The Apache Ant frontend: ProjectHelper

What is a ProjectHelper?

The ProjectHelper in Apache Ant is responsible for parsing the build fileand creating java instances representing the build workflow. It also signals whichkind of file it can parse, and which file name it expects as default input file.

Ant' default ProjectHelper(org.apache.tools.ant.helper.ProjectHelper2) parses theusual build.xml files. And if no build file is specified on the command line, itwill expect to find a file named build.xml.

The immediate benefit of a such abstraction it that it is possible to make Antunderstand other kind of descriptive languages than XML. Some experiments havebeen done around a pure java frontend, and a groovy one too (ask the dev mailinglist for further info about these).

Since Ant 1.8.2, the import task will alsotry to use the proper helper to parse the imported file. So it is possible towrite different build files in different languages and have them import eachother.

How is Ant is selecting the proper ProjectHelper

Ant knows about several implementations of ProjectHelperand has to decide which to use for each build file.

At startup Ant lists the all implementations found and keeps themin the same order they've been found in an internal 'repository':

• the first to be searched for is the one declared by the system property org.apache.tools.ant.ProjectHelper (see Java System Properties);
• then it searches with its class loader for a ProjectHelper service declarations in the META-INF: it searches in the classpath for a file META-INF/services/org.apache.tools.ant.ProjectHelper. This file will just contain the fully qualified name of the implementation of ProjectHelper to instanciate;
• it will also search with the system class loader for ProjectHelper service declarations in the META-INF;
• last but not least it will add its default ProjectHelper that can parse classical build.xml files.

When Ant is expected to parse a file, it will ask theProjectHelper repository to find an implementation that will beable to parse the input file. Actually it will just iterate over the ordered listand the first implementation that returns true tosupportsBuildFile(File buildFile) will be selected.

When Ant is started and no input file has been specified, it will search fora default input file. It will iterate over list of ProjectHelpersand will select the first one that expects a default file that actually exist.

Writing your own ProjectHelper

The class org.apache.tools.ant.ProjectHelper is the API expected tobe implemented. So write your own ProjectHelper by extending thatabstract class. You are then expected to implement at least the functionparse(Project project, Object source). Note also that yourimplementation will be instanciated by Ant, and it is expecting a defaultconstructor with no arguments.

There are some functions that will help you define what your helper iscapable of and what is is expecting:

• getDefaultBuildFile(): defines which file name is expected if none provided
• supportsBuildFile(File buildFile): defines if your parser can parse the input file
• canParseAntlibDescriptor(URL url): whether your implementation is capable of parsing a given Antlib descriptor. The base class returns false
• parseAntlibDescriptor(Project containingProject, URL source): invoked to actually parse the Antlib descriptor if your implementation returned true for the previous method.

Now that you have your implementation ready, you have to declare it to Ant. Threesolutions here:

• use the system property org.apache.tools.ant.ProjectHelper (see also the Java System Properties);
• use the service file in META-INF: in the jar you will build with your implementation, add a file META-INF/services/org.apache.tools.ant.ProjectHelper. And then in this file just put the fully qualified name of your implementation
• use the projecthelper task (since Ant 1.8.2) which will install dynamically an helper in the internal helper 'repository'. Then your helper can be used on the next call to the import task.