JavaVM & JNIEnv

JNIEnv提供了大多数的JNI函数。你的本地方法都会接收JNIEnv作为第一个参数。
JNIEnv用于本地线程存储。因此，你不能在线程间共享同一个JNIEnv。
如果一个代码段没有其他方式获取它自身线程的JNIEnv，你可以共享JavaVM，用GetEnv来获取线程的JNIEnv。(假设这个线程有一个JavaVM;参见下面的AttachCurrentThread。)

static JavaVM *myVm;

/*
 * This is called by the VM when the shared library is first loaded.
 */
extern "C" jint JNI_OnLoad(JavaVM* vm, void* reserved) {
    myVm = vm;

    jint result = -1;
    JNIEnv* env = NULL;
    LOGI("JNI_OnLoad");
    if (vm->GetEnv((void **)&env, JNI_VERSION_1_4) != JNI_OK) {
        LOGE("ERROR: GetEnv failed");
        goto bail;
    }

//    initClassHelper(env, "com/example/camera/AnObject", &gCameraViewObject);

    if (registerNatives(env) != JNI_TRUE) {
        LOGE("ERROR: registerNatives failed");
        goto bail;
    }

    result = JNI_VERSION_1_4;
bail:
    return result;
}

void JNI_OnUnload(JavaVM* vm, void* reserved)
{
//    JNIEnv* env = NULL;
    LOGI("JNI_OnUnload");
//    if (vm->GetEnv((void **)&env, JNI_VERSION_1_4) != JNI_OK) {
//        LOGE("ERROR: GetEnv failed");
//        return;
//    }
//
//    if (gCameraViewObject != NULL) {
//        env->DeleteGlobalRef(gCameraViewObject);
//        gCameraViewObject = NULL;
//    }
}

JNIEnv* Android_JNI_GetEnv(void)
{
    /* From http://developer.android.com/guide/practices/jni.html
     * All threads are Linux threads, scheduled by the kernel.
     * They're usually started from managed code (using Thread.start), but they can also be created elsewhere and then
     * attached to the JavaVM. For example, a thread started with pthread_create can be attached with the
     * JNI AttachCurrentThread or AttachCurrentThreadAsDaemon functions. Until a thread is attached, it has no JNIEnv,
     * and cannot make JNI calls.
     * Attaching a natively-created thread causes a java.lang.Thread object to be constructed and added to the "main"
     * ThreadGroup, making it visible to the debugger. Calling AttachCurrentThread on an already-attached thread
     * is a no-op.
     * Note: You can call this function any number of times for the same thread, there's no harm in it
     */

    JNIEnv *env;
    int status = (*myVm)->AttachCurrentThread(myVm, &env, NULL);
    if(status < 0) {
        LOGE("failed to attach current thread");
        return 0;
    }

    /* From http://developer.android.com/guide/practices/jni.html
     * Threads attached through JNI must call DetachCurrentThread before they exit. If coding this directly is awkward,
     * in Android 2.0 (Eclair) and higher you can use pthread_key_create to define a destructor function that will be
     * called before the thread exits, and call DetachCurrentThread from there. (Use that key with pthread_setspecific
     * to store the JNIEnv in thread-local-storage; that way it'll be passed into your destructor as the argument.)
     * Note: The destructor is not called unless the stored value is != NULL
     * Note: You can call this function any number of times for the same thread, there's no harm in it
     *       (except for some lost CPU cycles)
     */
    pthread_setspecific(mThreadKey, (void*) env);

    return env;
}