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<link rel="stylesheet" type="text/css" href="../stylesheets/style.css">
<title>Custom Components</title>
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<body>
<h2>Custom Components</h2>
<h3>Overview</h3>
<p>
Custom components are conditions, selectors, filters and other
objects that are defined outside Apache Ant core.
</p>
<p>
In Ant 1.6 custom conditions, selectors and filters has
been overhauled.
</p>
<p>
It is now possible to define custom conditions, selectors and filters
that behave like Ant Core components.
This is achieved by allowing datatypes defined in build scripts
to be used as custom components if the class of the datatype
is compatible, or has been adapted by an adapter class.
</p>
<p>
The old methods of defining custom components are still supported.
</p>
<h3>Definition and use</h3>
<p>
A custom component is a normal Java class that implements a particular
interface or extends a particular class, or has been adapted to the
interface or class.
</p>
<p>
It is exactly like writing a
<a href="../develop.html#writingowntask">custom task</a>.
One defines attributes and nested elements by writing <i>setter</i>
methods and <i>add</i> methods.
</p>
<p>
After the class has been written, it is added to the ant system
by using <code><typedef></code>.
</p>
<h3><a name="customconditions">Custom Conditions</a></h3>
<p>
Custom conditions are datatypes that implement
<code>org.apache.tools.ant.taskdefs.condition.Condition</code>.
For example a custom condition that returns true if a
string is all upper case could be written as:
</p>
<blockquote>
<pre>
package com.mydomain;
import org.apache.tools.ant.BuildException;
import org.apache.tools.ant.taskdefs.condition.Condition;
public class AllUpperCaseCondition implements Condition {
private String value;
// The setter for the "value" attribute
public void setValue(String value) {
this.value = value;
}
// This method evaluates the condition
public boolean eval() {
if (value == null) {
throw new BuildException("value attribute is not set");
}
return value.toUpperCase().equals(value);
}
}
</pre>
</blockquote>
<p>
Adding the condition to the system is achieved as follows:
</p>
<blockquote>
<pre>
<typedef
name="alluppercase"
classname="com.mydomain.AllUpperCaseCondition"
classpath="${mydomain.classes}"/>
</pre>
</blockquote>
<p>
This condition can now be used wherever a Core Ant condition
is used.
</p>
<blockquote>
<pre>
<condition property="allupper">
<alluppercase value="THIS IS ALL UPPER CASE"/>
</condition>
</pre>
</blockquote>
<h3><a name="customselectors">Custom Selectors</a></h3>
<p>
Custom selectors are datatypes that implement
<code>org.apache.tools.ant.types.selectors.FileSelector</code>.
</p>
<p>There is only one method required.
<code>public boolean isSelected(File basedir, String filename,
File file)</code>.
It returns true
or false depending on whether the given file should be
selected or not.
</p>
<p>
An example of a custom selection that selects filenames ending
in ".java" would be:
</p>
<blockquote>
<pre>
package com.mydomain;
import java.io.File;
import org.apache.tools.ant.types.selectors.FileSelector;
public class JavaSelector implements FileSelector {
public boolean isSelected(File b, String filename, File f) {
return filename.toLowerCase().endsWith(".java");
}
}
</pre>
</blockquote>
<p>
Adding the selector to the system is achieved as follows:
</p>
<blockquote>
<pre>
<typedef
name="javaselector"
classname="com.mydomain.JavaSelector"
classpath="${mydomain.classes}"/>
</pre>
</blockquote>
<p>
This selector can now be used wherever a Core Ant selector
is used, for example:
</p>
<blockquote>
<pre>
<copy todir="to">
<fileset dir="src">
<javaselector/>
</fileset>
</copy>
</pre>
</blockquote>
<p>
One may use
<code>org.apache.tools.ant.types.selectors.BaseSelector</code>,
a convenience class that provides reasonable default
behaviour.
It has 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
<code>isSelected()</code> method call <code>validate()</code> and
a BuildException will be thrown with the contents of your error
message. The <code>validate()</code> method also gives you a
last chance to check your settings for consistency because it
calls <code>verifySettings()</code>. Override this method and
call <code>setError()</code> within it if you detect any
problems in how your selector is set up.
</p>
<p>
To write custom selector containers one should extend
<code>org.apache.tools.ant.types.selectors.BaseSelectorContainer</code>.
Implement the
<code>public boolean isSelected(File baseDir, String filename, File file)</code>
method to do the right thing. Chances are you'll want to iterate
over the selectors under you, so use
<code>selectorElements()</code> to get an iterator that will do
that.
</p>
<p>
For example to create a selector container that will select files
if a certain number of contained selectors select, one could write
a selector as follows:
</p>
<blockquote>
<pre>
public class MatchNumberSelectors extends BaseSelectorContainer {
private int number = -1;
public void setNumber(int number) {
this.number = number;
}
public void verifySettings() {
if (number < 0) {
throw new BuildException("Number attribute should be set");
}
}
public boolean isSelected(File baseDir, String filename, File file) {
validate();
int numberSelected = 0;
for (Enumeration e = selectorElements(); e.hasNextElement();) {
FileSelector s = (FileSelector) e.nextElement();
if (s.isSelected(baseDir, filename, file)) {
numberSelected++;
}
}
return numberSelected == number;
}
}
</pre>
</blockquote>
<p>
To define and use this selector one could do:
</p>
<blockquote>
<pre>
<typedef name="numberselected"
classname="com.mydomain.MatchNumberSelectors"/>
...
<fileset dir="${src.path}">
<numberselected number="2">
<contains text="script" casesensitive="no"/>
<size value="4" units="Ki" when="more"/>
<javaselector/>
</numberselected>
</fileset>
</pre>
</blockquote>
<p>
<i>The custom selector</i>
</p>
<p>
The custom selector was the pre ant 1.6 way of defining custom selectors.
This method is still supported for backward compatibility.
</p>
<p>You can write your own selectors and use them within the selector
containers by specifying them within the <code><custom></code> tag.</p>
<p>To create a new Custom Selector, you have to create a class that
implements
<code>org.apache.tools.ant.types.selectors.ExtendFileSelector</code>.
The easiest way to do that is through the convenience base class
<code>org.apache.tools.ant.types.selectors.BaseExtendSelector</code>,
which provides all of the methods for supporting
<code><param></code> tags. First, override the
<code>isSelected()</code> method, and optionally the
<code>verifySettings()</code> method. If your custom
selector requires parameters to be set, you can also override
the <code>setParameters()</code> 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.</p>
<p>Once that is written, you include it in your build file by using
the <code><custom></code> tag.
</p>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
<td valign="top"><b>Attribute</b></td>
<td valign="top"><b>Description</b></td>
<td align="center" valign="top"><b>Required</b></td>
</tr>
<tr>
<td valign="top">classname</td>
<td valign="top">The name of your class that implements
<code>org.apache.tools.ant.types.selectors.FileSelector</code>.
</td>
<td valign="top" align="center">Yes</td>
</tr>
<tr>
<td valign="top">classpath</td>
<td valign="top">The classpath to use in order to load the
custom selector class. If neither this classpath nor the
classpathref are specified, the class will be
loaded from the classpath that Ant uses.
</td>
<td valign="top" align="center">No</td>
</tr>
<tr>
<td valign="top">classpathref</td>
<td valign="top">A reference to a classpath previously
defined. If neither this reference nor the
classpath above are specified, the class will be
loaded from the classpath that Ant uses.
</td>
<td valign="top" align="center">No</td>
</tr>
</table>
<p>Here is how you use <code><custom></code> to
use your class as a selector:
</p>
<blockquote><pre>
<fileset dir="${mydir}" includes="**/*">
<custom classname="com.mydomain.MySelector">
<param name="myattribute" value="myvalue"/>
</custom>
</fileset>
</pre></blockquote>
<p>The core selectors that can also be used as custom selectors
are</p>
<ul>
<li><a href="selectors.html#containsselect">Contains Selector</a> with
classname <code>org.apache.tools.ant.types.selectors.ContainsSelector</code>
</li>
<li><a href="selectors.html#dateselect">Date Selector</a> with
classname <code>org.apache.tools.ant.types.selectors.DateSelector</code>
</li>
<li><a href="selectors.html#depthselect">Depth Selector</a> with
classname <code>org.apache.tools.ant.types.selectors.DepthSelector</code>
</li>
<li><a href="selectors.html#filenameselect">Filename Selector</a> with
classname <code>org.apache.tools.ant.types.selectors.FilenameSelector</code>
</li>
<li><a href="selectors.html#sizeselect">Size Selector</a> with
classname <code>org.apache.tools.ant.types.selectors.SizeSelector</code>
</li>
</ul>
<p>Here is the example from the Depth Selector section rewritten
to use the selector through <code><custom></code>.</p>
<blockquote><pre>
<fileset dir="${doc.path}" includes="**/*">
<custom classname="org.apache.tools.ant.types.selectors.DepthSelector">
<param name="max" value="1"/>
</custom>
</fileset>
</pre></blockquote>
<p>Selects all files in the base directory and one directory below
that.</p>
<h3><a name="filterreaders">Custom Filter Readers</a></h3>
<p>
Custom filter readers selectors are datatypes that implement
<code>org.apache.tools.ant.types.filters.ChainableReader</code>.
</p>
<p>There is only one method required.
<code>Reader chain(Reader reader)</code>.
This returns a reader that filters input from the specified
reader.
</p>
<p>
For example a filterreader that removes every second character
could be:
</p>
<blockquote>
<pre>
public class RemoveOddCharacters implements ChainableReader {
public Reader chain(Reader reader) {
return new BaseFilterReader(reader) {
int count = 0;
public int read() throws IOException {
while (true) {
int c = in.read();
if (c == -1) {
return c;
}
count++;
if ((count % 2) == 1) {
return c;
}
}
}
}
}
}
</pre>
</blockquote>
<p>
For line oriented filters it may be easier to extend
<code>ChainableFilterReader</code> an inner class of
<code>org.apache.tools.ant.filters.TokenFilter</code>.
</p>
<p>
For example a filter that appends the line number could be
</p>
<blockquote>
<pre>
public class AddLineNumber extends ChainableReaderFilter {
private void lineNumber = 0;
public String filter(String string) {
lineNumber++;
return "" + lineNumber + "\t" + string;
}
}
</pre>
</blockquote>
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