Android中JNI调用时出现accessed stale local reference的问题

之前在做一个native的模块时遇到这样一个问题：

代码运行在android2.3上没有任何问题，可是在4.2上运行时报出了：JNI ERROR (app bug): accessed stale local reference 的错误。

后来在StackOverflow上找到了问题的答案。简单来说就是  4.0以上的android系统GC在垃圾回收时为了减少内存碎片，会对内存进行整理，整理时必然会移动对象的内存地址，这时C代码的指针还指向原来对象的地址，这时该对象已经被移动到了其他位置，因此会出现问题。

解决办法见原文：

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