Android中JNI調用時出現accessed stale local reference的問題

        以前在作一個native的模塊時遇到這樣一個問題：

        代碼運行在android2.3上沒有任何問題，但是在4.2上運行時報出了：JNI ERROR (app bug): accessed stale local reference 的錯誤。

        後來在StackOverflow上找到了問題的答案。簡單來講就是  4.0以上的android系統GC在垃圾回收時爲了減小內存碎片，會對內存進行整理，整理時必然會移動對象的內存地址，這時C代碼的指針還指向原來對象的地址，這時該對象已經被移動到了其餘位置，所以會出現問題。

解決辦法見原文：

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