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<chapter id='init_scripts' xmlns:xi="http://www.w3.org/2001/XInclude">
    <title>Initialization Scripts</title>
    <para>Gradle provides a powerful mechanism to allow customizing the build based on the current environment.  This
        mechanism also supports tools that wish to integrate with Gradle.
    </para>
    <section id='sec:basic_usage'>
        <title>Basic usage</title>
        <para>Initialization scripts (a.k.a. <firstterm>init scripts</firstterm>) are similar to other scripts in Gradle.
            These scripts, however, are run before the build starts.  Here are several possible uses:
            <itemizedlist>
                <listitem>
                    <para>Set up properties based on the current environment (such as a developer's machine vs. a
                        continuous integration server).
                    </para>
                </listitem>
                <listitem>
                    <para>Supply personal information about the user to the build, such as repository or database
                        authentication credentials.
                    </para>
                </listitem>
                <listitem>
                    <para>Define machine specific details, such as where JDKs are installed.
                    </para>
                </listitem>
                <listitem>
                    <para>Register build listeners.  External tools that wish to listen to Gradle events might find
                        this useful.
                    </para>
                </listitem>
                <listitem>
                    <para>Register build loggers.  You might wish to customise how Gradle logs the events that it generates.
                    </para>
                </listitem>
            </itemizedlist>
            One main limitation of init scripts is that they cannot access classes in the buildSrc project (see
            <xref linkend='sec:build_sources'/> for details of this feature).
        </para>
        <para>There are two ways to use init scripts.  Either put a file called <filename>init.gradle</filename> in
            <filename><replaceable>USER_HOME</replaceable>/.gradle</filename>,
            or specify the file on the command line.  The command line option is <option>-I</option> or
            <option>--init-script</option> followed by the path to the script.  The command line option can appear
            more than once, each time adding another init script. If more than one init script is found they will all be
            executed. This allows for a tool to specify an init script and the user to put home in their home directory
            for defining the environment and both scripts will run when gradle is executed.
        </para>
    </section>
    <section>
        <title>Writing an init script</title>
        <para>
            <para>Similar to a Gradle build script, an init script is a groovy script. Each init script has a
                <apilink class="org.gradle.api.invocation.Gradle"/> instance associated with it. Any property reference
                and method call in the init script will delegate to this <classname>Gradle</classname> instance.
            </para>
            <para>Each init script also implements the <apilink class="org.gradle.api.Script"/> interface.</para>
        </para>
    </section>
    <section id='sec:custom_classpath'>
        <title>External dependencies for the init script</title>
        <para>In <xref linkend='sec:external_dependencies'/> is was explained how to add external dependencies to a
            build script. Init scripts can similarly have external dependencies defined.  You do this using the
            <literal>initscript()</literal> method, passing in a closure which declares the init script classpath.
        </para>
        <sample id="declareExternalInitDependency" dir="userguide/initScripts/externalDependency" title="Declaring external dependencies for an init script">
            <sourcefile file="init.gradle" snippet="declare-classpath"/>
        </sample>
        <para>The closure passed to the <literal>initscript()</literal> method configures a
            <apilink class="org.gradle.api.initialization.dsl.ScriptHandler"/> instance. You declare the init script
            classpath by adding dependencies to the <literal>classpath</literal> configuration. This is the same way
            you declare, for example, the Java compilation classpath. You can use any of the dependency types described
            in <xref linkend='sec:how_to_declare_your_dependencies'/>, except project dependencies.</para>
        <para>Having declared the init script classpath, you can use the classes in your init script as you would
            any other classes on the classpath. The following example adds to the previous example, and uses classes
            from the init script classpath.</para>
        <sample id="externalInitDependency" dir="userguide/initScripts/externalDependency" title="An init script with external dependencies">
            <sourcefile file="init.gradle"/>
            <output args="--init-script init.gradle -q doNothing"/>
        </sample>
    </section>
</chapter>
