<?xml version='1.0'?>
<!DOCTYPE sconsdoc [
    <!ENTITY % scons SYSTEM "../scons.mod">
    %scons;

    <!ENTITY % builders-mod SYSTEM "../generated/builders.mod">
    %builders-mod;
    <!ENTITY % functions-mod SYSTEM "../generated/functions.mod">
    %functions-mod;
    <!ENTITY % tools-mod SYSTEM "../generated/tools.mod">
    %tools-mod;
    <!ENTITY % variables-mod SYSTEM "../generated/variables.mod">
    %variables-mod;
]>

<chapter id="chap-java"
         xmlns="http://www.scons.org/dbxsd/v1.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 http://www.scons.org/dbxsd/v1.0/scons.xsd">
<title>Java Builds</title>

<!--
SPDX-License-Identifier: MIT

Copyright The SCons Foundation
-->

  <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>

    <scons_example name="java_java">
      <file name="SConstruct" printme="1">
Java('classes', 'src')
      </file>
      <file name="src/Example1.java">
public class Example1
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
      <file name="src/Example2.java">
public class Example2
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
      <file name="src/Example3.java">
public class Example3
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="java_java" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <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>

    <scons_example name="java_java-classes">
      <file name="SConstruct" printme="1">
Java('classes', 'src')
      </file>
      <file name="src/Example1.java">
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");
  }
}
      </file>
      <file name="src/Example2.java">
public class Example2
{
  class Inner2 {
    public static void main(String[] args)
    {
      System.out.println("Hello Java world!\n");
    }
  }
}
      </file>
      <file name="src/Example3.java">
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");
  }
}
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="java_java-classes" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
      <scons_output_command>scons -Q classes</scons_output_command>
    </scons_output>

    <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>

    <scons_output example="java_java-classes" suffix="2">
      <scons_output_command>scons -Q</scons_output_command>
      <scons_output_command>scons -Q -c classes</scons_output_command>
    </scons_output>

    <para>

    To ensure correct handling of <filename>.class</filename>
    dependencies in all cases, you need to tell &SCons; which Java
    version is being used.  This is needed because Java 1.5 changed
    the <filename>.class</filename> file names for nested anonymous
    inner classes.  Use the <varname>JAVAVERSION</varname> construction
    variable to specify the version in use.  With Java 1.6, the
    one-liner example can then be defined like this:

    </para>

    <sconstruct>
Java('classes', 'src', JAVAVERSION='1.6')
    </sconstruct>

    <para>
    See <varname>JAVAVERSION</varname> in the man page for more information.
    </para>

  </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.
    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>

    <scons_example name="java_jar1">
      <file name="SConstruct" printme="1">
Java(target='classes', source='src')
Jar(target='test.jar', source='classes')
      </file>
      <file name="src/Example1.java">
public class Example1
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
      <file name="src/Example2.java">
public class Example2
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
      <file name="src/Example3.java">
public class Example3
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="java_jar1" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <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>

    <scons_example name="java_jar2">
      <file name="SConstruct" printme="1">
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)
      </file>
      <file name="prog1/Example1.java">
public class Example1
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
      <file name="prog1/Example2.java">
public class Example2
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
      <file name="prog2/Example3.java">
public class Example3
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
      <file name="prog2/Example4.java">
public class Example4
{
  public static void main(String[] args)
  {
    System.out.println("Hello Java world!\n");
  }
}
      </file>
    </scons_example>

    <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>

    <scons_output example="java_jar2" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

  </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>

    <scons_example name="java_javah">
      <file name="SConstruct" printme="1">
classes = Java(target='classes', source='src/pkg/sub')
JavaH(target='native', source=classes)
      </file>
      <file name="src/pkg/sub/Example1.java">
package pkg.sub;
public class Example1
{
  public static void main(String[] args)
  {
  }
}
      </file>
      <file name="src/pkg/sub/Example2.java">
package pkg.sub;
public class Example2
{
  public static void main(String[] args)
  {
  }
}
      </file>
      <file name="src/pkg/sub/Example3.java">
package pkg.sub;
public class Example3
{
  public static void main(String[] args)
  {
  }
}
      </file>
    </scons_example>

    <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>

    <scons_output example="java_javah" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <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-link-Java; Builder
    as the source of a call to the &b-link-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>

    <scons_example name="java_JAVACLASSDIR">
      <file name="SConstruct" printme="1">
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')
      </file>
      <file name="src/pkg/sub/Example1.java">
package pkg.sub;
public class Example1
{
  public static void main(String[] args)
  {
  }
}
      </file>
      <file name="src/pkg/sub/Example2.java">
package pkg.sub;
public class Example2
{
  public static void main(String[] args)
  {
  }
}
      </file>
      <file name="src/pkg/sub/Example3.java">
package pkg.sub;
public class Example3
{
  public static void main(String[] args)
  {
  }
}
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="java_JAVACLASSDIR" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <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>

    <scons_example name="java_javah_file">
      <file name="SConstruct" printme="1">
classes = Java(target='classes', source='src/pkg/sub')
JavaH(target=File('native.h'), source=classes)
      </file>
      <file name="src/pkg/sub/Example1.java">
package pkg.sub;
public class Example1
{
  public static void main(String[] args)
  {
  }
}
      </file>
      <file name="src/pkg/sub/Example2.java">
package pkg.sub;
public class Example2
{
  public static void main(String[] args)
  {
  }
}
      </file>
      <file name="src/pkg/sub/Example3.java">
package pkg.sub;
public class Example3
{
  public static void main(String[] args)
  {
  }
}
      </file>
    </scons_example>

    <para>

    Because &SCons; assumes by default
    that the target of the &b-link-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>

    <scons_output example="java_javah_file" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    Note that the the <command>javah</command> command was
    removed from the JDK as of JDK 10, and the approved method
    (available since JDK 8) is to use <command>javac</command>
    to generate native headers at the same time as the Java source
    code is compiled..  As such the &b-link-JavaH; builder
    is of limited utility in later Java versions.

    </para>

  </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>

    <scons_example name="java_RMIC">
      <file name="SConstruct" printme="1">
classes = Java(target='classes', source='src/pkg/sub')
RMIC(target='outdir', source=classes)
      </file>
      <file name="src/pkg/sub/Example1.java">
package pkg.sub;
public class Example1
{
  public static void main(String[] args)
  {
  }
}
      </file>
      <file name="src/pkg/sub/Example2.java">
package pkg.sub;
public class Example2
{
  public static void main(String[] args)
  {
  }
}
      </file>
    </scons_example>

    <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>

    <scons_output example="java_RMIC" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <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>

</chapter>
