en/devices/tech/dalvik/index.html

page.title=ART and Dalvik
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  <h2 id="Contents">In this document</h2>
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<p>Android runtime (ART) is the managed runtime used by applications and some system
services on Android. ART and its predecessor Dalvik were originally created
specifically for the Android project. ART as the runtime executes the Dalvik
Executable format and Dex bytecode specification.</p>

<p>ART and Dalvik are compatible runtimes running Dex bytecode, so apps
developed for Dalvik should work when running with ART. However, some
techniques that work on Dalvik do not work on ART. For information about the
most important issues, see <a
href="http://developer.android.com/guide/practices/verifying-apps-art.html">Verifying
App Behavior on the Android Runtime (ART)</a>.</p>

<h2 id="features">ART Features</h2>

<p>Here are some of the major features implemented by ART.</p>

<h3 id="AOT_compilation">Ahead-of-time (AOT) compilation</h3>

<p>ART introduces ahead-of-time (AOT) compilation, which can improve app
performance. ART also has tighter install-time verification than Dalvik.</p>

<p>At install time, ART compiles apps using the on-device
<strong>dex2oat</strong> tool. This utility accepts <a
href="http://source.android.com/devices/tech/dalvik/dex-format.html">DEX</a> files as input and
generates a compiled app executable for the target device. The utility should be
able to compile all valid DEX files without difficulty. However, some
post-processing tools produce invalid files that may be tolerated by Dalvik but
cannot be compiled by ART. For more information, see  <a
href="http://developer.android.com/guide/practices/verifying-apps-art.html#GC_Migration">Addressing
Garbage Collection Issues</a>.</p>

<h3 id="Improved_GC">Improved garbage collection</h3>

<p>Garbage collection (GC) can impair an app's performance, resulting in choppy
display, poor UI responsiveness, and other problems. ART improves garbage
collection in several ways:</p>

<ul>
  <li>One GC pause instead of two</li>
  <li>Parallelized processing during the remaining GC pause</li>
  <li>Collector with lower total GC time for the special case of cleaning up
  recently-allocated, short-lived objects</li>
  <li>Improved garbage collection ergonomics, making concurrent garbage
  collections more timely, which makes <a
  href="http://developer.android.com/tools/debugging/debugging-memory.html#LogMessages"><code>GC_FOR_ALLOC</code></a>
  events extremely rare in typical use cases</li>
  <li>Compacting GC to reduce background memory usage and fragmentation</li>
</ul>

<h3 id="Debugging_Imp">Development and debugging improvements</h3>

<p>ART offers a number of features to improve app development and debugging.</p>

<h4 id="Sampling_Profiler">Support for sampling profiler</h4>

<p>Historically, developers have used the <a
href=" http://developer.android.com/tools/help/traceview.html">Traceview</a>
tool (designed for tracing
application execution) as a profiler. While Traceview gives useful information,
its results on Dalvik have been skewed by the per-method-call overhead, and use
of the tool noticeably affects run time performance.</p>

<p>ART adds support for a dedicated sampling profiler that does not have these
limitations. This gives a more accurate view of app execution without
significant slowdown. Sampling support was added to Traceview for
Dalvik in the KitKat release.</p>

<h4 id="Debugging_Features">Support for more debugging features</h4>

<p>ART supports a number of new debugging options, particularly in monitor- and
garbage collection-related functionality. For example, you can:</p>

<ul>
  <li>See what locks are held in stack traces, then jump to the thread that
      holds a lock.</li>
  <li>Ask how many live instances there are of a given class, ask to see the
      instances, and see what references are keeping an object live.</li>
  <li>Filter events (like breakpoint) for a specific instance.</li>
  <li>See the value returned by a method when it exits (using “method-exit”
      events).</li>
  <li>Set field watchpoint to suspend the execution of a program when a specific
      field is accessed and/or modified.</li>
</ul>

<h4 id="Crash_Reports">Improved diagnostic detail in exceptions and crash reports</h4>

<p>ART gives you as much context and detail as possible when runtime exceptions
occur.  ART provides expanded exception detail for <code><a
href="http://developer.android.com/reference/java/lang/ClassCastException.html">java.lang.ClassCastException</a></code>,
<code><a
href="http://developer.android.com/reference/java/lang/ClassNotFoundException.html">java.lang.ClassNotFoundException</a></code>,
and <code><a
href="http://developer.android.com/reference/java/lang/NullPointerException.html">java.lang.NullPointerException</a></code>.
(Later versions of Dalvik provided expanded exception detail for <code><a
href="http://developer.android.com/reference/java/lang/ArrayIndexOutOfBoundsException.html">java.lang.ArrayIndexOutOfBoundsException</a></code>
and <code><a
href="http://developer.android.com/reference/java/lang/ArrayStoreException.html">java.lang.ArrayStoreException</a></code>,
which now include the size of the array and the out-of-bounds offset, and ART
does this as well.)</p>

<p>For example, <code><a
href="http://developer.android.com/reference/java/lang/NullPointerException.html">java.lang.NullPointerException</a></code>
now shows information about what the app was trying to do with the null pointer,
such as the field the app was trying to write to, or the method it was trying to
call. Here are some typical examples:</p>

<pre class="no-pretty-print">
java.lang.NullPointerException: Attempt to write to field 'int
android.accessibilityservice.AccessibilityServiceInfo.flags' on a null object
reference</pre>

<pre class="no-pretty-print">
java.lang.NullPointerException: Attempt to invoke virtual method
'java.lang.String java.lang.Object.toString()' on a null object reference</pre>

<p>ART also provides improved context information in app native crash reports,
by including both Java and native stack information. </p>

<h2 id="Reporting_Problems">Reporting Problems</h2>

<p>If you run into any issues that aren’t due to app JNI issues, please report
them via the Android Open Source Project Issue Tracker at <a
href="http://b.android.com">http://b.android.com</a>.
Please include an <code>"adb bugreport"</code> and link to the app in Google
Play store if available. Otherwise, if possible, attach an APK that reproduces
the issue. Please note that issues (including attachments) are publicly
visible.</p>