1 <html devsite> 2 <head> 3 <title>ART and Dalvik</title> 4 <meta name="project_path" value="/_project.yaml" /> 5 <meta name="book_path" value="/_book.yaml" /> 6 </head> 7 <body> 8 <!-- 9 Copyright 2017 The Android Open Source Project 10 11 Licensed under the Apache License, Version 2.0 (the "License"); 12 you may not use this file except in compliance with the License. 13 You may obtain a copy of the License at 14 15 http://www.apache.org/licenses/LICENSE-2.0 16 17 Unless required by applicable law or agreed to in writing, software 18 distributed under the License is distributed on an "AS IS" BASIS, 19 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 20 See the License for the specific language governing permissions and 21 limitations under the License. 22 --> 23 24 25 26 <p>Android runtime (ART) is the managed runtime used by applications and some system 27 services on Android. ART and its predecessor Dalvik were originally created 28 specifically for the Android project. ART as the runtime executes the Dalvik 29 Executable format and Dex bytecode specification.</p> 30 31 <p>ART and Dalvik are compatible runtimes running Dex bytecode, so apps 32 developed for Dalvik should work when running with ART. However, some 33 techniques that work on Dalvik do not work on ART. For information about the 34 most important issues, see <a 35 href="http://developer.android.com/guide/practices/verifying-apps-art.html">Verifying 36 App Behavior on the Android Runtime (ART)</a>.</p> 37 38 <h2 id="features">ART Features</h2> 39 40 <p>Here are some of the major features implemented by ART.</p> 41 42 <h3 id="AOT_compilation">Ahead-of-time (AOT) compilation</h3> 43 44 <p>ART introduces ahead-of-time (AOT) compilation, which can improve app 45 performance. ART also has tighter install-time verification than Dalvik.</p> 46 47 <p>At install time, ART compiles apps using the on-device 48 <strong>dex2oat</strong> tool. This utility accepts <a 49 href="http://source.android.com/devices/tech/dalvik/dex-format.html">DEX</a> files as input and 50 generates a compiled app executable for the target device. The utility should be 51 able to compile all valid DEX files without difficulty. However, some 52 post-processing tools produce invalid files that may be tolerated by Dalvik but 53 cannot be compiled by ART. For more information, see <a 54 href="http://developer.android.com/guide/practices/verifying-apps-art.html#GC_Migration">Addressing 55 Garbage Collection Issues</a>.</p> 56 57 <h3 id="Improved_GC">Improved garbage collection</h3> 58 59 <p>Garbage collection (GC) can impair an app's performance, resulting in choppy 60 display, poor UI responsiveness, and other problems. ART improves garbage 61 collection in several ways:</p> 62 63 <ul> 64 <li>One GC pause instead of two</li> 65 <li>Parallelized processing during the remaining GC pause</li> 66 <li>Collector with lower total GC time for the special case of cleaning up 67 recently-allocated, short-lived objects</li> 68 <li>Improved garbage collection ergonomics, making concurrent garbage 69 collections more timely, which makes <a 70 href="http://developer.android.com/tools/debugging/debugging-memory.html#LogMessages"><code>GC_FOR_ALLOC</code></a> 71 events extremely rare in typical use cases</li> 72 <li>Compacting GC to reduce background memory usage and fragmentation</li> 73 </ul> 74 75 <h3 id="Debugging_Imp">Development and debugging improvements</h3> 76 77 <p>ART offers a number of features to improve app development and debugging.</p> 78 79 <h4 id="Sampling_Profiler">Support for sampling profiler</h4> 80 81 <p>Historically, developers have used the <a 82 href=" http://developer.android.com/tools/help/traceview.html">Traceview</a> 83 tool (designed for tracing 84 application execution) as a profiler. While Traceview gives useful information, 85 its results on Dalvik have been skewed by the per-method-call overhead, and use 86 of the tool noticeably affects run time performance.</p> 87 88 <p>ART adds support for a dedicated sampling profiler that does not have these 89 limitations. This gives a more accurate view of app execution without 90 significant slowdown. Sampling support was added to Traceview for 91 Dalvik in the KitKat release.</p> 92 93 <h4 id="Debugging_Features">Support for more debugging features</h4> 94 95 <p>ART supports a number of new debugging options, particularly in monitor- and 96 garbage collection-related functionality. For example, you can:</p> 97 98 <ul> 99 <li>See what locks are held in stack traces, then jump to the thread that 100 holds a lock.</li> 101 <li>Ask how many live instances there are of a given class, ask to see the 102 instances, and see what references are keeping an object live.</li> 103 <li>Filter events (like breakpoint) for a specific instance.</li> 104 <li>See the value returned by a method when it exits (using method-exit 105 events).</li> 106 <li>Set field watchpoint to suspend the execution of a program when a specific 107 field is accessed and/or modified.</li> 108 </ul> 109 110 <h4 id="Crash_Reports">Improved diagnostic detail in exceptions and crash reports</h4> 111 112 <p>ART gives you as much context and detail as possible when runtime exceptions 113 occur. ART provides expanded exception detail for <code><a 114 href="http://developer.android.com/reference/java/lang/ClassCastException.html">java.lang.ClassCastException</a></code>, 115 <code><a 116 href="http://developer.android.com/reference/java/lang/ClassNotFoundException.html">java.lang.ClassNotFoundException</a></code>, 117 and <code><a 118 href="http://developer.android.com/reference/java/lang/NullPointerException.html">java.lang.NullPointerException</a></code>. 119 (Later versions of Dalvik provided expanded exception detail for <code><a 120 href="http://developer.android.com/reference/java/lang/ArrayIndexOutOfBoundsException.html">java.lang.ArrayIndexOutOfBoundsException</a></code> 121 and <code><a 122 href="http://developer.android.com/reference/java/lang/ArrayStoreException.html">java.lang.ArrayStoreException</a></code>, 123 which now include the size of the array and the out-of-bounds offset, and ART 124 does this as well.)</p> 125 126 <p>For example, <code><a 127 href="http://developer.android.com/reference/java/lang/NullPointerException.html">java.lang.NullPointerException</a></code> 128 now shows information about what the app was trying to do with the null pointer, 129 such as the field the app was trying to write to, or the method it was trying to 130 call. Here are some typical examples:</p> 131 132 <pre class="no-pretty-print"> 133 java.lang.NullPointerException: Attempt to write to field 'int 134 android.accessibilityservice.AccessibilityServiceInfo.flags' on a null object 135 reference</pre> 136 137 <pre class="no-pretty-print"> 138 java.lang.NullPointerException: Attempt to invoke virtual method 139 'java.lang.String java.lang.Object.toString()' on a null object reference</pre> 140 141 <p>ART also provides improved context information in app native crash reports, 142 by including both Java and native stack information. </p> 143 144 <h2 id="Reporting_Problems">Reporting Problems</h2> 145 146 <p>If you run into any issues that arent due to app JNI issues, please report 147 them via the Android Open Source Project Issue Tracker at <a 148 href="http://b.android.com">http://b.android.com</a>. 149 Please include an <code>"adb bugreport"</code> and link to the app in Google 150 Play store if available. Otherwise, if possible, attach an APK that reproduces 151 the issue. Please note that issues (including attachments) are publicly 152 visible.</p> 153 154 </body> 155 </html> 156
