## 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 Tests and Rules—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

- `org.apache.tools.ant.Project.MSG_ERR`
- `org.apache.tools.ant.Project.MSG_WARN`
- `org.apache.tools.ant.Project.MSG_INFO` (default)
- `org.apache.tools.ant.Project.MSG_VERBOSE`
- `org.apache.tools.ant.Project.MSG_DEBUG`