docs/html/preview/j8-jack.jd

page.title=Using Java 8 Language Features
page.keywords="android N", "Java 8", "Jack"
@jd:body

<div id="qv-wrapper">
  <div id="qv">
    <h2>
      In this document
    </h2>

    <ol>
      <li>
        <a href="#supported-features">Supported Java 8 Language Features and APIs</a>
      </li>

      <li>
        <a href="#configuration">Enabling Java 8 Features and the Jack Toolchain</a>
      </li>
    </ol>

    <h2>
      See also
    </h2>

    <ol>
      <li>
        <a class="external-link" href=
        "https://source.android.com/source/jack.html">Jack (Java Android Compiler
        Kit)</a>
      </li>
    </ol>
  </div>
</div>

<p>
  The N Developer Preview introduces newly supported Java 8 language features
  that you can use when developing apps that target the Preview. To start using
  these features, you need to download and set up Android Studio 2.1 (preview)
  and the N Preview SDK, which include the required Jack toolchain and updated
  Android Plugin for Gradle.
</p>

<p>
  To learn how to set up Android Studio for the Preview, <a href=
  "{@docRoot}preview/setup-sdk.html">read the guide</a>. If you instead want to
  simply test your app's forward-compatibility, follow the guide to <a href=
  "{@docRoot}preview/run-app.html">run your app on
  the N Preview</a>.
</p>

<p>
  This document will discuss the new language features supported in the
  Preview, how to properly set up your project to use them, and any known
  issues you may encounter.
</p>

<p class="note">
  <strong>Note:</strong> Using the new Java 8 language features is not a
  requirement for developing apps that target the N Developer Preview. After
  following the set up instructions in the <a href=
  "{@docRoot}preview/setup-sdk.html">Preview guide</a>, you may use
  the Java 7 language features already supported by Android 6.0 (API level 23)
  and below.
</p>

<h2 id="supported-features">
  Supported Java 8 Language Features and APIs
</h2>

<p>
  Android does not currently support all Java 8 language features. However, the
  following features are now available when developing apps targeting the N
  Developer Preview:
</p>

<ul>
  <li>
    <a class="external-link" href=
    "https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html">Default
    and static interface methods</a>
  </li>

  <li>
    <a class="external-link" href=
    "https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html">
    Lambda expressions</a>
  </li>

  <li>
    <a class="external-link" href=
    "https://docs.oracle.com/javase/tutorial/java/annotations/repeating.html">Repeatable
    annotations</a>
  </li>

  <li>
    <a class="external-link" href=
    "https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html">
    Method References</a>
  </li>

  <li>
    <a class="external-link" href=
    "https://docs.oracle.com/javase/tutorial/java/IandI/interfaceAsType.html">Type
    Interface</a>
  </li>
</ul>


<p>
  Additionally, the following Java 8 language feature APIs are now available
  with the Preview:
</p>

<ul>
  <li>Reflection and language-related APIs:
  </li>

  <li style="list-style: none; display: inline">
    <ul>
      <li>{@code java.lang.FunctionalInterface}
      </li>

      <li>{@code java.lang.annotation.Repeatable}
      </li>

      <li>{@code java.lang.reflect.Method.isDefault()}
      </li>

      <li>and Reflection APIs associated with repeatable annotations, such as
      {@code AnnotatedElement.getAnnotationsByType(Class)}
      </li>
    </ul>
  </li>
  <li>Utility APIs:
  </li>

  <li style="list-style: none; display: inline">
    <ul>
      <li>{@code java.util.function}
      </li>
    </ul>
  </li>
</ul>

<p class="note">
  <strong>Note:</strong> The N Developer Preview bases its implementation of
  lambda expressions on anonymous classes. This approach allows them to be
  backwards compatible and executable on earlier versions of Android. To test
  lambda expressions on earlier versions, remember to go to your {@code
  build.gradle} file, and set {@code compileSdkVersion} and {@code
  targetSdkVersion} to 23 or lower.
</p>

<h2 id="configuration">
  Enabling Java 8 Features and the Jack Toolchain
</h2>

<p>
  In order to use the new Java 8 language features, you need to also use the
  new <a class="external-link" href=
  "https://source.android.com/source/jack.html">Jack toolchain</a>. This new
  Android toolchain compiles Java language source into Android-readable dex
  bytecode, has its own {@code .jack} library format, and provides most tool chain
  features as part of a single tool: repackaging, shrinking, obfuscation and
  multidex.
</p>

<p>Here we compare the two toolchains used to build Android DEX files:</p>
<ul>
  <li>Legacy: javac ({@code .java} &gt; {@code .class}) &gt; dx ({@code
  .class} &gt; {@code .dex})
  </li>

  <li>Modern: Jack ({@code .java} &gt; {@code .jack} &gt; {@code .dex})
  </li>
</ul>

<h3>
  Configuring Gradle
</h3>

<p>
  To enable the Java 8 language features and Jack for your project, enter the
  following in your module-specific {@code build.gradle} file:
</p>

<pre>
android {
...
  defaultConfig {
  ...
    jackOptions {
      enabled true
    }
  }
  compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
  }
}
</pre>

<h3>
  Known Issues
</h3>

<p>
  Instant Run, introduced in Android Studio 2.0 (Beta), does not currently work
  with Jack and will be disabled while using the new toolchain.
</p>

<p>
  Since Jack does not generate intermediate class files when compiling an app,
  tools that depend on these files do not currently work with Jack. Some examples of
  these tools are:
</p>

<ul>
  <li>Lint detectors that operate on class files
  </li>

  <li>Tools and libraries that require the app’s class files (e.g. JaCoCo and Mockito)
  </li>
</ul>

If you find other problems while using Jack, <a class="external-link" href=
"http://tools.android.com/filing-bugs">please report bugs</a>.