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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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See the License for the specific language governing permissions and |
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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>Targets and Target-Groups</title> |
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</head> |
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<body> |
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<h1><a name="targets">Targets</a></h1> |
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<p>A target is a container of tasks 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 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, for example, 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 <i>depends on</i> the compile target.</p> |
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<p>Ant tries to execute the targets in the <code>depends</code> |
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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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<blockquote> |
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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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</blockquote> |
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<p>Suppose we want to execute target D. From its |
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<code>depends</code> 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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<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 <u>not</u> 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 <i>sense</i> this property, you should add |
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the <code>if</code> (or <code>unless</code>) 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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<blockquote> |
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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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</blockquote> |
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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. <i>false</i>), 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 propertyname can be specified in the if/unless |
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clause. If you want to check multiple conditions, you can use a |
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dependend target for computing the result for the check:</p> |
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<blockquote><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> |
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</pre></blockquote> |
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<p>If no <code>if</code> and no <code>unless</code> attribute is |
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present, the target will always be executed.</p> |
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<p><b>Important:</b> the <code>if</code> and <code>unless</code> |
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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 <code>description</code> 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 <code>-projecthelp</code> 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 <code>-verbose</code> or <code>-debug</code> |
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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="CoreTasks/tstamp.html">tstamp</a> tasks in a |
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so-called <i>initialization</i> 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 |
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name <code>"init"</code>.</p> |
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<p>If the depends attribute and the if/unless attribute are set, the |
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depends attribute is executed first.</p> |
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<p>A target has the following attributes:</p> |
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<table border="1" cellpadding="2" cellspacing="0"> |
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<tr> |
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<td valign="top"><b>Attribute</b></td> |
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<td valign="top"><b>Description</b></td> |
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<td align="center" valign="top"><b>Required</b></td> |
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</tr> |
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<tr> |
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<td valign="top">name</td> |
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<td valign="top">the name of the target.</td> |
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<td align="center" valign="top">Yes</td> |
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</tr> |
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<tr> |
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<td valign="top">depends</td> |
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<td valign="top">a comma-separated list of names of targets on |
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which this target depends.</td> |
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<td align="center" valign="top">No</td> |
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</tr> |
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<tr> |
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<td valign="top">if</td> |
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<td valign="top">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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true</a>.</td> |
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<td align="center" valign="top">No</td> |
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</tr> |
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<tr> |
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<td valign="top">unless</td> |
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<td valign="top">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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false</a>.</td> |
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<td align="center" valign="top">No</td> |
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</tr> |
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<tr> |
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<td valign="top">description</td> |
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<td valign="top">a short description of this target's function.</td> |
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<td align="center" valign="top">No</td> |
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</tr> |
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<tr> |
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<td valign="top">group</td> |
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<td valign="top">Adds the current target to the depends list of |
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the named <a href="#target-groups">target-group</a>. |
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<em>since Ant 1.8.0.</em></td> |
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<td align="center" valign="top">No</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 "" is in this |
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set, as is comma "," and space " ". Please |
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avoid using these, as they will not be supported in future Ant |
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versions because of all the confusion they cause. IDE support of |
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unusual target names, or any target name containing spaces, varies |
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with the IDE.</p> |
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<p>Targets beginning with a hyphen such |
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as <code>"-restart"</code> are valid, and can be used to |
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name targets that should not be called directly from the command |
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line.</p> |
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<h1><a name="target-groups">Target-Groups</a></h1> |
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<p><em>since Ant 1.8.0.</em></p> |
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<p>Target-Groups are similar to targets in that they have a name and |
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a depends list and can be executed from the command line. Just |
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like targets they represent a state during the build 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 depends list.</p> |
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<p>Targets can add themselves to a target-group's depends list via |
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their group attribute. The targets that add themselves will be |
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added after the targets of the explicit depends-attribute of the |
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target-group, if multiple targets add themselves, their relative |
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order is not defined.</p> |
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<p>The main purpose of a target-group is to act as an extension |
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point for build files designed to |
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be <a href="CoreTasks\import.html">imported</a>. In the imported |
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file a target-groups defines a state that must be reached and |
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targets from other build files can join the depends list of said |
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target-group in order to contribute to that 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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<blockquote><pre> |
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<target name="create-directory-layout"> |
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... |
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</target> |
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<target-group 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> |
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</pre></blockquote> |
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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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<blockquote><pre> |
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<target name="generate-sources" |
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group="ready-to-compile"> |
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... |
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</target> |
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</pre></blockquote> |
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<p>This will ensure that the <em>generate-sources</em> target is |
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executed before the <em>compile</em> target.</p> |
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<p>Don't rely on the order of the depends list, |
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if <em>generate-sources</em> depends |
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on <em>create-directory-layout</em> 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> |
Stefan Bodewig