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android – JNI ERROR (app bug): accessed stale local reference 0xbc00021 (index 8 in a table of size 8)

Posted by: admin May 14, 2020 Leave a comment

Questions:

I made hello world application from book Android apps for Absolute Beginners and Temperature Convertor app from here
Both is running fine on Emulator but when I try to run it on Samsung Note 2 following error is coming on LogCat

02-08 07:22:18.665: E/dalvikvm(30944): JNI ERROR (app bug): accessed stale local reference 0xbc00021 (index 8 in a table of size 8)
02-08 07:22:18.665: E/dalvikvm(30944): VM aborting
02-08 07:22:18.665: A/libc(30944): Fatal signal 11 (SIGSEGV) at 0xdeadd00d (code=1), thread 30944 (oid.temperature)

Both applications do open shows layout with title but do not shows any other views in layout

Samples runs fine

device: note 2 Samsung-gt_n7100

IDE:Eclipse version 3.8

OS: 64bit Windows 7

How to&Answers:

Since android 4.0 garbage collector was changed. Now it moves object around during garbage collection, which can cause a lot of problems.

Imagine that you have a static variable pointing to an object, and then this object gets moved by gc. Since android uses direct pointers for java objects, this would mean that your static variable is now pointing to a random address in the memory, unoccupied by any object or occupied by an object of different sort. This will almost guarantee that you’ll get EXC_BAD_ACCESS next time you use this variable.

So android gives you JNI ERROR (app bug) error to prevent you from getting undebugable EXC_BAD_ACCESS. Now there are two ways to avoid this error.

  1. You can set targetSdkVersion in your manifest to version 11 or less. This will enable JNI bug compatibility mode and prevent any problems altogether. This is the reason why your old examples are working.

  2. You can avoid using static variables pointing to java objects or make jobject references global before storing them by calling env->NewGlobalRef(ref).
    Perhaps on of the biggest examples here is keeping jclass objects. Normally, you’ll initialize static jclass variable during JNI_OnLoad, since class objects remain in the memory as long as the application is running.

This code will lead to a crash:

static jclass myClass;

JNIEXPORT jint JNICALL JNI_OnLoad (JavaVM * vm, void * reserved) {  
    myClass = env->FindClass("com/example/company/MyClass");  
    return JNI_VERSION_1_6;  
}

While this code will run fine:

static jclass myClass;

JNIEXPORT jint JNICALL JNI_OnLoad (JavaVM * vm, void * reserved) {  
    jclass tmp = env->FindClass("com/example/company/MyClass");  
    myClass = (jclass)env->NewGlobalRef(tmp);
    return JNI_VERSION_1_6;  
}

For more examples see link provided by Marek Sebera: http://android-developers.blogspot.cz/2011/11/jni-local-reference-changes-in-ics.html