Building other, dependent projects with Ant

Let me preface by saying that I am new to Ant. As in, I just started learning it 2 days ago in order to accomplish this task.

What I'm trying to do is create a "master" ant script that calls several other ant scripts. For example:

Project A
 - contains build.xml, target "makeProjectAjar"
 - output: A.jar

Project B
 - contains build.xml, target "makeProjectBjar"
 - output: B.jar

Project C
 - contains build.xml, target "makeProjectCjar"
 - output: C.jar

Project D
 - contains build.xml, target "finalPackage"
 - Must first build Project A, Project B, Project C
 - Copy A.jar, B.jar, C.jar to Project D
 - Package all of this into D.jar

Right now I have all of the individual builds for A, B, and C working. By this, I mean I can run 'ant' from any of these folders and it will build the project and produce a jar file. If I build A, B, and C this way, I have 3 jar files. I can then copy them into D and package it into the final jar. That works fine.

But what I'd like is to be able to trigger the A, B, and C builds from within build.xml of Project D. I've tried this:

<?xml version="1.0" encoding="UTF-8"?>

<project name="CommonJava" default="makeCommonJavaJar" basedir=".">

    <import file="../Common/build.xml"/>
    <import file="../CommonAndroid/build.xml"/>

    <target name="makeCommonJavaJar" description="Create a jar for the CommonJava project"
        depends="clean">
        <mkdir dir="build" />
        <jar jarfile="./build/commonjava.jar" includes="*.class" />
    </target>

    <target name="clean">
        <delete dir="build" />
    </target>

    <target name="build+package" description="Build all prerequisites and package them">
        <antcall target="makeCommonJar"/>
        <antcall target="makeAndroidJar"/>
    </target>
</project>

makeCommonJar is a target in Common/build.xml (Project A) and makeAndroidJar is a target in ../CommonAndroid/build.xml (Project B). However, it seems that when using <import>, it runs from the context of the calling folder. Meaning resources referenced by Project B are unavailable since it's running from Project D. Does that make sense?

Long question short... using Ant, how can I call build.xml files in other projects in order to build those projects first?

Solution

Here are some general guidelines (not rules, Ant is quite liberal):

a build.xml is generally designed to be run on a specific project, so it is not 'imported' in other builds
the import task is generally used to share some comme piece of build mechanic, not a piece of build workflow like you are doing. For instance a target to build jar, parameterized by some properties like the target directory.
the subant and ant tasks are preferred used to trigger builds of other project

So here is what would look like a build.xml for project D:

<project name="CommonJava" default="makeCommonJavaJar" basedir=".">

    <target name="build-deps">
        <ant antfile="../Common/build.xml" target="makeCommonJar"/>
        <ant antfile="../CommonAndroid/build.xml" target="makeAndroidJar"/>
    </target>

    <target name="makeCommonJavaJar" depends="build-deps">
        <mkdir dir="build" />
        <jar jarfile="./build/commonjava.jar" includes="*.class" />
    </target>

</project>

See http://ant.apache.org/manual/Tasks/ant.html