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369 lines
18 KiB
HTML
369 lines
18 KiB
HTML
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The ASF licenses this file to You under the Apache License, Version 2.0
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<html lang="en">
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<head>
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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 Apache Ant tries to expand <code>${key}</code>
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to <code>value</code> at run time.</p>
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<p>There are many tasks that can set properties; the most common one is
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the <a href="Tasks/property.html">property</a> task. In addition properties can be defined
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via <a href="running.html">command line arguments</a> or similar mechanisms from outside of
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Ant.</p>
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<p>Normally property values can not be changed: once a property is set, most tasks will not allow
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its value to be modified. In 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 subsequently—it is not possible
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to set a property in a child build process created via
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the <a href="Tasks/ant.html">ant</a>, <a href="Tasks/antcall.html">antcall</a>
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or <a href="Tasks/subant.html">subant</a> tasks and make it available to the calling build
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process, though.</p>
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<p><em>Since Ant 1.8.0</em> the <a href="Tasks/local.html">local</a> task can be used to create
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properties that are locally scoped to a target or
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a <a href="Tasks/sequential.html">sequential</a> element like the one of
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the <a href="Tasks/macrodef.html">macrodef</a> task.</p>
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<h2 id="built-in-props">Built-in Properties</h2>
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<p>Ant provides access to all system properties as if they had been defined using
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a <code><property></code> task. For example, <samp>${os.name}</samp> expands to the name
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of the operating system.</p>
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<p>For a list of system properties,
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see <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/System.html#getProperties--"
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target="_top">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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<dl>
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<dt><code>basedir</code></dt>
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<dd>the absolute path of the project's basedir (as set with
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the <var>basedir</var> attribute
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of <a href="using.html#projects"><project></a>).</dd>
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<dt><code>ant.file</code></dt>
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<dd>the absolute path of the buildfile.</dd>
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<dt><code>ant.version</code></dt>
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<dd>the version of Ant</dd>
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<dt><code>ant.project.name</code></dt>
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<dd>the name of the project that is currently executing; it is set in the <var>name</var>
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attribute of <code><project></code>.</dd>
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<dt><code>ant.project.default-target</code></dt>
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<dd>the name of the currently executing project's default target; it is set via
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the <var>default</var> attribute of <code><project></code>.</dd>
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<dt><code>ant.project.invoked-targets</code></dt>
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<dd>a comma-separated list of the targets that have been specified (on the command line, inside
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of an IDE, by an <code><ant></code> task, etc...) when invoking the current
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project.<br/>This property is set properly when the first target is executed. If you use it in
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the implicit target (directly under the <code><project></code> tag) the list will be
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empty if no target has been specified while it will contain the project's default target in
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this case for tasks nested into targets.</dd>
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<dt><code>ant.java.version</code></dt>
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<dd>the JVM version Ant detected; currently it can hold the
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values <q>9</q>, <q>1.8</q>, <q>1.7</q>, <q>1.6</q>, <q>1.5</q>, <q>1.4</q>, <q>1.3</q>
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and <q>1.2</q>.</dd>
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<dt><code>ant.core.lib</code></dt>
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<dd>the absolute path of the <samp>ant.jar</samp> file.</dd>
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</dl>
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<p>There is also another property, but this is set by the launcher script and therefore maybe not
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set inside IDEs:</p>
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<dl>
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<dt><code>ant.home</code></dt>
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<dd>home directory of Ant</dd>
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</dl>
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<p>The following property is only set if Ant is started via the Launcher class (which means it may
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not be set inside IDEs either):</p>
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<dl>
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<dt><code>ant.library.dir</code></dt>
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<dd>the directory that has been used to load Ant's jars from. In most cases this
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is <samp>ANT_HOME/lib</samp>.</dd>
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</dl>
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<h1 id="propertyHelper">PropertyHelpers</h1>
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<p>Ant's property handling is accomplished by an instance
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of <code class="code">org.apache.tools.ant.PropertyHelper</code> associated with the current
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Project. You can learn more about this class by examining Ant's Java API. In Ant 1.8 the
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<code class="code">PropertyHelper</code> class was much reworked and now itself employs a number
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of helper classes (actually instances of
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the <code class="code">org.apache.tools.ant.PropertyHelper$Delegate</code> marker interface) to
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take care of discrete tasks such as property setting, retrieval, parsing, etc. This makes Ant's
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property handling highly extensible; also of interest is the
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new <a href="Tasks/propertyhelper.html">propertyhelper</a> task used to manipulate the
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PropertyHelper and its delegates from the context of the Ant buildfile.</p>
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<p>There are three sub-interfaces of <code class="code">Delegate</code> that may be useful to
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implement:</p>
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<ul>
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<li><code>org.apache.tools.ant.property.PropertyExpander</code> is responsible for finding the
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property name inside a string in the first place (the default extracts <samp>foo</samp>
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from <samp>${foo}</samp>).
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<p>This is the interface you'd implement if you wanted to invent your own property
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syntax—or allow nested property expansions since the default implementation doesn't
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balance braces
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(see <a href="https://gitbox.apache.org/repos/asf?p=ant-antlibs-props.git;a=blob;f=src/main/org/apache/ant/props/NestedPropertyExpander.java;hb=HEAD"
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target="_top"><code class="code">NestedPropertyExpander</code> in the <samp>props</samp> Antlib</a> for
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an example).</p>
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</li>
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<li><code class="code">org.apache.tools.ant.PropertyHelper$PropertyEvaluator</code> is used to
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expand <samp>${some-string}</samp> into an <code>Object</code>.
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<p>This is the interface you'd implement if you want to provide your own storage independent
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of Ant's project instance—the interface represents the reading end. An example for
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this would be <code class="code">org.apache.tools.ant.property.LocalProperties</code> which
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implements storage for <a href="Tasks/local.html">local properties</a>.</p>
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<p>Another reason to implement this interface is if you wanted to provide your own "property
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protocol" like expanding <code>toString:foo</code> by looking up the project
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reference <samp>foo</samp> and invoking <code class="code">toString()</code> on it (which is
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already implemented in Ant, see below).</p>
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</li>
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<li><code class="code">org.apache.tools.ant.PropertyHelper$PropertySetter</code> is responsible
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for setting properties.
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<p>This is the interface you'd implement if you want to provide your own storage independent
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of Ant's project instance—the interface represents the writing end. An example for
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this would be <code class="code">org.apache.tools.ant.property.LocalProperties</code> which
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implements storage for <a href="Tasks/local.html">local properties</a>.</p>
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</li>
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<li><code class="code">org.apache.tools.ant.PropertyHelper$PropertyEnumerator</code>
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is responsible for enumerating property names.
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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
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instance—the interface represents part of the reading
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end. An example for this would
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be <code class="code">org.apache.tools.ant.property.LocalProperties</code>
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which implements storage for <a href="Tasks/local.html">local
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properties</a>.</p>
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<p><em>This interface has been added with Ant 1.10.9.</em></p>
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</li>
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</ul>
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<p>The default <code class="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("${", index) == index) {
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int end = s.indexOf('}', index);
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if (end < 0) {
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throw new BuildException("Syntax error in property: " + s);
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}
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int start = index + 2;
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pos.setIndex(end + 1);
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return s.substring(start, end);
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}
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return null;
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}
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}</pre>
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<p>The logic that replaces <samp>${toString:<i>some-id</i>}</samp> with the stringified
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representation of the object with <var>id</var> <samp><i>some-id</i></samp> inside the current
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build is contained in a <code class="code">PropertyEvaluator</code> similar to the following
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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(
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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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}</pre>
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<h1>Property Expansion</h1>
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<p>When Ant encounters a construct <samp>${some-text}</samp> the exact parsing semantics are
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subject to the configured property helper delegates.</p>
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<h2><code>$$</code> Expansion</h2>
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<p>In its default configuration Ant will expand the text <q>$$</q> to a single <q>$</q> and
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suppress the normal property expansion mechanism for the text immediately following it,
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i.e., <samp>$${key}</samp> expands to <samp>${key}</samp> and not <code>value</code> even though
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a property named <code>key</code> was defined and had the value <samp>value</samp>. This can be
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used to escape literal <q>$</q> characters and is useful in constructs that only look like
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property expansions or when you want to provide diagnostic output like in</p>
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<pre><echo>$${builddir}=${builddir}</echo></pre>
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<p>which will echo this message:</p>
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<pre class="output">${builddir}=build/classes</pre>
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<p>if the property <code>builddir</code> has the value <samp>build/classes</samp>.</p>
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<p>In order to maintain backward compatibility with older Ant releases, a single <q>$</q>
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character encountered apart from a property-like construct (including a matched pair of french
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braces) will be interpreted literally, that is, as <q>$</q>. The "correct" way to specify this
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literal character, however, is by using the escaping mechanism unconditionally, so
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that <q>$$</q> is obtained by specifying <q>$$$$</q>. Mixing the two approaches yields
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unpredictable results, as <q>$$$</q> results in <q>$$</q>.</p>
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<h2>Nesting of Braces</h2>
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<p>In its default configuration Ant will not try to balance braces in property expansions, it will
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only consume the text up to the first closing brace when creating a property name. I.e. when
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expanding something like <samp>${a${b}}</samp> it will be translated into two parts:</p>
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<ol>
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<li>the expansion of property <samp>a${b</samp>—likely nothing useful.</li>
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<li>the literal text <samp>}</samp> resulting from the second closing brace</li>
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</ol>
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<p>This means you can't use easily expand properties whose names are stored in properties, but
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there are <a href="https://ant.apache.org/faq.html#propertyvalue-as-name-for-property"
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target="_top">some workarounds</a> for older versions of Ant. <em>Since Ant 1.8.0</em> using
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the <a href="https://ant.apache.org/antlibs/props/" target="_top">props Antlib</a> you can
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configure Ant to use the <code class="code">NestedPropertyExpander</code> defined there if you
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need such a feature.</p>
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<h2>Expanding a Property Reference</h2>
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<p>In its most simple form <samp>${key}</samp> is supposed to look up a property
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named <code>key</code> and expand to the value of the property.
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Additional <code>PropertyEvaluator</code>s may result in a different interpretation
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of <code>key</code>, though.</p>
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<p>The <a href="https://ant.apache.org/antlibs/props/" target="_top">props Antlib</a> provides a
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few interesting evaluators but there are also a few built-in ones.</p>
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<h3 id="toString">Getting the value of a Reference with <samp>${toString:}</samp></h3>
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<p>Any Ant type item which has been declared with a reference can also its string value extracted
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by using the <samp>${toString:}</samp> operation, with the name of the reference listed after
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the <code>toString:</code> text. The <code class="code">toString()</code> method of the Java
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class instance that is referenced is invoked—all built in types strive to produce useful
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and relevant output 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 = ${toString:sourcefiles} </echo></pre>
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<p>There is no guarantee that external types provide meaningful information in such a
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situation</p>
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<h3 id="ant.refid">Getting the value of a Reference with <samp>${ant.refid:}</samp></h3>
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<p>Any Ant type item which has been declared with a reference can also be used as a property by
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using the <samp>${ant.refid:}</samp> operation, with the name of the reference listed after
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the <code>ant.refid:</code> text. The difference between this operation
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and <a href="#toString"><samp>${toString:}</samp></a> is that <samp>${ant.refid:}</samp> will
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expand to the referenced object itself. In most circumstances
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the <code class="code">toString()</code> method will be invoked anyway, for example if
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the <samp>${ant.refid:}</samp> is surrounded by other text.</p>
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<p>This syntax is most useful when using a task with attribute setters that accept objects other
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than <code class="code">String</code>. For example, if the setter accepts
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a <code class="code">Resource</code> object as in</p>
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<pre>public void setAttr(Resource r) { ... }</pre>
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<p>then the syntax can be used to pass in resource subclasses previously defined as references
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like</p>
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<pre>
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<url url="https://ant.apache.org/" id="anturl"/>
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<my:task attr="${ant.refid:anturl}"/></pre>
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<h2 id="if+unless">If/Unless Attributes</h2>
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<p>
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The <code><target></code> element and various tasks (such as <code><fail></code>)
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and task elements (such as <code><test></code> in <code><junit></code>)
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support <var>if</var> and <var>unless</var> attributes which can be used to control whether the
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item is run or otherwise takes effect.
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</p>
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<p>
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In Ant 1.7.1 and earlier, these attributes could only be property names. The item was enabled
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if a property with that name was defined—even to be the empty string
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or <q>false</q>—and disabled if the property was not defined. For example, the following
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works but there is no way to override the file existence check negatively (only positively):
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</p>
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<pre>
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<target name="-check-use-file">
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<available property="file.exists" file="some-file"/>
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</target>
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<target name="use-file" depends="-check-use-file" <strong>if="file.exists"</strong>>
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<!-- do something requiring that file... -->
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</target>
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<target name="lots-of-stuff" depends="use-file,other-unconditional-stuff"/></pre>
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<p>
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<em>Since Ant 1.8.0</em>, you may instead use property expansion; a value of <q>true</q>
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(or <q>on</q> or <q>yes</q>) will enable the item, while <q>false</q> (or <q>off</q>
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or <q>no</q>) will disable it. Other values are still assumed to be property names and so the
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item is enabled only if the named property is defined.
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</p>
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<p>
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Compared to the older style, this gives you additional flexibility, because you can override the
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condition from the command line or parent scripts:
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</p>
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<pre>
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<target name="-check-use-file" <strong>unless="file.exists"</strong>>
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<available property="file.exists" file="some-file"/>
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</target>
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<target name="use-file" depends="-check-use-file" <strong>if="${file.exists}"</strong>>
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<!-- do something requiring that file... -->
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</target>
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<target name="lots-of-stuff" depends="use-file,other-unconditional-stuff"/></pre>
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<p>
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Now <kbd>ant -Dfile.exists=false lots-of-stuff</kbd> will run <q>other-unconditional-stuff</q>
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but not <q>use-file</q>, as you might expect, and you can disable the condition from another
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script too:
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</p>
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<pre>
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<antcall target="lots-of-stuff">
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<param name="file.exists" value="false"/>
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</antcall></pre>
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<p>
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Similarly, an <var>unless</var> attribute disables the item if it is either the name of property
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which is defined, or if it evaluates to a <q>true</q>-like value. For example, the following
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allows you to define <code>skip.printing.message=true</code> in <samp>my-prefs.properties</samp>
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with the results you might expect:
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</p>
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<pre>
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<property file="my-prefs.properties"/>
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<target name="print-message" <strong>unless="${skip.printing.message}"</strong>>
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<echo>hello!</echo>
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</target></pre>
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</body>
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</html>
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