176 lines
5.9 KiB
HTML
176 lines
5.9 KiB
HTML
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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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-->
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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>Local Task</title>
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</head>
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<body>
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<h2>Local</h2>
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<p><em>Since Ant 1.8</em></p>
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<h3>Description</h3>
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<p>Adds a local property to the current scope. Property scopes exist at Apache Ant's various "block"
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levels. These include targets as well as the <a href="parallel.html">Parallel</a>
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and <a href="sequential.html">Sequential</a> task containers
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(including <a href="macrodef.html">Macrodef</a> bodies). A local property at a given scope "shadows"
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properties of the same name at higher scopes, including the global scope. Note that using
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the <code>Local</code> task at the global level effectively makes the property local to the
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"anonymous target" in which top-level operations are carried out; it will not be defined for other
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targets in the buildfile.</p>
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<p>A property is made local if the <code><local></code> task precedes its definition. See the
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examples section.</p>
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<h3>Parameters</h3>
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<table class="attr">
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<tr>
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<th scope="col">Attribute</th>
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<th scope="col">Description</th>
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<th scope="col">Required</th>
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</tr>
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<tr>
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<td>name</td>
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<td>The property to declare in the current scope</td>
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<td>Yes</td>
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</tr>
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</table>
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<h3>Examples</h3>
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<h4>Temporarily shadow a global property's value</h4>
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<pre>
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<property name="foo" value="foo"/>
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<target name="step1">
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<echo>Before local: foo is ${foo}</echo>
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<local name="foo"/>
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<property name="foo" value="bar"/>
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<echo>After local: foo is ${foo}</echo>
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</target>
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<target name="step2" depends="step1">
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<echo>In step2: foo is ${foo}</echo>
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</target></pre>
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<p>outputs</p>
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<pre class="output">
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step1:
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[echo] Before local: foo is foo
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[echo] After local: foo is bar
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step2:
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[echo] In step2: foo is foo</pre>
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<p>here the <code>local</code> task shadowed the global definition of <code>foo</code> for the
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remainder of the target <q>step1</q>.</p>
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<h4>Creating thread local properties</h4>
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<pre>
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<property name="foo" value="foo"/>
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<parallel>
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<echo>global 1: foo is ${foo}</echo>
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<sequential>
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<local name="foo"/>
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<property name="foo" value="bar.1"/>
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<echo>First sequential: foo is ${foo}</echo>
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</sequential>
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<sequential>
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<sleep seconds="1"/>
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<echo>global 2: foo is ${foo}</echo>
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</sequential>
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<sequential>
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<local name="foo"/>
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<property name="foo" value="bar.2"/>
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<echo>Second sequential: foo is ${foo}</echo>
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</sequential>
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<echo>global 3: foo is ${foo}</echo>
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</parallel></pre>
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<p>outputs something similar to</p>
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<pre class="output">
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[echo] global 3: foo is foo
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[echo] global 1: foo is foo
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[echo] First sequential: foo is bar.1
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[echo] Second sequential: foo is bar.2
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[echo] global 2: foo is foo</pre>
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<h4>Use inside <code>macrodef</code></h4>
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<p>This probably is where <code>local</code> can be applied in the most useful way. If you needed a
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"temporary property" inside a <code>macrodef</code> in Ant prior to Ant 1.8.0 you had to try to come
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up with a property name that would be unique across macro invocations.</p>
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<p>Say you wanted to write a macro that created the parent directory of a given file. A naive
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approach would be:</p>
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<pre>
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<macrodef name="makeparentdir">
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<attribute name="file"/>
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<sequential>
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<dirname property="parent" file="@{file}"/>
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<mkdir dir="${parent}"/>
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</sequential>
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</macrodef>
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<makeparentdir file="some-dir/some-file"/></pre>
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<p>but this would create a global property <code>parent</code> on the first invocation—and
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since properties are not mutable, any subsequent invocation will see the same value and try to
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create the same directory as the first invocation.</p>
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<p>The recommendation prior to Ant 1.8.0 was to use a property name based on one of the macro's
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attributes, like</p>
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<pre>
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<macrodef name="makeparentdir">
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<attribute name="file"/>
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<sequential>
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<dirname property="parent.@{file}" file="@{file}"/>
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<mkdir dir="${parent.@{file}}"/>
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</sequential>
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</macrodef></pre>
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<p>Now invocations for different files will set different properties and the directories will get
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created. Unfortunately this "pollutes" the global properties space. In addition, it may be hard to
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come up with unique names in some cases.</p>
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<p>Enter <code><local></code>:</p>
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<pre>
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<macrodef name="makeparentdir">
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<attribute name="file"/>
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<sequential>
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<local name="parent"/>
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<dirname property="parent" file="@{file}"/>
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<mkdir dir="${parent}"/>
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</sequential>
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</macrodef></pre>
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<p>Each invocation gets its own property named <code>parent</code> and there will be no global
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property of that name at all.</p>
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</body>
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</html>
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