拯救JNIEnv的最佳方法是什么*

Question

我和JNI有一个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，我想在java类中调用另一个函数。但是，为了调用这个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) {...}

当调用init(..)时，是否应该将cpp文件JNIEnv*保存为成员变量？当回调发生的时候再用它？

对不起，我是JNI的初学者。

Answer 1

缓存JNIEnv*不是一个特别好的主意，因为您不能跨多个线程使用相同的JNIEnv*，甚至可能无法将它用于同一线程上的多个本机调用(参见http://android-developers.blogspot.se/2011/11/jni-local-reference-changes-in-ics.html)。

编写一个获取JNIEnv*并在必要时将当前线程附加到VM的函数并不太困难：

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();
}

您可以将其封装在一个类中，类在构造时附加，在破坏时分离，RAII样式：

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时间运行的本机线程，在多个场合需要一个JNIEnv*。在这种情况下，您可能希望避免不断地将线程附加到JVM或从JVM中分离出来，但仍然需要确保在线程销毁时分离线程。

您可以通过只附加线程一次，然后将线程保持连接，以及使用pthread_key_create和pthread_setspecific设置线程销毁回调来完成这一任务，这将负责调用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.
        }
    }
}

如果您可以使用__cxa_thread_atexit，那么您可能可以通过在析构函数中调用DetachCurrentThread的某个thread_local对象来完成同样的任务。

Answer 2

@Michael，很好地概述了如何通过缓存JVM来最佳地检索JNI。对于那些不想使用c++ 11或更高的线程(或者因为你在Windows上而不能使用)并且使用更高版本的人来说，thread_local存储是最好的选择。

Bellow是关于如何实现包装器方法的粗略示例，该方法正确地附加到线程上，并在线程退出时自动清除。

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;
}