Why does ant compile all classes each run?

Question

I'm more accustomed to make, so I'm confused why ant recompiles classes when the source hasn't been changed. I've read that there is a requirement to recompile in some cases where generics are used, but I'm not sure that this will be necessary for my project.

Also, in the javac task, I've set includeDestClasses="true"

Here's some of the targets I'm using

<target name="init">
        <mkdir dir="${build}"/>
        <mkdir dir="${dist}"/>
    </target>
    <target name="compile" depends="init,util,semantics" description=""/>
    <target name="util" depends="" description="">
        <javac destdir="${build}" classpath="project.class.path" debug="on" srcdir="${src}/util" includeDestClasses="true" source="1.5">
            <classpath refid="project.class.path"/>
        </javac>
    </target>

Answer 1

Try modifying the opening tag of the javac task to include both a srcdir attribute and an includes attribute:

<javac destdir="${build}" classpath="project.class.path" debug="on" srcdir="${src}" includes="util/**" includeDestClasses="true" source="1.5">

Answer 2

Your src & dest directories are not equivalent, so ant is not able to effectively stat the output files to compare them.

This is an FAQ: http://ant.apache.org/faq.html#always-recompiles

Answer 3

In my experience the javac target will not compile all the classes, only the ones in need of it, even without the includeDestClasses attribute. In fact I usually set up two (or more) compile targets, one that does a complete compile (forced by deleting the output directory) and one that does a quick updating compile, much like your javac line. Are you sure that one of your dependencies isn't deleting the output directory?