如何获得异步调用的 JNI 接口指针(JNIEnv *)

我了解到 JNI 接口指针(JNIEnv *)仅在当前线程中有效。假设我在本机方法中启动了一个新线程; 它如何异步地将事件发送到 Java 方法？因为这个新线程不能引用(JNIEnv *)。存储(JNIEnv *)的全局变量显然不会起作用？

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You can obtain a pointer to the JVM (JavaVM*) with JNIEnv->GetJavaVM. You can safely store that pointer as a global variable. Later, in the new thread, you can either use AttachCurrentThread to attach the new thread to the JVM if you created it in C/C++ or simply GetEnv if you created the thread in java code which I do not assume since JNI would pass you a JNIEnv* then and you wouldn't have this problem.

    // JNIEnv* env; (initialized somewhere else)
JavaVM* jvm;
env->GetJavaVM(&jvm);
// now you can store jvm somewhere


// in the new thread:
JNIEnv* myNewEnv;
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
jvm->AttachCurrentThread((void**)&myNewEnv, &args);
// And now you can use myNewEnv
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最佳答案

Within synchronous calls using JNI from Java to C++ the "environment" has already been setup by the JVM, however going in the other direction from an arbitrary C++ thread it may not have been

Therefore you need to follow these steps

  • get hold of the JVM environment context using GetEnv
  • attach the context if necessary using AttachCurrentThread
  • call the method as normal using CallVoidMethod
  • detach using DetachCurrentThread

Full example. Note I have written about this in the past in more detail on my blog

JavaVM* g_vm;
env->GetJavaVM(&g_vm);


void callback(int val) {
JNIEnv * g_env;
// double check it's all ok
int getEnvStat = g_vm->GetEnv((void **)&g_env, JNI_VERSION_1_6);
if (getEnvStat == JNI_EDETACHED) {
std::cout << "GetEnv: not attached" << std::endl;
if (g_vm->AttachCurrentThread((void **) &g_env, NULL) != 0) {
std::cout << "Failed to attach" << std::endl;
}
} else if (getEnvStat == JNI_OK) {
//
} else if (getEnvStat == JNI_EVERSION) {
std::cout << "GetEnv: version not supported" << std::endl;
}


g_env->CallVoidMethod(g_obj, g_mid, val);


if (g_env->ExceptionCheck()) {
g_env->ExceptionDescribe();
}


g_vm->DetachCurrentThread();
}

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