Programming your own Selectors

Selector Programming API

Want to define your own selectors? It's easy!

First, pick the type of selector that you want to define. There are three types, and a recipe for each one follows. Chances are you'll want to work with the first one, Custom Selectors.

  1. Custom Selectors

    This is the category that Apache Ant provides specifically for you to define your own Selectors. Anywhere you want to use your selector you use the <custom> element and specify the class name of your selector within it. See the Custom Selectors section of the Selector page for details. The <custom> element can be used anywhere the core selectors can be used. It can be contained within Selector Containers, for example.

    To create a new Custom Selector, you have to create a class that implements org.apache.tools.ant.types.selectors.ExtendFileSelector. The easiest way to do that is through the convenience base class org.apache.tools.ant.types.selectors.BaseExtendSelector, which provides all of the methods for supporting <param> tags. First, override the isSelected() method, and optionally the verifySettings() method. If your custom selector requires parameters to be set, you can also override the setParameters() method and interpret the parameters that are passed in any way you like. Several of the core selectors demonstrate how to do that because they can also be used as custom selectors.

  2. Core Selectors

    These are the selectors used by Ant itself. To implement one of these, you will have to alter some of the classes contained within Ant.

    • First, create a class that implements org.apache.tools.ant.types.selectors.FileSelector. You can either choose to implement all methods yourself from scratch, or you can extend org.apache.tools.ant.types.selectors.BaseSelector instead, a convenience class that provides reasonable default behaviour for many methods.

      There is only one method required. public boolean isSelected(File basedir, String filename, File file) is the real purpose of the whole exercise. It returns true or false depending on whether the given file should be selected from the list or not.

      If you are using org.apache.tools.ant.types.selectors.BaseSelector there are also some predefined behaviours you can take advantage of. Any time you encounter a problem when setting attributes or adding tags, you can call setError(String errmsg) and the class will know that there is a problem. Then, at the top of your isSelected() method call validate() and a BuildException will be thrown with the contents of your error message. The validate() method also gives you a last chance to check your settings for consistency because it calls verifySettings(). Override this method and call setError() within it if you detect any problems in how your selector is set up.

      You may also want to override toString().

    • Put an add method for your selector in org.apache.tools.ant.types.selectors.SelectorContainer. This is an interface, so you will also have to add an implementation for the method in the classes which implement it, namely org.apache.tools.ant.types.AbstractFileSet, org.apache.tools.ant.taskdefs.MatchingTask and org.apache.tools.ant.types.selectors.BaseSelectorContainer. Once it is in there, it will be available everywhere that core selectors are appropriate.

  3. Selector Containers

    Got an idea for a new Selector Container? Creating a new one is no problem:

    • Create a new class that implements org.apache.tools.ant.types.selectors.SelectorContainer. This will ensure that your new Container can access any new selectors that come along. Again, there is a convenience class available for you called org.apache.tools.ant.types.selectors.BaseSelectorContainer.

    • Implement the public boolean isSelected(String filename, File file) method to do the right thing. Chances are you'll want to iterate over the selectors under you, so use selectorElements() to get an iterator that will do that.

    • Again, put an add method for your container in org.apache.tools.ant.types.selectors.SelectorContainer and its implementations org.apache.tools.ant.types.AbstractFileSet and org.apache.tools.ant.types.selectors.BaseSelectorContainer.

Testing Selectors

For a robust component (and selectors are (Project)Components) tests are necessary. For testing Tasks we use JUnit TestCases - more specific org.apache.tools.ant.BuildFileRule extends org.junit.rules.ExternalResource. Some of its features like configure the (test) project by reading its buildfile and execute targets we need for selector tests also. Therefore we use that BuildFileRule. But testing selectors requires some more work: having a set of files, instantiate and configure the selector, check the selection work and more. Because we usually extend BaseExtendSelector its features have to be tested also (e.g. setError()).

That's why we have a test rule for doing our selector tests: org.apache.tools.ant.types.selectors.BaseSelectorRule.

This class extends ExternalResource and therefore can included in the set of Ant's unit tests. It holds an instance of preconfigured BuildFileRule. Configuration is done by parsing the src/etc/testcases/types/selectors.xml. BaseSelectorRule then gives us helper methods for handling multiple selections.

Because the term "testcase" or "testenvironment" are so often used, this special testenvironment got a new name: bed. The setup and cleanup of the bed is all handled by the BaseSelectorRule so any test only has to handle the actual test scenarios

A usual test scenario is:

  1. instantiate the selector
  2. configure the selector
  3. let the selector do some work
  4. verify the work

An example test would be:

package org.apache.tools.ant.types.selectors;

public class MySelectorTest {

    @Rule
    public final BaseSelectorRule selectorRule = new BaseSelectorRule();

    @Test
    public void testCase1() {


        // Configure the selector
        MySelector s = new MySelector();
        s.addParam("key1", "value1");
        s.addParam("key2", "value2");
        s.setXX(true);
        s.setYY("a value");

        // do the tests
        assertEquals("FTTTTTTTT", selectorRule.selectionString(s));
    }
}
As an example of an error JUnit could log
    [junit]     FAILED
    [junit] Error for files: .;copy.filterset.filtered;tar/gz/asf-logo.gif.tar.gz
    [junit] expected:<FTTTFTTTF...> but was:<TTTTTTTTT...>
    [junit] junit.framework.ComparisonFailure: Error for files: .;copy.filterset.filtered;tar/gz/asf-logo.gif.tar.gz
    [junit] expected:<FTTTFTTTF...> but was:<TTTTTTTTT...>
    [junit]     at junit.framework.Assert.assertEquals(Assert.java:81)
    [junit]     at org.apache.tools.ant.types.selectors.BaseSelectorTest.performTest(BaseSelectorTest.java:194)

Described above the test class should provide a getInstance() method. But that isn't used here. The used getSelector() method is implemented in the base class and gives an instance of an Ant Project to the selector. This is usually done inside normal build file runs, but not inside this special environment, so this method gives the selector the ability to use its own Project object (getProject()), for example for logging.

Logging

During development and maybe later you sometimes need the output of information. Therefore Logging is needed. Because the selector extends BaseExtendSelector or directly BaseSelector it is an Ant DataType and therefore a ProjectComponent.
That means that you have access to the project object and its logging capability. ProjectComponent itself provides log methods which will do the access to the project instance. Logging is therefore done simply with: 

        log( "message" );
or 
        log( "message" , loglevel );
where the loglevel is one of the values