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From "Bill Burton" <>
Subject [PROPOSAL] Class dependency checking for optional tasks
Date Sun, 21 Jan 2001 03:01:39 GMT

I've been thinking about ways to improve usability of Ant, particulary for
first time users.  Here's one idea in that direction.

One of the questions that comes up a lot on the ant-user list is, "Where
do a find the classes for ftp support, etc?"  As a result, it was good to
see Nico's patches to clarify the docs in this area.  I've been thinking
about this problem from a little different perspective.


Each task requiring external classes should provide a way the task
developer can declare the required jar file(s) by specifing a class within
each jar to check for the availability of the jar.  If the class isn't
available, the task will throw a BuildException with the name of the
missing jar and the URL where it can be obtained.

This feature is conceptually similar to the <available classname=... />
task but would force the build to stop if the specified class is not
available by throwing a BuildException.

Implementation Details would be modified by adding a method similar to the following

     * Tries to load the specified class ensuring any associated jar 
     * file is available.  If the class is not available, a BuildException
     * is thrown with the specified comment.  The comment should concisely 
     * provide information indicating what's missing and where it can be 
     * obtained.
     * @param name name of class to check in the classpath
     * @param loader class loader or null for the default 
     *        class loader
     * @param comment text describing the name of the class or 
     *        associated jar file and the URL or location where it 
     *        can be obtained
    public void checkForRequiredClass(String name, ClassLoader loader,
String comment)
        throws BuildExeception
        try {
            if (loader == null)
                loader = this.getClass().getClassLoader();
            Class.forName(name, true, loader);
        } catch (ClassNotFoundException cnfe) {
            // Maybe this should also output the location too?
            throw (new BuildException("Missing class " + name + ": " +
        // Need to catch other exceptions here?

The task developer could use it in either the init() or execute() methods
like this:
    checkForRequiredClass("", null
                          "Requires ORO NetComponents available at");

The <script> task could have many calls to this method, one for BSF and
one for each of the supported scripting languages.

If the consensus is that this is a good idea, I'll work on a patch for
this functionality.

-Bill Burton

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