1page.title=Introducing ART 2@jd:body 3 4<!-- 5 Copyright 2013 The Android Open Source Project 6 7 Licensed under the Apache License, Version 2.0 (the "License"); 8 you may not use this file except in compliance with the License. 9 You may obtain a copy of the License at 10 11 http://www.apache.org/licenses/LICENSE-2.0 12 13 Unless required by applicable law or agreed to in writing, software 14 distributed under the License is distributed on an "AS IS" BASIS, 15 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 See the License for the specific language governing permissions and 17 limitations under the License. 18--> 19 20 21<div id="qv-wrapper"> 22<div id="qv"> 23 <h2 id="Contents">In this document</h2> 24 <ol id="auto-toc"> 25 </ol> 26 <strong>See also</strong> 27 <ol> 28 <li><a 29 href="http://developer.android.com/guide/practices/verifying-apps-art.html">Verifying 30 App Behavior on the Android Runtime (ART)</a></li> 31 </ol> 32</div> 33</div> 34 35<p> 36ART is a new Android runtime being introduced experimentally in the 4.4 37release. This is a preview of work in progress in KitKat that can be turned on 38in <strong>Settings</strong> > <strong>developer options</strong>. This is 39available for the purpose of obtaining early developer and partner feedback.</p> 40 41<p class="caution"><strong>Important:</strong> Dalvik must remain the default 42runtime or you risk breaking your Android implementations and third-party 43applications.</p> 44 45<p>Most existing apps should just work when running with ART. However, some 46techniques that work on Dalvik do not work on ART. For information about the 47most important issues, see <a 48href="http://developer.android.com/guide/practices/verifying-apps-art.html">Verifying 49App Behavior on the Android Runtime (ART)</a>.</p> 50 51<h2 id="features">ART Features</h2> 52 53<p>Here are some of the major new features implemented by ART.</p> 54 55<h3 id="AOT_compilation">Ahead-of-time (AOT) compilation</h3> 56 57<p>ART introduces ahead-of-time (AOT) compilation, which can improve app 58performance. ART also has tighter install-time verification than Dalvik.</p> 59 60<p>At install time, ART compiles apps using the on-device 61<strong>dex2oat</strong> tool. This utility accepts <a 62href="http://source.android.com/devices/tech/dalvik/dex-format.html">DEX</a> files as input and 63generates a compiled app executable for the target device. The utility should be 64able to compile all valid DEX files without difficulty. However, some 65post-processing tools produce invalid files that may be tolerated by Dalvik but 66cannot be compiled by ART. For more information, see <a 67href="http://developer.android.com/guide/practices/verifying-apps-art.html#GC_Migration">Addressing 68Garbage Collection Issues</a>.</p> 69 70<h3 id="Improved_GC">Improved garbage collection</h3> 71 72<p>Garbage collection (GC) can impair an app's performance, resulting in choppy 73display, poor UI responsiveness, and other problems. ART improves garbage 74collection in several ways:</p> 75 76<ul> 77 <li>One GC pause instead of two</li> 78 <li>Parallelized processing during the remaining GC pause</li> 79 <li>Collector with lower pause time for the special case of cleaning up 80 recently-allocated, short-lived objects</li> 81 <li>Improved garbage collection ergonomics, making concurrent garbage 82 collections more timely, which makes <a 83 href="http://developer.android.com/tools/debugging/debugging-memory.html#LogMessages"><code>GC_FOR_ALLOC</code></a> 84 events extremely rare in typical use cases</li> 85</ul> 86 87<p>ART currently does not use compacting GC, but this feature is under 88development in the <a href="https://source.android.com">Android Open Source 89Project (AOSP)</a>. In the meantime, don't perform operations that are 90incompatible with compacting GC, such as storing pointers to object fields. For 91more information, see <a 92href="http://developer.android.com/guide/practices/verifying-apps-art.html#GC_Migration">Addressing 93Garbage Collection Issues</a>.</p> 94 95<h3 id="Debugging_Imp">Development and debugging improvements</h3> 96 97<p>ART offers a number of features to improve app development and debugging.</p> 98 99<h4 id="Sampling_Profiler">Support for sampling profiler</h4> 100 101<p>Historically, developers have used the <a 102href=" http://developer.android.com/tools/help/traceview.html">Traceview</a> 103tool (designed for tracing 104application execution) as a profiler. While Traceview gives useful information, 105its results on Dalvik have been skewed by the per-method-call overhead, and use 106of the tool noticeably affects run time performance.</p> 107 108<p>ART adds support for a dedicated sampling profiler that does not have these 109limitations. This gives a more accurate view of app execution without 110significant slowdown. Sampling support has also been added to Traceview for 111Dalvik.</p> 112 113<h4 id="Debugging_Features">Support for more debugging features</h4> 114 115<p>ART supports a number of new debugging options, particularly in monitor- and 116garbage collection-related functionality. For example, you can:</p> 117 118<ul> 119 <li>See what locks are held in stack traces, then jump to the thread that 120 holds a lock.</li> 121 <li>Ask how many live instances there are of a given class, ask to see the 122 instances, and see what references are keeping an object live.</li> 123 <li>Filter events (like breakpoint) for a specific instance.</li> 124 <li>See the value returned by a method when it exits (using “method-exit” 125 events).</li> 126 <li>Set field watchpoint to suspend the execution of a program when a specific 127 field is accessed and/or modified.</li> 128</ul> 129 130<h4 id="Crash_Reports">Improved diagnostic detail in exceptions and crash reports</h4> 131 132<p>ART gives you as much context and detail as possible when runtime exceptions 133occur. ART provides expanded exception detail for <code><a 134href="http://developer.android.com/reference/java/lang/ClassCastException.html">java.lang.ClassCastException</a></code>, 135<code><a 136href="http://developer.android.com/reference/java/lang/ClassNotFoundException.html">java.lang.ClassNotFoundException</a></code>, 137and <code><a 138href="http://developer.android.com/reference/java/lang/NullPointerException.html">java.lang.NullPointerException</a></code>. 139(Later versions of Dalvik provided expanded exception detail for <code><a 140href="http://developer.android.com/reference/java/lang/ArrayIndexOutOfBoundsException.html">java.lang.ArrayIndexOutOfBoundsException</a></code> 141and <code><a 142href="http://developer.android.com/reference/java/lang/ArrayStoreException.html">java.lang.ArrayStoreException</a></code>, 143which now include the size of the array and the out-of-bounds offset, and ART 144does this as well.)</p> 145 146<p>For example, <code><a 147href="http://developer.android.com/reference/java/lang/NullPointerException.html">java.lang.NullPointerException</a></code> 148now shows information about what the app was trying to do with the null pointer, 149such as the field the app was trying to write to, or the method it was trying to 150call. Here are some typical examples:</p> 151 152<pre class="no-pretty-print"> 153java.lang.NullPointerException: Attempt to write to field 'int 154android.accessibilityservice.AccessibilityServiceInfo.flags' on a null object 155reference</pre> 156 157<pre class="no-pretty-print"> 158java.lang.NullPointerException: Attempt to invoke virtual method 159'java.lang.String java.lang.Object.toString()' on a null object reference</pre> 160 161<p>ART also provides improved context information in app native crash reports, 162by including both Java and native stack information. </p> 163 164<h2 id="Known_Issues">Known Issues</h2> 165 166<p>The following known issues are present in the 4.4.1 implementation of ART.</p> 167 168<ul> 169 170 <li><em>Compile-time issue:</em> As noted above, ART flags unbalanced 171 <code>monitorenter</code>/<code>moniterexit</code> instructions. We relaxed 172 this check in 4.4.1 but intend to restore this verification in the future once 173 tools are fixed, as this check is necessary for certain compiler 174 optimizations. <a 175 href="https://code.google.com/p/android/issues/detail?id=61916">https://code.google.com/p/android/issues/detail?id=61916</a></li> 176 177 <li><em>Run-time issue:</em> There was an issue where JNI 178 <code>GetFieldID</code> and <code>GetStaticFieldID</code> were using the wrong 179 class loader on unattached threads, often leading to later CheckJNI errors or 180 NoSuchFieldError exceptions. <a 181 href="http://code.google.com/p/android/issues/detail?id=63970">http://code.google.com/p/android/issues/detail?id=63970</a></li> 182 183 <li><em>Run-time issue:</em> Calling JNI <code>NewDirectByteBuffer()</code> 184 with byte size of <code>0</code> led to the following CheckJNI error: <pre 185 class="no-pretty-print"> JNI DETECTED ERROR IN APPLICATION: capacity must be 186 greater than 0: 0</pre> <a 187 href="http://code.google.com/p/android/issues/detail?id=63055">http://code.google.com/p/android/issues/detail?id=63055</a></li> 188 189</ul> 190 191<h3 id="Fixed_Issues">Fixed issues</h3> 192 193<ul> 194 195 <li><em>Compile-time issue:</em> Overly aggressive verification and 196 compilation of unused portions of dex files lead to corrupt package messages. 197 This was addressed in AOSP with: <a 198 href="https://android-review.googlesource.com/#/c/72374/">https://android-review.googlesource.com/#/c/72374/</a></li> 199 200 <li><em>Debug-time issue:</em> Interactive debugging performance was slow, 201 even in code without breakpoints. This has been addressed in the latest AOSP 202 code.</li> 203 204</ul> 205 206<h2 id="building">Enabling ART in Android Build</h2> 207 208<p> Two runtimes are now available, the existing Dalvik runtime 209(<code>libdvm.so</code>) and the ART runtime (<code>libart.so</code>). A device 210can be built using either or both runtimes. (You can dual boot from 211<strong>Developer options</strong> if both runtimes are installed.) See 212runtime_common.mk. That is included from build/target/product/runtime_libdvm.mk 213or build/target/product/runtime_libdvm.mk or both.</p> 214 215<p> The <code>dalvikvm</code> command line tool can run with either runtime now. 216It will default to using the runtime specified in <strong>developer 217options</strong>. The default can be overridden by specifying the desired 218runtime library, for example with <code>-XXlib:libart.so</code> </p> 219 220<p> 221A new <code>PRODUCT_RUNTIMES</code> variable controls which runtimes 222are included in a build. Include it within either 223<code>build/target/product/core_minimal.mk</code> or 224<code>build/target/product/core_base.mk</code>. 225</p> 226 227<p> 228Add this to the device makefile to have both runtimes 229built and installed, with Dalvik as the default: 230</br> 231<code>PRODUCT_RUNTIMES := runtime_libdvm_default</code> 232</br> 233<code>PRODUCT_RUNTIMES += runtime_libart</code> 234</p> 235 236<h2 id="Reporting_Problems">Reporting Problems</h2> 237 238<p>If you run into any issues that aren’t due to app JNI issues, please report 239them via the Android Open Source Project Issue Tracker at <a 240href="https://code.google.com/p/android/issues/list">https://code.google.com/p/android/issues/list</a>. 241Please include an <code>"adb bugreport"</code> and link to the app in Google 242Play store if available. Otherwise, if possible, attach an APK that reproduces 243the issue. Please note that issues (including attachments) are publicly 244visible.</p> 245