wangwei
/
antliscence
forked from youys/ant

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  <title>Targets and Extension-Points</title>
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<body>
  <h1 id="targets">Targets</h1>

  <p>A target is a container of tasks that cooperate to reach a
    desired state during the build process.</p>

  <p>Targets can depend on other targets and Apache Ant ensures that these
    other targets have been executed before the current target. For
    example you might have a target for compiling and a
    target for creating a distributable. You can only build a
    distributable when you have compiled first, so the distribute
    target <i>depends on</i> the compile target.</p>

  <p>Ant tries to execute the targets in the <code>depends</code>
    attribute in the order they appear (from left to right). Keep in
    mind that it is possible that a target can get executed earlier
    when an earlier target depends on it:</p>

  <pre>&lt;target name=&quot;A&quot;/&gt;
&lt;target name=&quot;B&quot; depends=&quot;A&quot;/&gt;
&lt;target name=&quot;C&quot; depends=&quot;B&quot;/&gt;
&lt;target name=&quot;D&quot; depends=&quot;C,B,A&quot;/&gt;</pre>

  <p>Suppose we want to execute target D. From its
    <code>depends</code> attribute, you might think that first target
    C, then B and then A is executed.  Wrong! C depends on B, and B
    depends on A, so first A is executed, then B, then C, and finally
    D.</p>

  <pre><b>Call-Graph:</b>  A --> B --> C --> D</pre>

  <p>In a chain of dependencies stretching back from a given target
    such as D above, each target gets executed only once, even when
    more than one target depends on it. Thus, executing the D target
    will first result in C being called, which in turn will first call
    B, which in turn will first call A. After A, then B, then C have
    executed, execution returns to the dependency list of D, which
    will <u>not</u> call B and A, since they were already called in
    process of dependency resolution for C and B respectively as
    dependencies of D. Had no such dependencies been discovered in
    processing C and B, B and A would have been executed after C in
    processing D's dependency list.</p>

  <p>A target also has the ability to perform its execution if (or
    unless) a property has been set. This allows, for example, better
    control on the building process depending on the state of the
    system (java version, OS, command-line property defines, etc.).
    To make a target <i>sense</i> this property, you should add
    the <code>if</code> (or <code>unless</code>) attribute with the
    name of the property that the target should react
    to. <strong>Note:</strong> In the most simple case Ant will only
    check whether the property has been set, the value doesn't matter,
    but using property expansions you can build more complex
    conditions.  See
    <a href="properties.html#if+unless">the properties page</a> for
    more details.  For example:</p>

  <pre>&lt;target name=&quot;build-module-A&quot; if=&quot;module-A-present&quot;/&gt;</pre>
  <pre>&lt;target name=&quot;build-own-fake-module-A&quot; unless=&quot;module-A-present&quot;/&gt;</pre>

  <p>In the first example, if the <code>module-A-present</code>
    property is set (to any value, e.g. <i>false</i>), the target will
    be run. In the second example, if
    the <code>module-A-present</code> property is set (again, to any
    value), the target will not be run.</p>

  <p>Only one propertyname can be specified in the if/unless
    clause. If you want to check multiple conditions, you can use a
    dependent target for computing the result for the check:</p>

<pre>
&lt;target name="myTarget" depends="myTarget.check" if="myTarget.run"&gt;
    &lt;echo&gt;Files foo.txt and bar.txt are present.&lt;/echo&gt;
&lt/target&gt;

&lt;target name="myTarget.check"&gt;
    &lt;condition property="myTarget.run"&gt;
        &lt;and&gt;
            &lt;available file="foo.txt"/&gt;
            &lt;available file="bar.txt"/&gt;
        &lt;/and&gt;
    &lt;/condition&gt;
&lt/target&gt;
</pre>

  <pre><b>Call-Graph:</b>  myTarget.check --> maybe(myTarget)</pre>

  <p>If no <code>if</code> and no <code>unless</code> attribute is
    present, the target will always be executed.</p>

  <p><b>Important</b>: the <code>if</code> and <code>unless</code>
    attributes only enable or disable the target to which they are
    attached. They do not control whether or not targets that a
    conditional target depends upon get executed.  In fact, they do
    not even get evaluated until the target is about to be executed,
    and all its predecessors have already run.

  <p>The optional <code>description</code> attribute can be used to
    provide a one-line description of this target, which is printed by
    the <code>-projecthelp</code> command-line option. Targets without
    such a description are deemed internal and will not be listed,
    unless either the <code>-verbose</code> or <code>-debug</code>
    option is used.</p>

  <p>It is a good practice to place
    your <a href="Tasks/tstamp.html">tstamp</a> tasks in a
    so-called <i>initialization</i> target, on which all other targets
    depend. Make sure that target is always the first one in the
    depends list of the other targets. In this manual, most
    initialization targets have the name <code>&quot;init&quot;</code>.</p>
    <pre>
    &lt;project&gt;
        &lt;target name=&quot;init&quot;&gt;
            &lt;tstamp/&gt;
        &lt;/target&gt;
        &lt;target name=&quot;otherTarget&quot; depends=&quot;init&quot;&gt;
            ...
        &lt;/target&gt;
    &lt;/project&gt;
    </pre>

  <p>Especially if you only have a few tasks you also could place these
    tasks directly under the project tag (since Ant 1.6.0):</p>
    <pre>
    &lt;project&gt;
        &lt;tstamp/&gt;
    &lt;/project&gt;
    </pre>

  <p>If the depends attribute and the if/unless attribute are set, the
    depends attribute is executed first.</p>

  <p>A target has the following attributes:</p>

  <table>
    <tr>
      <td valign="top"><b>Attribute</b></td>
      <td valign="top"><b>Description</b></td>
      <td align="center" valign="top"><b>Required</b></td>
    </tr>
    <tr>
      <td valign="top">name</td>
      <td valign="top">the name of the target.</td>
      <td align="center" valign="top">Yes</td>
    </tr>
    <tr>
      <td valign="top">depends</td>
      <td valign="top">a comma-separated list of names of targets on
        which this target depends.</td>
      <td align="center" valign="top">No</td>
    </tr>
    <tr>
      <td valign="top">if</td>
      <td valign="top">the name of the property that must be set in
        order for this target to execute,
        or <a href="properties.html#if+unless">something evaluating to
          true</a>.</td>
      <td align="center" valign="top">No</td>
    </tr>
    <tr>
      <td valign="top">unless</td>
      <td valign="top">the name of the property that must not be set
        in order for this target to execute,
        or <a href="properties.html#if+unless">something evaluating to
        false</a>.</td>
      <td align="center" valign="top">No</td>
    </tr>
    <tr>
      <td valign="top">description</td>
      <td valign="top">a short description of this target's function.</td>
      <td align="center" valign="top">No</td>
    </tr>
    <tr>
      <td valign="top">extensionOf</td>
      <td valign="top">Adds the current target to the depends list of
        the named <a href="#extension-points">extension-point</a>.
        <em>since Ant 1.8.0.</em></td>
      <td align="center" valign="top">No</td>
    </tr>
    <tr>
      <td valign="top">onMissingExtensionPoint</td>
      <td valign="top">What to do if this target tries to extend a
        missing
        <a href="#extension-points">extension-point</a>. ("fail",
        "warn", "ignore").
        <em>since Ant 1.8.2.</em></td>
      <td align="center" valign="top">No. Not allowed unless
        <code>extensionOf</code> is present. Defaults to <code>fail</code>.
      </td>
    </tr>
  </table>

  <p>A target name can be any alphanumeric string valid in the
    encoding of the XML file. The empty string &quot;&quot; is in this
    set, as is comma &quot;,&quot; and space &quot; &quot;.  Please
    avoid using these, as they will not be supported in future Ant
    versions because of all the confusion they cause on command line and IDE. IDE support of
    unusual target names, or any target name containing spaces, varies
    with the IDE.</p>

  <p>Targets beginning with a hyphen such
    as <code>&quot;-restart&quot;</code> are valid, and can be used to
    name targets that should not be called directly from the command
    line. <br>
    For Ants main class every option starting with hyphen is an
    option for Ant itself and not a target. For that reason calling these
    target from command line is not possible. On the other hand IDEs usually
    don't use Ants main class as entry point and calling them from the IDE
    is usually possible.</p>

  <h1 id="extension-points">Extension-Points</h1>

  <p><em>since Ant 1.8.0.</em></p>

  <p>Extension-Points are similar to targets in that they have a name and
    a depends list and can be executed from the command line.  Just
    like targets they represent a state during the build process.</p>

  <p>Unlike targets they don't contain any tasks, their main purpose
    is to collect targets that contribute to the desired state in
    their depends list.</p>

  <p>Targets can add themselves to an extension-point's depends list via
    their extensionOf attribute.  The targets that add themselves will be
    added after the targets of the explicit depends attribute of the
    extension-point, if multiple targets add themselves, their relative
    order is not defined.</p>

  <p>The main purpose of an extension-point is to act as an extension
    point for build files designed to
    be <a href="Tasks/import.html">imported</a>.  In the imported
    file, an extension-point defines a state that must be reached and
    targets from other build files can join the depends list of said
    extension-point in order to contribute to that state.</p>

  <p>For example your imported build file may need to compile code, it
    might look like:</p>
<pre>
&lt;target name="create-directory-layout"&gt;
   ...
&lt;/target&gt;
&lt;extension-point name="ready-to-compile"
              depends="create-directory-layout"/&gt;
&lt;target name="compile" depends="ready-to-compile"&gt;
   ...
&lt;/target&gt;
</pre>

  <pre><b>Call-Graph:</b>  create-directory-layout --> 'empty slot' --> compile</pre>

  <p>And you need to generate some source before compilation, then in
    your main build file you may use something like</p>
<pre>
&lt;target name="generate-sources"
        extensionOf="ready-to-compile"&gt;
   ...
&lt;/target&gt;
</pre>

  <pre><b>Call-Graph:</b>  create-directory-layout --> generate-sources  --> compile</pre>

  <p>This will ensure that the <em>generate-sources</em> target is
    executed before the <em>compile</em> target.</p>

  <p>Don't rely on the order of the depends list,
    if <em>generate-sources</em> depends
    on <em>create-directory-layout</em> then it must explicitly depend
    on it via its own depends attribute.</p>
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