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

<p>If you haven't yet installed the Android N Preview SDK, follow the guide to
<a href="{@docRoot}preview/setup-sdk.html">Set Up to Develop for Android
N</a>.</p>


<p>
  This document describes the new language features supported in the Android N
  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 Android N 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>
</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 Android N 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>.