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<chapter id='java_library_plugin'>
    <title>The Java Library Plugin</title>

    <para>The Java Library plugin expands the capabilities of the Java plugin by providing specific knowledge about Java libraries. In particular, a Java
        library exposes an API to consumers (i.e., other projects using the Java or the Java Library plugin). All the source sets, tasks and configurations
        exposed by the Java plugin are implicitly available when using this plugin.
    </para>

    <section id="sec:java_library_usage">
        <title>Usage</title>
        <para>To use the Java Library plugin, include the following in your build script:</para>
        <sample id="useJavaLibraryPlugin" dir="java-library/quickstart" title="Using the Java Library plugin">
            <sourcefile file="build.gradle" snippet="use-plugin"/>
        </sample>
    </section>

    <section id="sec:java_library_separation">
        <title>API and implementation separation</title>
        <para>The key difference between the standard Java plugin and the Java Library plugin is that the latter introduces the concept of
            an <firstterm>API</firstterm> exposed to consumers. A library is a Java component meant to be consumed by other components.
            It's a very common use case in multi-project builds, but also as soon as you have external dependencies.
        </para>
        <para>
            The plugin exposes two <link linkend="configurations">configurations</link> that can be used to declare dependencies:
            <literal>api</literal>
            and <literal>implementation</literal>. The <literal>api</literal> configuration should be used to declare dependencies which
            are exported by the library API, whereas the <literal>implementation</literal> configuration should be used to declare dependencies
            which are internal to the component.
        </para>
        <sample id="useJavaLibraryPlugin" dir="java-library/quickstart" title="Declaring API and implementation dependencies">
            <sourcefile file="build.gradle" snippet="dependencies"/>
        </sample>
        <para>Dependencies appearing in the <literal>api</literal> configurations will be transitively exposed to consumers of the library, and
            as such will appear on the compile classpath of consumers. Dependencies found in the <literal>implementation</literal> configuration will,
            on the other hand, not be exposed to consumers, and therefore not leak into the consumers' compile classpath. This comes with several benefits:
        </para>
        <itemizedlist>
            <listitem>dependencies do not leak into the compile classpath of consumers anymore, so you will never accidentally depend on a transitive dependency</listitem>
            <listitem>faster compilation thanks to reduced classpath size</listitem>
            <listitem>less recompilations when implementation dependencies change: consumers would not need to be recompiled</listitem>
            <listitem>cleaner publishing: when used in conjunction with the new <literal>maven-publish</literal> plugin, Java libraries produce POM files that distinguish exactly between what
                is required to compile against the library and what is required to use the library at runtime (in other words, don't mix what is needed to compile
                the library itself and what is needed to compile against the library).
            </listitem>
        </itemizedlist>
        <note>The <literal>compile</literal> configuration still exists but should not be used as it will not offer the guarantees that the
            <literal>api</literal>
            and <literal>implementation</literal> configurations provide.
        </note>
    </section>

    <section id="sec:java_library_recognizing_dependencies">
        <title>Recognizing API and implementation dependencies</title>
        <para>This section will help you spot API and Implementation dependencies in your code using simple rules of thumb.
            Basically, an API dependency is a type that is exposed in the library binary interface, often referred to ABI (Application Binary Interface).
            This includes, but is not limited to:
        </para>
        <itemizedlist>
            <listitem>types used in super classes or interfaces</listitem>
            <listitem>types used in public method parameters, including generic parameter types (where <firstterm>public</firstterm> is something that is visible to compilers.
                I.e. , <firstterm>public</firstterm>, <firstterm>protected</firstterm> and <firstterm>package private</firstterm> members in the Java world)
            </listitem>
            <listitem>types used in public fields</listitem>
            <listitem>public annotation types</listitem>
        </itemizedlist>
        <para>In opposition, any type that is used in the following list is irrelevant to the ABI, and therefore should be declared as <literal>implementation</literal> dependency:
        </para>
        <itemizedlist>
            <listitem>types exclusively used in method bodies</listitem>
            <listitem>types exclusively used in private members</listitem>
            <listitem>types exclusively found in internal classes (future versions of Gradle will let you declare which packages belong to the public API)</listitem>
        </itemizedlist>
        <para>In the following sample, we can make the difference between an API dependency and an implementation dependency:</para>
        <sample id="useJavaLibraryPlugin" dir="java-library/quickstart" title="Making the difference between API and implementation">
            <sourcefile file="src/main/java/org/gradle/HttpClientWrapper.java" snippet="sample"/>
        </sample>
        <para>We can see that our class imports third party classes, but imports alone won't tell us if a dependency is an API or implementation dependency.
            For this, we need to look at the methods. The public constructor of <literal>HttpClientWrapper</literal> uses <literal>HttpClient</literal> as a parameter,
            so it exposed to consumers and therefore belongs to the API.
        </para>
        <para>On the other hand, the <literal>ExceptionUtils</literal> type, coming from the <literal>commons-lang</literal> library, is only used in a method
            body, so it's an implementation dependency.
        </para>
        <para>Therefore, we can deduce that <literal>commons-httpclient</literal> is an API dependency, whereas <literal>commons-lang</literal> is an implementation
            dependency, which directly translates into the build file:
        </para>
        <sample id="useJavaLibraryPlugin" dir="java-library/quickstart" title="Declaring API and implementation dependencies">
            <sourcefile file="build.gradle" snippet="dependencies"/>
        </sample>
        <para>As a guideline, you should prefer the <literal>implementation</literal> configuration first: leakage of implementation types to consumers would
            then directly lead to a compile error of consumers, which would be solved either by removing the type from the public API, or promoting the dependency as an
            API dependency instead.
        </para>
    </section>

    <section id="sec:java_library_configurations_graph">
        <title>The Java Library plugin configurations</title>
        <para>The following graph describes the main configurations setup when the Java Library plugin is in use.
        </para>
        <imageobject>
            <imagedata fileref="img/java-library-ignore-deprecated-main.png"/>
        </imageobject>
        <itemizedlist>
            <listitem>The configurations in <firstterm>green</firstterm> are the ones a user should use to declare dependencies
            </listitem>
            <listitem>The configurations in <firstterm>pink</firstterm> are the ones used when a component compiles, or runs against the library
            </listitem>
            <listitem>The configurations in <firstterm>blue</firstterm> are internal to the component, for its own use
            </listitem>
            <listitem>The configurations in <firstterm>white</firstterm> are configurations inherited from the Java plugin
            </listitem>
        </itemizedlist>
        <para>And the next graph describes the test configurations setup:
        </para>
        <imageobject>
            <imagedata fileref="img/java-library-ignore-deprecated-test.png"/>
        </imageobject>
        <note>
            The <firstterm>compile</firstterm>, <firstterm>testCompile</firstterm>, <firstterm>runtime</firstterm> and <firstterm>testRuntime</firstterm> configurations
            inherited from the Java plugin are still available but are deprecated. You should avoid using them, as they are only kept for backwards compatibility.
        </note>
        <para>The role of each configuration is described in the following tables:</para>
        <table>
            <title>Java Library plugin - configurations used to declare dependencies</title>
            <thead>
                <tr>
                    <td>Configuration name</td>
                    <td>Role</td>
                    <td>Can be consumed</td>
                    <td>Can be resolved</td>
                    <td>Description</td>
                </tr>
            </thead>
            <tr>
                <td>api</td>
                <td>Declaring API dependencies</td>
                <td>no</td>
                <td>no</td>
                <td>This is where you should declare dependencies which are transitively exported to consumers, for compile.</td>
            </tr>
            <tr>
                <td>implementation</td>
                <td>Declaring implementation dependencies</td>
                <td>no</td>
                <td>no</td>
                <td>This is where you should declare dependencies which are purely internal and not meant to be exposed to consumers.</td>
            </tr>
            <tr>
                <td>compileOnly</td>
                <td>Declaring compile only dependencies</td>
                <td>yes</td>
                <td>yes</td>
                <td>This is where you should declare dependencies which are only required at compile time, but should not leak into the runtime. This typically includes
                    dependencies which are shaded when found at runtime.
                </td>
            </tr>
            <tr>
                <td>runtimeOnly</td>
                <td>Declaring runtime dependencies</td>
                <td>no</td>
                <td>no</td>
                <td>This is where you should declare dependencies which are only required at runtime, and not at compile time.</td>
            </tr>
            <tr>
                <td>testImplementation</td>
                <td>Test dependencies</td>
                <td>no</td>
                <td>no</td>
                <td>This is where you should declare dependencies which are used to compile tests.</td>
            </tr>
            <tr>
                <td>testCompileOnly</td>
                <td>Declaring test compile only dependencies</td>
                <td>yes</td>
                <td>yes</td>
                <td>This is where you should declare dependencies which are only required at test compile time, but should not leak into the runtime. This typically includes
                    dependencies which are shaded when found at runtime.
                </td>
            </tr>
            <tr>
                <td>testRuntimeOnly</td>
                <td>Declaring test runtime dependencies</td>
                <td>no</td>
                <td>no</td>
                <td>This is where you should declare dependencies which are only required at test runtime, and not at test compile time.</td>
            </tr>
        </table>
        <table>
            <title>Java Library plugin - configurations used by consumers</title>
            <thead>
                <tr>
                    <td>Configuration name</td>
                    <td>Role</td>
                    <td>Can be consumed</td>
                    <td>Can be resolved</td>
                    <td>Description</td>
                </tr>
            </thead>
            <tr>
                <td>apiElements</td>
                <td>For compiling against this library</td>
                <td>yes</td>
                <td>no</td>
                <td>This configuration is meant to be used by consumers, to retrieve all the elements necessary to compile against this library. Unlike
                    the <literal>default</literal> configuration, this doesn't leak implementation or runtime dependencies.
                </td>
            </tr>
            <tr>
                <td>runtimeElements</td>
                <td>For executing this library</td>
                <td>yes</td>
                <td>no</td>
                <td>This configuration is meant to be used by consumers, to retrieve all the elements necessary to run against this library.</td>
            </tr>
        </table>
       <table>
            <title>Java Library plugin - configurations used by the library itself</title>
            <thead>
                <tr>
                    <td>Configuration name</td>
                    <td>Role</td>
                    <td>Can be consumed</td>
                    <td>Can be resolved</td>
                    <td>Description</td>
                </tr>
            </thead>
            <tr>
                <td>compileClasspath</td>
                <td>For compiling this library</td>
                <td>no</td>
                <td>yes</td>
                <td>This configuration contains the compile classpath of this library, and is therefore used when invoking the java compiler to compile it.</td>
            </tr>
            <tr>
                <td>runtimeClasspath</td>
                <td>For executing this library</td>
                <td>no</td>
                <td>yes</td>
                <td>This configuration contains the runtime classpath of this library</td>
            </tr>
            <tr>
                <td>testCompileClasspath</td>
                <td>For compiling the tests of this library</td>
                <td>no</td>
                <td>yes</td>
                <td>This configuration contains the test compile classpath of this library.</td>
            </tr>
            <tr>
                <td>testRuntimeClasspath</td>
                <td>For executing tests of this library</td>
                <td>no</td>
                <td>yes</td>
                <td>This configuration contains the test runtime classpath of this library</td>
            </tr>
        </table>

    </section>

    <section id="sec:java_library_known_issues">
    <title>Known issues</title>
        <section id="sec:java_library_known_issues_compat">
            <title>Compatibility with other plugins</title>
            <para>At the moment the Java Library plugin is only wired to behave correctly with the <literal>java</literal> plugin. Other plugins, such as
            the Groovy plugin, may not behave correctly. In particular, if the Groovy plugin is used in addition to the <literal>java-library</literal> plugin,
            then consumers may not get the Groovy classes when they consume the library. To workaround this, you need to explicitly wire the Groovy compile
            dependency, like this:</para>
            <sample id="useGroovyPlugin" dir="java-library/with-groovy" title="Configuring the Groovy plugin to work with Java Library">
                <sourcefile file="a/build.gradle" snippet="configure-groovy"/>
            </sample>
        </section>
        <section id="sec:java_library_known_issues_memory">
            <title>Increased memory usage for consumers</title>
            <para>When a project uses the Java Library plugin, consumers will use the output classes directory of this project directly on their compile
            classpath, instead of the jar file if the project uses the Java plugin. An indirect consequence is that up-to-date checking will require more
            memory, because Gradle will snapshot individual class files instead of a single jar. This may lead to increased memory consumption for large
            projects.
            </para>
        </section>
    </section>
</chapter>
