最近，在APP測試過程中發現一個crash，檢視log發現如下錯誤：

07-30 14:15:24.053: E/dalvikvm(23567): JNI ERROR (app bug): accessed stalelocal reference 0x2bf00005 (index 1 in a table of size 0)

07-30 14:15:24.053: E/dalvikvm(23567): VM aborting

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:

<pre name="code" class="java">

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:

<pre name="code" class="java">

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;  
}