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CREATING A BASIC ANT TASK SUBCLASS
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classpath="${build.dir}" /> <property name="the.message" value="blue scooter"/> <message message="${the.message}"/> </target>
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messagetask: [message] blue scooter
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This task is a bare-bones task similar to <echo> and does nothing but log (at the MSG_INFO level) the value assigned to the message attribute. It is an example of using Ant s attribute introspection and population, and demonstrates the use of logging through the Task.log method. An important fact to note is that Ant handles property expansion in XML attributes automatically for you, as you can see, since our task got the value of the.message property rather than the text ${the.message} . 19.4.2 Handling element text Element text is handed to its containing object, typically the task itself, or possibly a nested element, using the addText method. Of note is that the text is provided as is, and no property references are expanded. Here is a simple example of a variant of the original MessageTask to take the message text as the element data rather than from an attribute:
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package org.example.antbook.tasks; import org.apache.tools.ant.Task; public class MessageTask2 extends Task { private String message = ""; public void addText(String message) { this.message = message; } public void execute() { log(message); } }
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Special method to accept element text
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Our build file fragment using this task is:
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<property name="another.message" value="light up ahead"/> <message2>${another.message}</message2>
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It generates this output:
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[message2] ${another.message}
CHAPTER 1 9
WRITING ANT TASKS
Having the unaltered body text provided to the task is beneficial for data that may contain such strings that appear like property references, but if you need those references resolved, it s a simple matter of adding a call to a Project method. Our execute method now becomes:
public void execute() { log(getProject().replaceProperties(message)); }
The results now have the property references resolved:
[message2] light up ahead
OPERATING ON A FILESET
Ant s datatypes make writing tasks that deal with many of the typical Java build domain objects such as paths and filesets much simpler. If you are writing a task to process files in a single directory tree and would like your task to act as an implicit fileset, the base class org.apache.tools.ant.taskdefs.MatchingTask can save a lot of work. Listing 19.4 shows a task to process a set of files.
Listing 19.4 A task to act upon an implicit fileset
package org.example.antbook.tasks; import import import import import import java.io.File; org.apache.tools.ant.BuildException; org.apache.tools.ant.DirectoryScanner; org.apache.tools.ant.Task; org.apache.tools.ant.Project; org.apache.tools.ant.taskdefs.MatchingTask;
public class FileProcTask extends MatchingTask { private File dir; public void setDir (File dir) { this.dir = dir; } public void execute() throws BuildException { if (dir == null) { throw new BuildException("dir must be specified"); } log("dir = " + dir, Project.MSG_DEBUG); DirectoryScanner ds = getDirectoryScanner(dir); String[] files = ds.getIncludedFiles(); for (int i = 0; i < files.length; i++) { log("file: " + files[i]);
OPERATING ON A FILESET
} dir = null; } }
Using MatchingTask provides some nice freebies that mirror the <fileset> datatype:
includes/excludes attributes defaultexcludes attribute <include>/<exclude> / <includesfile> / <excludesfile> elements <patternset> element
The one piece that you must provide in your own code is the directory to use with the implicit fileset. In our example we implemented a setDir method and required that a dir attribute be specified by throwing a BuildException in execute if it was not specified. The MatchingTask base class provides a getDirectoryScanner(File baseDir) method to get a DirectoryScanner instance, taking into account all the specified inclusion and exclusion rules. Although many Ant tasks are derived from MatchingTask, the current trend is away from this task; explicit filesets have proven to be more flexible. If you are writing your own task, the ease of using MatchingTask as a base class still makes it appealing. Tasks that extend from MatchingTask should only deal with a single implicit fileset. Tasks that need to support multiple nested filesets should extend from Task instead.
ERROR HANDLING
It is up to you, as a task developer, to decide how to handle abnormal conditions that may occur during the configuration or execution of your task. Ant will catch exceptions that are thrown from the task methods it invokes and make the build fail at that point. Throw Ant s org.apache.tools.ant.BuildException, which is a RuntimeException subclass, when you wish a build to fail for any reason. You may want the failure of a build to be user-specified, in which case a simple pattern to follow is adding a failonerror attribute similar to many of Ant s core tasks like <java>. Here is a simple Task class allowing the build file to control if a build failure occurs or not:
package org.example.antbook.tasks; import org.apache.tools.ant.Task; import org.apache.tools.ant.BuildException; public class ConditionalFailTask extends Task { private boolean failOnError = true;
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