCons; will then pass that directory
    to the &jar; command,
    which will collect all of the underlying
    <filename>.class</filename> files:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
      jar cf test.jar classes
    </screen>

    <para>

    If you want to keep all of the
    <filename>.class</filename> files
    for multiple programs in one location,
    and only archive some of them in
    each <filename>.jar</filename> file,
    you can pass the &b-Jar; builder a
    list of files as its source.
    It's extremely simple to create multiple
    <filename>.jar</filename> 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:

    </para>

    <programlisting>
      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)
    </programlisting>

    <para>

    This will then create
    <filename>prog1.jar</filename>
    and <filename>prog2.jar</filename>
    next to the subdirectories
    that contain their <filename>.java</filename> files:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      javac -d classes -sourcepath prog1 prog1/Example1.java prog1/Example2.java
      javac -d classes -sourcepath prog2 prog2/Example3.java prog2/Example4.java
      jar cf prog1.jar classes/Example1.class classes/Example2.class
      jar cf prog2.jar classes/Example3.class classes/Example4.class
    </screen>

  </section>

  <section>
  <title>Building C Header and Stub Files:  the &b-JavaH; Builder</title>

    <para>

    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:

    </para>

    <programlisting>
      classes = Java(target = 'classes', source = 'src/pkg/sub')
      JavaH(target = 'native', source = classes)
    </programlisting>

    <para>

    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 <option>-d</option>
    when &SCons; runs &javah;:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      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
    </screen>

    <para>

    In this case,
    the call to &javah;
    will generate the header files
    <filename>native/pkg_sub_Example1.h</filename>,
    <filename>native/pkg_sub_Example2.h</filename>
    and
    <filename>native/pkg_sub_Example3.h</filename>.
    Notice that &SCons; remembered that the class
    files were generated with a target directory of
    <filename>classes</filename>,
    and that it then specified that target directory
    as the <option>-classpath</option> option
    to the call to &javah;.

    </para>

    <para>

    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 <emphasis>can</emphasis>
    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;:

    </para>

    <programlisting>
      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')
    </programlisting>

    <para>

    The &cv-JAVACLASSDIR; value then
    gets converted into the <option>-classpath</option>
    when &SCons; runs &javah;:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      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
    </screen>

    <para>

    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:

    </para>

    <programlisting>
      classes = Java(target = 'classes', source = 'src/pkg/sub')
      JavaH(target = File('native.h'), source = classes)
    </programlisting>

    <para>

    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 <filename>native.h</filename>.
    When a file is used, though,
    &SCons; correctly converts the file name
    into the &javah; <option>-o</option> option:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      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
    </screen>

  </section>

  <section>
  <title>Building RMI Stub and Skeleton Class Files:  the &b-RMIC; Builder</title>

    <para>

    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 <filename>_Stub.class</filename>
    and <filename>_Skel.class</filename> files will
    be placed:

    </para>

    <programlisting>
      classes = Java(target = 'classes', source = 'src/pkg/sub')
      RMIC(target = 'outdir', source = classes)
    </programlisting>

    <para>

    As it did with the &b-link-JavaH; Builder,
    &SCons; remembers the class directory
    and passes it as the <option>-classpath</option> option
    to &rmic;:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java
      rmic -d outdir -classpath classes pkg.sub.Example1 pkg.sub.Example2
    </screen>

    <para>

    This example would generate the files
    <filename>outdir/pkg/sub/Example1_Skel.class</filename>,
    <filename>outdir/pkg/sub/Example1_Stub.class</filename>,
    <filename>outdir/pkg/sub/Example2_Skel.class</filename> and
    <filename>outdir/pkg/sub/Example2_Stub.class</filename>.

    </para>

  </section>