Apache Ant Task Design Guidelines

This document covers how to write ant tasks to a standard required to be incorporated into the Ant distribution. You may find it useful when writing tasks for personal use as the issues it addresses are still there in such a case.

Use built in helper classes

Ant includes helper tasks to simplify mauch of your work. Be warned that these helper classes will look very different in ant2.0 from these 1.x versions. However it is still better to use them than roll your own, for development, maintenance and code size reasons.

Execute

Execute will spawn off separate programs under all the platforms which ant supports, dealing with Java version issues as well as platform issues. Always use this task to invoke other programs.

Java, ExecuteJava

These classes can be used to spawn Java programs in a separate VM (they use execute) or in the same VM -with or without a different classloader. When deriving tasks from this, it often benefits users to permit the classpath to be specified, and for forking to be an optional attribute.

Project

Project has some helper functions to touch a file, to copy a file and the like. Use these instead of trying to code them yourself -or trying to use tasks which may be less stable and fiddlier to use.

Obey the Sun/Java style guidelines

The Ant codebase aims to have a single unified coding standard, and that standard is the Sun Java coding guidelines

It's not that they are better than any alternatives, but they are a standard and they are what is consistently used in the rest of the tasks. Code will not be incorporated into the database until it complies with these.

If you are writing a task for your personal or organisational use, you are free to use whatever style you like. But using the Sun Java style will help you to become comfortable with the rest of the Ant source, which may be important.

One important rule is 'no tabs'. Use four spaces instead. Not two, not eight, four. Even if your editor is configured to have a tab of four spaces, lots of others aren't -spaces have more consistency across editors and platforms.

Recommended Names for attributes and elements

The ant1.x tasks are very inconsistent regarding naming of attributes -some tasks use source, others src. Here is a list of preferred attribute names.

failonerror	boolean to control whether failure to execute should throw a `BuildException` or just print an error. Parameter validation failures should always throw an error, regardless of this flag
destdir	destination directory for output

Yes, this is a very short list. Try and be vaguely consistent with the core tasks, at the very least.

Design for controlled re-use

Keep member variables private. If read access by subclasses is required. add accessor methods rather than change the accessiblity of the member. This enables subclasses to access the contents, yet still be decoupled from the actual implementation.

The other common re-use mechanism in ant is for one task to create and configure another. This is fairly simple.

Do your own Dependency Checking

Make has the edge over Ant in its integrated dependency checking: the command line apps make invokes dont need to do their own work. Ant tasks do have to do their own dependency work, but if this can be done then it can be done well. A good dependency aware task can work out the dependencies without explicit dependency information in the build file, and be smart enough to work out the real dependencies, perhaps through a bit of file parsing. The depends task is the best example of this. Some of the zip/jar tasks are pretty good too, as they can update the archive when needed. Most tasks just compare source and destination timestamps and work from there. Tasks which don't do any dependency checking do not help users as much as they can, because their needless work can trickle through the entire build, test and deploy process.

Support Java 1.1 through Java 1.4

Ant is designed to support Java1.1: to build on it, to run on it. Sometimes functionality of tasks have to degrade in that environment -<touch> is a case in point- this is usually due to library limitations; such behaviour change must always be noted in the documentation.

What is problematic is code which is dependent on Java1.2 features -Collections, Reader and Writer classes, extra methods in older classes. These can not be used directly by any code and still be able to compile and run on a Java 1.1 system. So please stick to the older collection classes, and the older IO classes. If a new method in an existing class is to be used, it must be used via reflection and the NoSuchMethodException handled somehow.

What if code simply does not work on Java1.1? It can happen. It will probably be OK to have the task as an optional task, with compilation restricted to Java1.2 or later through build.xml modifications. Better still, use reflection to link to the classes at run time.

Java 1.4 adds a new optional change to the language itself, the assert keyword, which is only enabled if the compiler is told to compile 1.4 version source. Clearly with the 1.1 compatibility requirement, Ant tasks can not use this keyword. They also need to move away from using the JUnit assert() method and start calling assertTrue() instead.

Refactor

If the changes made to a task are making it too unwieldy, split it up into a cleaner design, refactor the code and submit not just feature creep but cleaner tasks. A common design pattern which tends to occur in the ant process is the adoption of the adapter pattern, in which a base class (say Javac or Rmi) starts off simple, then gets convoluted with support for multiple back ends -javac, jikes, jvc. A refactoring to split the programmable front end from the classes which provide the back end cleans up the design and makes it much easier to add new back ends. But to carry this off one needs to keep the interface and behaviour of the front end identical, and to be sure that no subclasses have been accessing data members directly -because these data members may not exist in the refactored design. Which is why having private data members is so important.

Test

Look in jakarta-ant/src/testcases and you will find JUnit tests for the shipping ant tasks, to see how it is done and what is expected of a new task. Most of them are rudimentary, and no doubt you could do better for your task -feel free to do so!

A well written set of test cases will break the Ant task while it is in development, until the code is actually complete. And every bug which surfaces later should have a test case added to demonstrate the problem, and to fix it.

The test cases are a great way of testing your task during development. A simple call to 'ant run-test' in the ant source tree will run all ant tests, to verify that your changes don't break anything. To test a single task, use the one shot ant run-single-test -Dtestcase=${testname} where ${testname} is the name of your test class.

The test cases are also used by the committers to verify that changes and patches do what they say. If you've got test cases it increases your credibility significantly.

Remember also that Ant 1.x is designed to compile and run on Java1.1, so you should test on Java 1.1 as well as any later version which you use. You can download an old SDK from Sun for this purpose.

Document

Without documentation, the task can't be used. So remember to provide a succint and clear html (soon, xml) page describing the task in a similar style to that of existing tasks. It should include a list of attributes and elements, and at least one working example of the task. Many users cut and paste the examples into their build files as a starting point, so make the examples practical and test them too.

Licensing and Copyright

Any code submitted to the Apache project must be compatible with the Apache Software License, and the act of submission must be viewed as an implicit transfer of ownership of the submitted code to the Apache Software Foundation.

This is important.

The fairly laissez-faire license of Apache is not compabitible with either the GPL or the Lesser GPL of the Free Software Foundation -the Gnu project. Their license requires all changes to the source to be made public, and give the licensee of any software the right to distribute copies. It also requires derivative works to be made available under the same license terms. None of these requirements are in the Apache Software Foundation license, which permits people and organisations to build commercial and closed source applications atop the Apache libraries and source -but not use the Apache, Ant or Jakarta Project names without permission.

Because the Gnu GPL license immediately extends to cover any larger application (or library, in the case of GLPL) into which it is incorporated, the Ant team can not incorporate any task based upon GPL or LGPL source into the Ant codebase. You are free to submit it, but it will be politely and firmly rejected.

Once ant-2 adds better dynamic task incorporation, it may be possible to provide a framework for supporting [L]GPL code, but still no tasks direcely subject to the Gnu licenses will ever be included in the Ant CVS tree.

Dont re-invent the wheel

We've all done it: written and submitted a task only to discover it was already implemented in a small corner of another task, or it has been submitted by someone else and not committed. You can avoid this by being aware of what is in the latest CVS tree -keep getting the daily source updates, look at manual changes and subscribe to the ant-dev mailing list.

If you are thinking of writing a task, posting a note on your thoughts to the list can be informative -you well get other peoples insight and maybe some half written task to do the basics, all without writing a line of code.

Submitting to Ant

The process for submitting an Ant task is documented on the jakarta web site. The basic mechanism is to mail it to the ant-dev mailing list. It helps to be on this list, as you will see other submissions, and any debate about your own submission.

Patches to existing files should be generated with cvs diff -u filename and save the output to a file. If you want to get the changes made to multiple files in a directory , just use cvs diff -u. The patches should be sent as an attachment to a message titled [PATCH] and distinctive one-line summary in the subject of the patch. The filename/task and the change usually suffices. It's important to include the changes as an attachment, as too many mailers reformat the text pasted in, which breaks the patch.

Then you wait for one of the committers to commit the patch, if it is felt appropriate to do so. Bug fixes go in quickly, other changes often spark a bit of discussion before a (perhaps revised) commit is made.

New submissions should be proceeded with [SUBMIT]. The mailer-daemon will reject any messages over 100KB, so any large update should be zipped up. If your submission is bigger than that, why not break it up into separate tasks.

If you hear nothing after a couple of weeks, remind the mailing list. Sometimes really good submissions get lost in the noise of other issues. This is particularly the case just prior to a new point release of the product. At that time anything other than bug fixes will tend to be neglected.

Checklists

These are the things you should verify before submitting patches and new tasks. Things don't have to be perfect, it may take a couple of iterations before a patch or submission is committed, and these items can be addressed in the process. But by the time the code is committed, everything including the documentation and some test cases will have been done, so by getting them out the way up front can save time. The committers look more favourably on patches and submissions with test cases, while documentation helps sell the reason for a task.

Checklist before submitting a patch

Added code complies with style guidelines
Code compiles and runs on Java1.1
New member variables are private, and provide public accessor methods if access is actually needed.
Existing test cases succeed.
New test cases written and succeed.
Documentation page extended as appropriate.
Example task declarations in the documentation tested.
Diff files generated using cvs diff -u
Message to ant-dev contains [PATCH], task name and patch reason in subject.
Message body contains a rationale for the patch.
Message attachment contains the patch file(s).

Checklist before submitting a new task

Java file begins with Apache copyright and license statement.
Task does not depend on GPL or LGPL code.
Source code complies with style guidelines
Code compiles and runs on Java1.1
Member variables are private, and provide public accessor methods if access is actually needed.
Maybe Task has failonerror attribute to control failure behaviour
New test cases written and succeed
Documentation page written
Example task declarations in the documentation tested.
Patch files generated using cvs diff -u
patch files include a patch to defaults.properties to register the tasks
patch files include a patch to coretasklist.html or optionaltasklist.html to link to the new task page
Message to ant-dev contains [SUBMIT] and task name in subject
Message body contains a rationale for the task
Message attachments contain the required files -source, documentation, test and patches