androi-Verifying App Behavior on the Android Runtime (ART)

With Android 4.4, we are beginning to roll out a new Android runtime, ART. This runtime offers a number of new features that improve performance and smoothness of the Android platform and apps.

At this time, all devices still use Dalvik as the default runtime. We encourage you to test your apps for ART compatibility and to take advantage of ART's new features. However, for the time being, you should also take care to maintain compatibility with Dalvik.

 Most apps should just work when running with ART. However, some techniques that work on Dalvik do not work on ART. 

>Under Dalvik, apps frequently find it useful to explicitly callSystem.gc() to prompt garbage collection (GC). This should be far less necessary with ART, particularly if you're invoking garbage collection to prevent GC_FOR_ALLOC-type occurrences or to reduce fragmentation. You can verify which runtime is in use by callingSystem.getProperty("java.vm.version"). If ART is in use, the property's value is "2.0.0" or higher.

Furthermore, a compacting garbage collector is under development in the Android Open-Source Project (AOSP) to improve memory management. Because of this, you should avoid using techniques that are incompatible with compacting GC (such as saving pointers to object instance data). This is particularly important for apps that make use of the Java Native Interface (JNI).

ART's JNI is somewhat stricter than Dalvik's. It is an especially good idea to use CheckJNI mode to catch common problems. If your app makes use of C/C++ code, you should review the following article:

Debugging Android JNI with CheckJNI

ART has a compacting garbage collector under development on the Android Open Source Project (AOSP). Once the compacting garbage collector is in use, objects may be moved in memory. If you use C/C++ code, do not perform operations that are incompatible with compacting GC. We have enhanced CheckJNI to identify some potential issues (as described in  JNI Local Reference Changes in ICS ).

One area to watch for in particular is the use of Get...ArrayElements() and Release...ArrayElements()functions. 

ART's JNI throws errors in a number of cases where Dalvik didn’t. (Once again, you can catch many such cases by testing with CheckJNI.)

>In addition, the JNI functions GetFieldID() and GetStaticFieldID() now properly throw NoSuchFieldErrorinstead of simply returning null. Similarly, GetMethodID() and GetStaticMethodID() now properly throwNoSuchMethodError. This can lead to CheckJNI failures because of the unhandled exceptions or the exceptions being thrown to Java callers of native code. This makes it particularly important to test ART-compatible apps with CheckJNI mode.

>

Dalvik had separate stacks for native and Java code, with a default Java stack size of 32KB and a default native stack size of 1MB. ART has a unified stack for better locality. Ordinarily, the ART Thread stack size should be approximately the same as for Dalvik. However, if you explicitly set stack sizes, you may need to revisit those values for apps running in ART.

• In Java, review calls to the Thread constructor that specify an explicit stack size. For example, you will need to increase the size if StackOverflowError occurs.
• In C/C++, review use of pthread_attr_setstack() and pthread_attr_setstacksize() for threads that also run Java code via JNI. Here is an example of the error logged when an app attempts to call JNIAttachCurrentThread() when the pthread size is too small:

  F/art: art/runtime/thread.cc:435]
      Attempt to attach a thread with a too-small stack (16384 bytes)

• Fixing AOT Compilation Issues

  ---

  ART's Ahead-Of-Time (AOT) Java compilation should work for all standard Java code. Compilation is performed by ART's dex2oat tool; if you encounter any issues related to dex2oat at install time, let us know (seeReporting Problems) so we can fix them as quickly as possible. A couple of issues to note:

  • ART does tighter bytecode verification at install time than Dalvik does. Code produced by the Android build tools should be fine. However, some post-processing tools (especially tools that perform obfuscation) may produce invalid files that are tolerated by Dalvik but rejected by ART. We have been working with tool vendors to find and fix such issues. In many cases, getting the latest versions of your tools and regenerating the DEX files can fix these problems.
  • Some typical problems that are flagged by the ART verifier include:
    • invalid control flow
    • unbalanced moniterenter/moniterexit
    • 0-length parameter type list size
  • Some apps have dependencies on the installed .odex file format in /system/framework, /data/dalvik-cache, or in DexClassLoader’s optimized output directory. These files are now ELF files and not an extended form of DEX files. While ART tries to follow the same naming and locking rules as Dalvik, apps should not depend on the file format; the format is subject to change without notice.

  Reporting Problems

  ---

  If you run into any issues that aren’t due to app JNI issues, report them via the Android Open Source Project Issue Tracker at https://code.google.com/p/android/issues/list. Include an "adb bugreport" and a link to the app in the Google Play store if available. Otherwise, if possible, attach an APK that reproduces the issue. Note that issues (including attachments) are publicly visible.