docs/html/preview/j8-jack.jd

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

<div id="qv-wrapper">
  <div id="qv">
    <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>
  </div>
</div>

<p>Android N introduces support for Java 8 language features
  that you can use when developing apps that target Android N.
  This page 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>To start using these features, you need to download and set up Android
Studio 2.1 and the Android N Preview SDK, which includes the required
Jack toolchain and updated Android Plugin for Gradle. If you haven't yet
installed the Android N Preview SDK, see <a href=
"{@docRoot}preview/setup-sdk.html">Set Up to Develop for Android N</a>.</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 platform. If you
  don't want to write code with Java 8 language features, you can keep your
  project's source and target compatibility values set to Java 7, but you still
  must compile with JDK 8 to build against the Android N platform.
</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
  Android N 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> (also available on API level 23 and lower)
  </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> (also available on API level 23 and lower)
  </li>

  <li>
    <a class="external-link" href=
    "https://docs.oracle.com/javase/tutorial/java/annotations/type_annotations.html">
    Type Annotations</a> (also available on API level 23 and lower)
  </li>
</ul>

<p class="note">
  <strong>Note:</strong> Type annotation information is only available at
  compile time, and not during runtime.
</p>

<p>
  To test lambda expressions, method references, and type annotations on
  earlier versions of Android, go to your {@code build.gradle} file, and set
  {@code compileSdkVersion} and {@code targetSdkVersion} to 23 or lower. You
  will still need to <a href="#configuration">enable the Jack toolchain</a> to
  use these Java 8 features.
</p>

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

<ul>
  <li>Reflection and language-related APIs:
    <ul>
      <li>
        <a class="external-link" href=
        "https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html">
        {@code java.lang.FunctionalInterface}</a>
      </li>

      <li>
        <a class="external-link" href=
        "https://docs.oracle.com/javase/8/docs/api/java/lang/annotation/Repeatable.html">
        {@code java.lang.annotation.Repeatable}</a>
      </li>

      <li>
        <a class="external-link" href=
        "https://docs.oracle.com/javase/8/docs/api/java/lang/reflect/Method.html#isDefault--">
        {@code java.lang.reflect.Method.isDefault()}</a>
      </li>

      <li>and Reflection APIs associated with repeatable annotations, such as
        <a class="external-link" href=
        "https://docs.oracle.com/javase/8/docs/api/java/lang/reflect/AnnotatedElement.html#getAnnotationsByType-java.lang.Class-">
        {@code AnnotatedElement.getAnnotationsByType(Class)}</a>
      </li>
    </ul>
  </li>

  <li>Utility APIs:
    <ul>
      <li>
        <a class="external-link" href=
        "https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html">
        {@code java.util.function}</a>
      </li>

      <li>
        <a class="external-link" href=
        "https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html">
        {@code java.util.stream}</a>
      </li>
    </ul>
  </li>
</ul>

<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 toolchain
  features as part of a single tool: repackaging, shrinking, obfuscation and
  multidex.
</p>

<p>Here is a comparison of the two toolchains used to build Android DEX files:</p>
<ul>
  <li>Legacy javac toolchain:<br>
  <b>javac</b> ({@code .java} --&gt; {@code .class}) --&gt; <b>dx</b> ({@code
  .class} --&gt; {@code .dex})
  </li>

  <li>New Jack toolchain:<br>
  <b>Jack</b> ({@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-level {@code build.gradle} file:
</p>

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

<h3 id="known-issues">
  Known Issues
</h3>

<p>
  <a href="{@docRoot}tools/building/building-studio.html#instant-run">Instant
  Run</a> does not currently work with Jack and will be disabled while using
  the new toolchain.
</p>

<p>Because 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 (such as
  instrumentation tests with JaCoCo)
  </li>
</ul>

<p>If you find other problems while using Jack, <a href=
"http://tools.android.com/filing-bugs">please file a bug</a>.</p>