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