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HTML
295 lines
12 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>Targets and Extension-Points</title>
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</head>
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<body>
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<h1 id="targets">Targets</h1>
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<p>A target is a container of tasks and datatypes that cooperate to reach a
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desired state during the build process.</p>
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<p>Targets can depend on other targets and Apache Ant ensures that these
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other targets have been executed before the current target. For
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example you might have a target for compiling and a
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target for creating a distributable. You can only build a
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distributable when you have compiled first, so the distribute
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target <em>depends on</em> the compile target.</p>
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<p>Ant tries to execute the targets in the <var>depends</var>
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attribute in the order they appear (from left to right). Keep in
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mind that it is possible that a target can get executed earlier
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when an earlier target depends on it:</p>
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<pre><target name="A"/>
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<target name="B" depends="A"/>
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<target name="C" depends="B"/>
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<target name="D" depends="C,B,A"/></pre>
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<p>Suppose we want to execute target D. From its
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<var>depends</var> attribute, you might think that first target
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C, then B and then A is executed. Wrong! C depends on B, and B
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depends on A, so first A is executed, then B, then C, and finally
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D.</p>
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<pre><b>Call-Graph:</b> A → B → C → D</pre>
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<p>In a chain of dependencies stretching back from a given target
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such as D above, each target gets executed only once, even when
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more than one target depends on it. Thus, executing the D target
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will first result in C being called, which in turn will first call
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B, which in turn will first call A. After A, then B, then C have
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executed, execution returns to the dependency list of D, which
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will <strong>not</strong> call B and A, since they were already called in
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process of dependency resolution for C and B respectively as
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dependencies of D. Had no such dependencies been discovered in
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processing C and B, B and A would have been executed after C in
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processing D's dependency list.</p>
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<p>A target also has the ability to perform its execution if (or
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unless) a property has been set. This allows, for example, better
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control on the building process depending on the state of the
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system (Java version, OS, command-line property defines, etc.).
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To make a target <em>sense</em> this property, you should add
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the <var>if</var> (or <var>unless</var>) attribute with the
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name of the property that the target should react
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to. <strong>Note:</strong> In the most simple case Ant will only
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check whether the property has been set, the value doesn't matter,
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but using property expansions you can build more complex
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conditions. See
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<a href="properties.html#if+unless">the properties page</a> for
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more details. For example:</p>
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<pre><target name="build-module-A" if="module-A-present"/></pre>
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<pre><target name="build-own-fake-module-A" unless="module-A-present"/></pre>
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<p>In the first example, if the <code>module-A-present</code>
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property is set (to any value, e.g. <q>false</q>), the target will
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be run. In the second example, if
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the <code>module-A-present</code> property is set (again, to any
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value), the target will not be run.</p>
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<p>Only one property name can be specified in
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the <var>if</var>/<var>unless</var> attribute. If you want to
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check multiple conditions, you can use a dependent target for
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computing the result for the check:</p>
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<pre>
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<target name="myTarget" depends="myTarget.check" if="myTarget.run">
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<echo>Files foo.txt and bar.txt are present.</echo>
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</target>
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<target name="myTarget.check">
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<condition property="myTarget.run">
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<and>
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<available file="foo.txt"/>
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<available file="bar.txt"/>
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</and>
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</condition>
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</target></pre>
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<pre><b>Call-Graph:</b> myTarget.check → maybe(myTarget)</pre>
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<p>If no <var>if</var> and no <var>unless</var> attribute is
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present, the target will always be executed.</p>
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<p><strong>Important</strong>: the <var>if</var> and <var>unless</var>
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attributes only enable or disable the target to which they are
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attached. They do not control whether or not targets that a
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conditional target depends upon get executed. In fact, they do
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not even get evaluated until the target is about to be executed,
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and all its predecessors have already run.
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<p>The optional <var>description</var> attribute can be used to
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provide a one-line description of this target, which is printed by
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the <kbd>-projecthelp</kbd> command-line option. Targets without
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such a description are deemed internal and will not be listed,
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unless either the <kbd>-verbose</kbd> or <kbd>-debug</kbd>
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option is used.</p>
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<p>It is a good practice to place
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your <a href="Tasks/tstamp.html">tstamp</a> tasks in a
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so-called <em>initialization</em> target, on which all other targets
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depend. Make sure that target is always the first one in the
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depends list of the other targets. In this manual, most
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initialization targets have the name <code>"init"</code>.</p>
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<pre>
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<project>
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<target name="init">
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<tstamp/>
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</target>
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<target name="otherTarget" depends="init">
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...
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</target>
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</project></pre>
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<p>Especially if you only have a few tasks you also could place these
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tasks directly under the project tag (<em>since Ant 1.6.0</em>):</p>
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<pre>
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<project>
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<tstamp/>
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</project></pre>
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<p>If the <var>depends</var> attribute and
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the <var>if</var>/<var>unless</var> attribute are set,
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the <var>depends</var> attribute is executed first.</p>
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<p>A target has the following attributes:</p>
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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 name of the target.</td>
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<td>Yes</td>
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</tr>
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<tr>
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<td>depends</td>
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<td>a comma-separated list of names of targets on
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which this target depends.</td>
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<td>No</td>
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</tr>
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<tr>
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<td>if</td>
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<td>the name of the property that must be set in
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order for this target to execute,
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or <a href="properties.html#if+unless">something evaluating to
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<q>true</q></a>.</td>
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<td>No</td>
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</tr>
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<tr>
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<td>unless</td>
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<td>the name of the property that must not be set
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in order for this target to execute,
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or <a href="properties.html#if+unless">something evaluating to
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<q>false</q></a>.</td>
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<td>No</td>
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</tr>
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<tr>
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<td>description</td>
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<td>a short description of this target's function.</td>
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<td>No</td>
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</tr>
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<tr>
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<td>extensionOf</td>
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<td>Adds the current target to the depends list of
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the named <a href="#extension-points">extension-point</a>.
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<em>since Ant 1.8.0</em>.</td>
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<td>No</td>
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</tr>
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<tr>
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<td>onMissingExtensionPoint</td>
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<td>What to do if this target tries to extend a
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missing
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<a href="#extension-points">extension-point</a>. (<q>fail</q>,
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<q>warn</q>, <q>ignore</q>).
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<em>since Ant 1.8.2</em>.</td>
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<td>No; not allowed unless
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<var>extensionOf</var> is present, defaults to <q>fail</q>.
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</td>
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</tr>
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</table>
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<p>A target name can be any alphanumeric string valid in the
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encoding of the XML file. The empty string <q></q> is in this set,
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as is comma <q>,</q> and space <q> </q>. Please avoid using
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these, as they will not be supported in future Ant versions
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because of all the confusion they cause on command line and
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IDE. IDE support of unusual target names, or any target name
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containing spaces, varies with the IDE.</p>
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<p>Targets beginning with a hyphen such as <q>-restart</q> are
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valid, and can be used to name targets that should not be called
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directly from the command line.<br/>
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For Ant's main class every option starting with hyphen is an
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option for Ant itself and not a target. For that reason calling
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these targets from command line is not possible. On the other
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hand, IDEs usually don't use Ant's main class as entry point and
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calling them from the IDE is usually possible.</p>
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<h1 id="extension-points">Extension-Points</h1>
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<p><em>Since Ant 1.8.0</em>.</p>
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<p>Extension-Points are similar to targets in that they have a name
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and a <var>depends</var> list and can be executed from the command
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line. Just like targets they represent a state during the build
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process.</p>
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<p>Unlike targets they don't contain any tasks, their main purpose
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is to collect targets that contribute to the desired state in
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their <var>depends</var> list.</p>
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<p>Targets can add themselves to an
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extension-point's <var>depends</var> list via
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their <var>extensionOf</var> attribute. The targets that add
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themselves will be added after the targets of the
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explicit <var>depends</var> attribute of the extension-point, if
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multiple targets add themselves, their relative order is not
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defined.</p>
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<p>The main purpose of an extension-point is to act as an extension
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point for build files designed to
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be <a href="Tasks/import.html">imported</a>. In the imported
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file, an extension-point defines a state that must be reached and
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targets from other build files can join the <var>depends</var>
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list of said extension-point in order to contribute to that
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state.</p>
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<p>For example your imported build file may need to compile code, it
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might look like:</p>
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<pre>
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<target name="create-directory-layout">
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...
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</target>
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<extension-point name="ready-to-compile"
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depends="create-directory-layout"/>
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<target name="compile" depends="ready-to-compile">
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...
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</target></pre>
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<pre><b>Call-Graph:</b> create-directory-layout → 'empty slot' → compile</pre>
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<p>And you need to generate some source before compilation, then in
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your main build file you may use something like</p>
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<pre>
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<target name="generate-sources"
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extensionOf="ready-to-compile">
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...
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</target></pre>
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<pre><b>Call-Graph:</b> create-directory-layout → generate-sources → compile</pre>
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<p>This will ensure that the <q>generate-sources</q> target is
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executed before the <q>compile</q> target.</p>
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<p>Don't rely on the order of the depends list,
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if <q>generate-sources</q> depends
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on <q>create-directory-layout</q> then it must explicitly depend
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on it via its own depends attribute.</p>
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</body>
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</html>
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