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Ant: Debugging Classpaths

At 11:14 PM on May 1, 2006, R.J. Lorimer wrote:

A lot of times when I'm working on an Ant build script, I'll run into compiler errors that I have no reason to believe are valid. You know the kind; you've written a script and you've set up all of your classpaths, and then you run the script and it starts complaining about classfiles missing that you *just know* are not missing.

This process can be a real pain to debug, particularly because Ant isn't exactly transparent in its running (Eclipse has the ability to debug an Ant script, but that's a different discussion). One technique I have used many, many times is to have Ant print out the classpath I'm getting ready to compile against so that I can see what class files, JAR files, and other minutia are actually on the classpath so that there is no doubt on my mind that the correct dependencies are being referenced.

To have Ant print out a classpath, you simply have to give your classpath a ref-id. This allows you to set up the correct variables later for printing. Here is an example. This is a snippet Ant file example that may look something similar to an ant script you have floating around:

<?xml version="1.0"?>
<project name="project" default="default">
	<property name="lib" value="web/WEB-INF/lib"/>
	<property name="src" value="src"/>
	<property name="dist" value="dist"/>
		
	<path id="classpath">
	  <fileset dir="${lib}">
		  <include name="**/*.jar"/>
	  </fileset>
	</path>

    <target name="default" description="--> description">
    	<javac srcdir="${src}" destdir="${dist}">
    		<classpath refid="classpath"/>
	</javac>
    </target>

</project>

If we wanted to debug the 'classpath' path attribute to make sure it had every JAR we anticipated, we could do this:

<?xml version="1.0"?>
<project name="project" default="default">
	<property name="lib" value="web/WEB-INF/lib"/>
	<property name="src" value="src"/>
	<property name="dist" value="dist"/>
		
	<path id="classpath">
	  <fileset dir="${lib}">
		  <include name="**/*.jar"/>
	  </fileset>
	</path>

    <target name="default" description="--> description">
	<!-- Capture the path as a delimited property using the refid attribute -->
	<property name="myclasspath" refid="classpath"/>
	<!-- Emit the property to the ant console -->
	<echo message="Classpath = ${myclasspath}"/>

    	<javac srcdir="${src}" destdir="${dist}">
    		<classpath refid="classpath"/>
	</javac>
    </target>

</project>

The refid attribute on the property tag serializes the object with the given reference ID onto the property (in this case, our classpath).

Note that after slinging together this tip, I also ran into this much more friendly version from Andrew Beacock's Blog: Pretty Printing Java Classpaths Using Ant's pathconvert Task . I wish I could take credit for this as it is much more fancy than mine .

Anyway, this proved to be an interesting problem that elicited help from several teams: JDT Core, Platform/JDT Text, JDT UI, and Platform/JDT Debug.

Until next time,

R.J. Lorimer
Contributing Editor - rj -at- javalobby.org
Author              - www.coffee-bytes.com
Software Consultant - www.numbersix.com

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