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327 lines
14 KiB
HTML
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<!--
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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this work for additional information regarding copyright ownership.
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<html>
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<head>
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<meta http-equiv="Content-Language" content="en-us"/>
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<link rel="stylesheet" type="text/css" href="stylesheets/style.css"/>
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<title>Properties and PropertyHelpers</title>
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</head>
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<body>
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<h1>Properties</h1>
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<p>Properties are key-value-pairs where Ant tries to
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expand <code>${key}</code> to <code>value</code> at runtime.</p>
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<p>There are many tasks that can set properties, the most common one
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is the <a href="CoreTasks/property.html">property</a> task. In
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addition properties can be defined
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via <a href="running.html">command line arguments</a> or similar
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mechanisms from outside of Ant.</p>
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<p>Normally property values can not be changed, once a property is
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set, most tasks will not allow its value to be modified. In
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general properties are of global scope, i.e. once they have been
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defined they are available for any task or target invoked
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subsequently - it is not possible to set a property in a child
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build process created via
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the <a href="CoreTasks/ant.html">ant</a>, antcall or subant tasks
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and make it available to the calling build process, though.</p>
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<p>Starting with Ant 1.8.0
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the <a href="CoreTasks/local.html">local</a> task can be used to
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create properties that are locally scoped to a target or
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a <a href="CoreTasks/sequential.html">sequential</a> element like
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the one of the <a href="CoreTasks/macrodef.html">macrodef</a>
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task.</p>
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<a name="built-in-props"><h2>Built-in Properties</h2></a>
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<p>Ant provides access to all system properties as if they had been
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defined using a <code><property></code> task. For
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example, <code>${os.name}</code> expands to the name of the
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operating system.</p>
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<p>For a list of system properties see
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<a href="http://java.sun.com/j2se/1.3/docs/api/java/lang/System.html#getProperties()">the Javadoc of System.getProperties</a>.
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</p>
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<p>In addition, Ant has some built-in properties:</p>
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<pre>
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basedir the absolute path of the project's basedir (as set
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with the basedir attribute of <a href="using.html#projects"><project></a>).
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ant.file the absolute path of the buildfile.
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ant.version the version of Ant
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ant.project.name the name of the project that is currently executing;
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it is set in the name attribute of <project>.
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ant.project.default-target
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the name of the currently executing project's
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default target; it is set via the default
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attribute of <project>.
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ant.project.invoked-targets
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a comma separated list of the targets that have
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been specified on the command line (the IDE,
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an <ant> task ...) when invoking the current
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project.
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ant.java.version the JVM version Ant detected; currently it can hold
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the values "1.2", "1.3",
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"1.4", "1.5" and "1.6".
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ant.core.lib the absolute path of the <code>ant.jar</code> file.
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</pre>
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<p>There is also another property, but this is set by the launcher
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script and therefore maybe not set inside IDEs:</p>
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<pre>
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ant.home home directory of Ant
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</pre>
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<p>The following property is only set if Ant is started via the
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Launcher class (which means it may not be set inside IDEs
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either):</p>
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<pre>
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ant.library.dir the directory that has been used to load Ant's
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jars from. In most cases this is ANT_HOME/lib.
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</pre>
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<a name="propertyHelper"><h1>PropertyHelpers</h1></a>
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<p>Ant's property handling is accomplished by an instance of
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<code>org.apache.tools.ant.PropertyHelper</code> associated with
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the current Project. You can learn more about this class by
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examining Ant's Java API. In Ant 1.8 the PropertyHelper class was
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much reworked and now itself employs a number of helper classes
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(actually instances of
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the <code>org.apache.tools.ant.PropertyHelper$Delegate</code>
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marker interface) to take care of discrete tasks such as property
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setting, retrieval, parsing, etc. This makes Ant's property
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handling highly extensible; also of interest is the
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new <a href="CoreTasks/propertyhelper.html">propertyhelper</a>
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task used to manipulate the PropertyHelper and its delegates from
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the context of the Ant buildfile.
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<p>There are three sub-interfaces of <code>Delegate</code> that may be
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useful to implement.</p>
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<ul>
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<li><code>org.apache.tools.ant.property.PropertyExpander</code> is
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responsible for finding the property name inside a string in the
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first place (the default extracts <code>foo</code>
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from <code>${foo}</code>).
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<p>This is the interface you'd implement if you wanted to invent
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your own property syntax - or allow nested property expansions
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since the default implementation doesn't balance braces
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(see <a href="http://svn.apache.org/viewvc/ant/antlibs/props/trunk/src/main/org/apache/ant/props/NestedPropertyExpander.java?view=log"><code>NestedPropertyExpander</code>
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in the "props" Antlib</a> for an example).</p>
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</li>
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<li><code>org.apache.tools.ant.PropertyHelper$PropertyEvaluator</code>
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is used to expand <code>${some-string}</code> into
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an <code>Object</code>.
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<p>This is the interface you'd implement if you want to provide
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your own storage independent of Ant's project instance - the
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interface represents the reading end. An example for this
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would
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be <code>org.apache.tools.ant.property.LocalProperties</code>
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which implements storage
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for <a href="CoreTasks/local.html">local properties</a>.</p>
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<p>Another reason to implement this interface is if you wanted
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to provide your own "property protocol" like
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expanding <code>toString:foo</code> by looking up the project
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reference foo and invoking <code>toString()</code> on it
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(which is already implemented in Ant, see below).</p>
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</li>
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<li><code>org.apache.tools.ant.PropertyHelper$PropertySetter</code>
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is responsible for setting properties.
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<p>This is the interface you'd implement if you want to provide
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your own storage independent of Ant's project instance - the
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interface represents the reading end. An example for this
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would
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be <code>org.apache.tools.ant.property.LocalProperties</code>
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which implements storage
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for <a href="CoreTasks/local.html">local properties</a>.</p>
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</li>
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</ul>
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<p>The default <code>PropertyExpander</code> looks similar to:</p>
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<pre>
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public class DefaultExpander implements PropertyExpander {
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public String parsePropertyName(String s, ParsePosition pos,
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ParseNextProperty notUsed) {
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int index = pos.getIndex();
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if (s.indexOf("${", index) == index) {
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int end = s.indexOf('}', index);
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if (end < 0) {
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throw new BuildException("Syntax error in property: " + s);
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}
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int start = index + 2;
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pos.setIndex(end + 1);
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return s.substring(start, end);
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}
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return null;
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}
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}
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</pre>
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<p>The logic that replaces <code>${toString:some-id}</code> with the
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stringified representation of the object with
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id <code>some-id</code> inside the current build is contained in a
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PropertyEvaluator similar to the following code:</p>
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<pre>
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public class ToStringEvaluator implements PropertyHelper.PropertyEvaluator {
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private static final String prefix = "toString:";
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public Object evaluate(String property, PropertyHelper propertyHelper) {
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Object o = null;
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if (property.startsWith(prefix) && propertyHelper.getProject() != null) {
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o = propertyHelper.getProject().getReference(property.substring(prefix.length()));
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}
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return o == null ? null : o.toString();
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}
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}
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</pre>
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<h1>Property Expansion</h1>
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<p>When Ant encounters a construct <code>${some-text}</code> the
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exact parsing semantics are subject to the configured property
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helper delegates.</p>
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<h2><code>$$</code> Expansion</h2>
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<p>In its default configuration Ant will expand the
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text <code>$$</code> to a single <code>$</code> and suppress the
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normal property expansion mechanism for the text immediately
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following it, i.e. <code>$${key}</code> expands
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to <code>${key}</code> and not <code>value</code> even though a
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property named <code>key</code> was defined and had the
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value <code>value</code>. This can be used to escape
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literal <code>$</code> characters and is useful in constructs that
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only look like property expansions or when you want to provide
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diagnostic output like in</p>
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<pre> <echo>$${builddir}=${builddir}</echo></pre>
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<p>which will echo this message:</p>
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<pre> ${builddir}=build/classes</pre>
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<p>if the property <code>builddir</code> has the
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value <code>build/classes</code>.</p>
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<p>In order to maintain backward compatibility with older Ant
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releases, a single '$' character encountered apart from a
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property-like construct (including a matched pair of french
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braces) will be interpreted literally; that is, as '$'. The
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"correct" way to specify this literal character, however, is by
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using the escaping mechanism unconditionally, so that "$$" is
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obtained by specifying "$$$$". Mixing the two approaches yields
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unpredictable results, as "$$$" results in "$$".</p>
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<h2>Nesting of Braces</h2>
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<p>In its default configuration Ant will not try to ballance braces
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in property expansions, it will only consume the text up to the
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first closing brace when creating a property name. I.e. when
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expanding something like <code>${a${b}}</code> it will be
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translated into two parts:</p>
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<ol>
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<li>the expansion of property <code>a${b</code> - likely nothing
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useful.</li>
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<li>the literal text <code>}</code> resulting from the second
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closing brace</li>
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</ol>
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<p>This means you can't use easily expand properties whose names are
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given by properties, but there
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are <a href="http://ant.apache.org/faq.html#propertyvalue-as-name-for-property">some
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workarounds</a> for older versions of Ant. With Ant 1.8.0 and the
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<a href="http://ant.apache.org/antlib/props/">the props Antlib</a>
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you can configure Ant to use
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the <code>NestedPropertyExpander</code> defined there if you need
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such a feature.</p>
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<h2>Expanding a "Property Name"</h2>
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<p>In its most simple form <code>${key}</code> is supposed to look
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up a property named <code>key</code> and expand to the value of
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the property. Additional <code>PropertyEvaluator</code>s may
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result in a different interpretation of <code>key</code>,
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though.</p>
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<p>The <a href="http://ant.apache.org/antlibs/props/">props
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Antlib</a> provides a few interesting evaluators but there are
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also a few built-in ones.</p>
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<a name="toString"><h3>Getting the value of a Reference with
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${toString:}</h3></a>
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<p>Any Ant type which has been declared with a reference can also
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its string value extracted by using the <code>${toString:}</code>
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operation, with the name of the reference listed after
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the <code>toString:</code> text. The <code>toString()</code>
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method of the Java class instance that is referenced is invoked
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-all built in types strive to produce useful and relevant output
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in such an instance.</p>
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<p>For example, here is how to get a listing of the files in a fileset,<p>
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<pre>
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<fileset id="sourcefiles" dir="src" includes="**/*.java" />
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<echo> sourcefiles = ${toString:sourcefiles} </echo>
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</pre>
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<p>There is no guarantee that external types provide meaningful
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information in such a situation</p>
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<h3><a name="ant.refid">Getting the value of a Reference with
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${ant.refid:}</a></h3>
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<p>Any Ant type which has been declared with a reference can also be
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used as a property by using the <code>${ant.refid:}</code>
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operation, with the name of the reference listed after
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the <code>ant.refid:</code> text. The difference between this
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operation and <a href="#toString"><code>${toString:}</code></a> is
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that <code>${ant.refid:}</code> will expand to the referenced
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object itself. In most circumstances the toString method will be
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invoked anyway, for example if the <code>${ant.refid:}</code> is
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surrounded by other text.</p>
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<p>This syntax is most useful when using a task with attribute
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setters that accept objects other than String. For example if the
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setter accepts a Resource object as in</p>
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<pre>
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public void setAttr(Resource r) { ... }
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</pre>
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<p>then the syntax can be used to pass in resource subclasses
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preciously defined as references like</p>
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<pre>
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<url url="http://ant.apache.org/" id="anturl"/>
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<my:task attr="${ant.refid:anturl}"/>
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</pre>
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</body>
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