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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="/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 through the <a href="/setup/report-bugs#platform">Android Open Source
 Project Issue Tracker</a>. 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.</p>

 <aside class="caution"><strong>Reminder</strong>: Issues (including attachments)
 are publicly visible.</aside>

   </body>
 </html>
	<html devsite>
	<head>
	<title>ART and Dalvik</title>
	<meta name="project_path" value="/_project.yaml" />
	<meta name="book_path" value="/_book.yaml" />
	</head>
	<body>
	<!--
	Copyright 2017 The Android Open Source Project

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->



	<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="/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 through the <a href="/setup/report-bugs#platform">Android Open Source
	Project Issue Tracker</a>. 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.</p>

	<aside class="caution"><strong>Reminder</strong>: Issues (including attachments)
	are publicly visible.</aside>

	</body>
	</html>