保存JNIEnv*的最佳方法是什么？

Question

我有一个JNI的Android项目.在实现侦听器类的CPP文件中,有一个回调x().当调用x()函数时,我想调用java类中的另一个函数.但是,为了调用那个java函数,我需要访问JNIEnv*.

我知道在回调的同一个cpp文件中,有一个函数:

static jboolean init (JNIEnv* env, jobject obj) {...}

我应该在cpp文件中保存JNIEnv*作为成员变量init(..)吗？并在以后回调发生时使用它？

对不起,我是JNI的初学者.

Answer 1

缓存a JNIEnv*并不是一个特别好的主意,因为你不能JNIEnv*在多个线程中使用它,甚至可能无法在同一个线程上将它用于多个本机调用(请参阅http://android-developers.blogspot. se/2011/11/jni-local-reference-changes-in-ics.html)

编写一个函数来获取JNIEnv*并在必要时将当前线程附加到VM并不太困难:

bool GetJniEnv(JavaVM *vm, JNIEnv **env) {
    bool did_attach_thread = false;
    *env = nullptr;
    // Check if the current thread is attached to the VM
    auto get_env_result = vm->GetEnv((void**)env, JNI_VERSION_1_6);
    if (get_env_result == JNI_EDETACHED) {
        if (vm->AttachCurrentThread(env, NULL) == JNI_OK) {
            did_attach_thread = true;
        } else {
            // Failed to attach thread. Throw an exception if you want to.
        }
    } else if (get_env_result == JNI_EVERSION) {
        // Unsupported JNI version. Throw an exception if you want to.
    }
    return did_attach_thread;
}

你使用它的方式是:

JNIEnv *env;
bool did_attach = GetJniEnv(vm, &env);
// Use env...
// ...
if (did_attach) {
   vm->DetachCurrentThread();
}

你可以将它包装在一个附着在构造上的类中,并在破坏时分离,RAII风格:

class ScopedEnv {
public:
    ScopedEnv() : attached_to_vm_(false) {
        attached_to_vm_ = GetJniEnv(g_vm, &env_);  // g_vm is a global
    }

    ScopedEnv(const ScopedEnv&) = delete;
    ScopedEnv& operator=(const ScopedEnv&) = delete;

    virtual ~ScopedEnv() {
        if (attached_to_vm_) {
            g_vm->DetachCurrentThread();
            attached_to_vm_ = false;
        }
    }

    JNIEnv *GetEnv() const { return env_; }

private:
    bool attached_to_env_;
    JNIEnv *env_;
};

// Usage:

{
    ScopedEnv scoped_env;
    scoped_env.GetEnv()->SomeJniFunction();
}
// scoped_env falls out of scope, the thread is automatically detached if necessary

编辑:有时你可能有一个长期运行的本机线程,需要JNIEnv*多次.在这种情况下,您可能希望避免不断地将线程连接到JVM或从JVM断开线程,但是仍然需要确保在线程销毁时分离线程.

您可以通过连接线只有一次,然后离开它连接,并通过设置使用线程销毁回调做到这一点pthread_key_create,并pthread_setspecific说将采取调用的照顾DetachCurrentThread.

/**
 * Get a JNIEnv* valid for this thread, regardless of whether
 * we're on a native thread or a Java thread.
 * If the calling thread is not currently attached to the JVM
 * it will be attached, and then automatically detached when the
 * thread is destroyed.
 */   
JNIEnv *GetJniEnv() {
    JNIEnv *env = nullptr;
    // We still call GetEnv first to detect if the thread already
    // is attached. This is done to avoid setting up a DetachCurrentThread
    // call on a Java thread.

    // g_vm is a global.
    auto get_env_result = g_vm->GetEnv((void**)&env, JNI_VERSION_1_6);
    if (get_env_result == JNI_EDETACHED) {
        if (g_vm->AttachCurrentThread(&env, NULL) == JNI_OK) {
            DeferThreadDetach(env);
        } else {
            // Failed to attach thread. Throw an exception if you want to.
        }
    } else if (get_env_result == JNI_EVERSION) {
        // Unsupported JNI version. Throw an exception if you want to.
    }
    return env;
}

void DeferThreadDetach(JNIEnv *env) {
    static pthread_key_t thread_key;

    // Set up a Thread Specific Data key, and a callback that
    // will be executed when a thread is destroyed.
    // This is only done once, across all threads, and the value
    // associated with the key for any given thread will initially
    // be NULL.
    static auto run_once = [] {
        const auto err = pthread_key_create(&thread_key, [] (void *ts_env) {
            if (ts_env) {
                g_vm->DetachCurrentThread();
            }
        });
        if (err) {
            // Failed to create TSD key. Throw an exception if you want to.
        }
        return 0;
    }();

    // For the callback to actually be executed when a thread exits
    // we need to associate a non-NULL value with the key on that thread.
    // We can use the JNIEnv* as that value.
    const auto ts_env = pthread_getspecific(thread_key);
    if (!ts_env) {
        if (pthread_setspecific(thread_key, env)) {
            // Failed to set thread-specific value for key. Throw an exception if you want to.
        }
    }
}

如果__cxa_thread_atexit您可以使用,则可以使用在析构函数thread_local中调用的某个对象来完成相同的操作DetachCurrentThread.