Java code compilation

Author: Kasper B. Graversen and Jakob Jenkov, 04/12/2007
Keywords: ANT, Build script, Compile
Abstract: This guide shows you how to compile Java code in ANT and introduces the most commonly used settings for compilations of real-life sized projects.
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Java code compilation

ANT provides a straight forward way to compile Java code. The below task assumes the properties dir.src and dir.dst 
<target name="compile">
    <delete dir="${dir.dst}" />
    <mkdir dir="${dir.dst}" />
    <javac
        srcdir="${dir.src}"
        destdir="${dir.dst}"
        source="5"
        target="5"
        optimize="true"
        debug="off" />
</target>
Often, however, you need a bit more fine-grained control over class paths and which files should be compiled. It is also a good idea to fork the javac process, to circumvent the memory settings the build script was executed with. Below shows you how 
<target name="compile">
    <delete dir="${dir.dst}" />
    <mkdir dir="${dir.dst}" />
    <javac destdir="${webapp.path}/WEB-INF/classes"
           optimize="on"
           debug="on"
           verbose="${flag.java.verbose}"
           deprecation="true"
           failonerror="true"
           encoding="utf-8"

           fork="true"
           memoryInitialSize="100m"
           memoryMaximumSize="512m"

           srcdir="${webapp.path}/WEB-INF/src"
           source="1.5" >
      <classpath>
        <pathelement location="${webapp.path}/WEB-INF/classes"/>
        <fileset dir="${webapp.path}/WEB-INF/lib">
          <include name="*.jar"/>
        </fileset>
        <pathelement location="${tomcat.home}/common/classes"/>
        <fileset dir="${tomcat.home}/common/lib">
          <include name="*.jar"/>
        </fileset>
        <pathelement location="${tomcat.home}/shared/classes"/>
        <fileset dir="${tomcat.home}/shared/lib">
          <include name="*.jar"/>
        </fileset>

        <fileset dir="${webapp.path}/WEB-INF/classes">
          <include name="**/*.jar"/>
        </fileset>

     </classpath>
     <include name="**" />
     <exclude name="tags/**" />

    </javac>
</target>
alternatively, you may assemble all the references to jar files the project depends on under a name such as path.jars. You can have as many fileset definitions you want nested inside the path. 
<path id="path.jars">
    <fileset dir="${lib.dir}">
       <include name="**/*.jar" />
    </fileset>
</path>

<javac fork="true" destdir="${workdir.dir}">
   <classpath refid="path.jars" />
</javac>
You should note that the javactasks supports a significant number of compiler flags (see compiler flags for a full list).

The most commonly used flags are to enable the compilation of deprecated classes, and to maximize the memory allocated to the compiler. You set the compiler flags using a combination of value and line to compilerarg. 
<javac
    ...>
    <compilerarg value="-Xlint:-unchecked" />
    <compilerarg value="-Xlint:-deprecation" />
    <compilerarg line="-Xmaxerrs 30000" />
    <compilerarg line="-Xmaxwarns 30000" />
</javac>
Finally, you should be careful with using the target flag. If you set the target to 5 (that is Java 5 compatible) but compiling using a JDK 6 or later, you will be unable to use the class files. Although there are all sorts of extra flags to support this "legacy compatible compilation", I found it easiest to execute the ANT script within Eclipse and setting the JDK in there to a freshly installed JDK 5.



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