@@ -36,6 +36,9 @@ level effectively makes the property local to the "anonymous target" in which
top-level operations are carried out; it will not be defined for other targets
in the buildfile. <b>Since Ant 1.8</b></p>
<p>A property is made local if the <code><local></code> task
preceedes its definition. See the examples section.</p>
<h3>Parameters</h3>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
@@ -50,6 +53,134 @@ in the buildfile. <b>Since Ant 1.8</b></p>
</tr>
</table>
<h3>Examples</h3>
<h4>Temporarily shadow a global property's value</h4>
<pre>
<property name="foo" value="foo"/>
<target name="step1">
<echo>Before local: foo is ${foo}</echo>
<local name="foo"/>
<property name="foo" value="bar"/>
<echo>After local: foo is ${foo}</echo>
</target>
<target name="step2" depends="step1">
<echo>In step2: foo is ${foo}</echo>
</target>
</pre>
<p>outputs</p>
<pre>
step1:
[echo] Before local: foo is foo
[echo] After local: foo is bar
step2:
[echo] In step2: foo is foo
</pre>
<p>here the local-task shadowed the global definition
of <code>foo</code> for the remainder of the target step1.</p>
<h4>Creating thread local properties</h4>
<pre>
<property name="foo" value="foo"/>
<parallel>
<echo>global 1: foo is ${foo}</echo>
<sequential>
<local name="foo"/>
<property name="foo" value="bar.1"/>
<echo>First sequential: foo is ${foo}</echo>
</sequential>
<sequential>
<sleep seconds="1"/>
<echo>global 2: foo is ${foo}</echo>
</sequential>
<sequential>
<local name="foo"/>
<property name="foo" value="bar.2"/>
<echo>Second sequential: foo is ${foo}</echo>
</sequential>
<echo>global 3: foo is ${foo}</echo>
</parallel>
</pre>
<p>outputs something similar to</p>
<pre>
[echo] global 3: foo is foo
[echo] global 1: foo is foo
[echo] First sequential: foo is bar.1
[echo] Second sequential: foo is bar.2
[echo] global 2: foo is foo
</pre>
<h4>Use inside macrodef</h4>
<p>This probably is where local can be applied in the most useful
way. If you needed a "temporary property" inside a macrodef in Ant
prior to Ant 1.8.0 you had to try to come up with a property name
that would be unique across macro invocations.</p>
<p>Say you wanted to write a macro that created the parent directory
of a given file. A naive approach would be:</p>
<pre>
<macrodef name="makeparentdir">
<attribute name="file"/>
<sequential>
<dirname property="parent" file="@{file}"/>
<mkdir dir="${parent}"/>
</sequential>
</macrodef>
<makeparentdir file="some-dir/some-file"/>
</pre>
<p>but this would create a global property "parent" on the first
invocation - and since properties are not mutable, any subsequent
invocation will see the same value and try to create the same
directory as the first invocation.</p>
<p>The recommendation prior to Ant 1.8.0 was to use a property name
based on one of the macro's attributes, like</p>
<pre>
<macrodef name="makeparentdir">
<attribute name="file"/>
<sequential>
<dirname property="parent.@{file}" file="@{file}"/>
<mkdir dir="${parent.@{file}}"/>
</sequential>
</macrodef>
</pre>
<p>Now invocations for different files will set different properties
and the directories will get created. Unfortunately this "pollutes"
the global properties space. In addition it may be hard to come up
with unique names in some cases.</p>
<p>Enter <code><local></code>:</p>
<pre>
<macrodef name="makeparentdir">
<attribute name="file"/>
<sequential>
<local name="parent"/>
<dirname property="parent" file="@{file}"/>
<mkdir dir="${parent}"/>
</sequential>
</macrodef>
</pre>
<p>Each invocation gets its own property name "parent" and there will
be no global property of that name at all.</p>
</body>
</html>
Stefan Bodewig