Question

我有一个使用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) {...}

在调用init(..)时，我应该将cpp文件JNIEnv *保存为成员变量吗？并在以后发生回调时使用它？

很抱歉，我是JNI的初学者。

Answer 1

缓存JNIEnv*并不是一个特别好的主意，因为你不能在多个线程中使用相同的JNIEnv*，甚至可能无法在同一个线程上使用它进行多个本机调用线程（见http://android-developers.blogspot.se/2011/11/jni-local-reference-changes-in-ics.html）

编写一个获取JNIEnv*的函数并在必要时将当前线程附加到VM上并不太困难：

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：有时您可能有一个长期运行的本机线程，需要多次JNIEnv*。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，但是您仍然需要确保在线程销毁时分离线程。

您可以通过仅将线程附加一次然后保持连接来完成此操作，并使用pthread_key_create和pthread_setspecific设置线程销毁回调来处理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可用，您可以使用在析构函数中调用thread_local的{{1}}对象完成相同的操作。

Answer 2

@Michael，很好地概述了如何通过缓存JVM来最好地检索JNI。对于那些不想使用pthread（或者因为您在Windows系统上而不能）的人，并且您使用的是c ++ 11或更高版本，那么thread_local存储是您的理想选择。

Bellow是一个粗略的示例，说明了如何实现正确地附加到线程并在线程退出时自动清除的包装器方法

JNIEnv* JNIThreadHelper::GetJniEnv() {

    // This method might have been called from a different thread than the one that created
    // this handler. Check to make sure that the JNI is attached and if not attach it to the 
    // new thread.

    // double check it's all ok
    int nEnvStat = m_pJvm->GetEnv(reinterpret_cast<void**>(&m_pJniEnv), JNI_VERSION_1_6);

    if (nEnvStat == JNI_EDETACHED) {

        std::cout << "GetEnv: not attached. Attempting to attach" << std::endl;

        JavaVMAttachArgs args;
        args.version = JNI_VERSION_1_6; // choose your JNI version
        args.name = NULL; // you might want to give the java thread a name
        args.group = NULL; // you might want to assign the java thread to a ThreadGroup

        if (m_pJvm->AttachCurrentThread(&m_pJniEnv, &args) != 0) {
            std::cout << "Failed to attach" << std::endl;
            return nullptr;
        }

        thread_local struct DetachJniOnExit {
            ~DetachJniOnExit() {
                m_pJvm->DetachCurrentThread();
            }
        };


        m_bIsAttachedOnAThread = true;

    }
    else if (nEnvStat == JNI_OK) {
        //
    }
    else if (nEnvStat == JNI_EVERSION) {

        std::cout << "GetEnv: version not supported" << std::endl;
        return nullptr;
    }


    return m_pJniEnv;
}

保存JNIEnv *的最佳方法是什么？

2 个答案: