How to obtain JNI interface pointer (JNIEnv *) for asynchronous calls

Question

I have learned that the JNI interface pointer (JNIEnv *) is only valid in the current thread. Suppose I started a new thread inside a native method; how it can asynchronously send events to a Java method? As this new thread can't have a reference of (JNIEnv *). Storing a global variable for (JNIEnv *) apparently will not work?

score 106 · Answer 1 · edited Oct 28 '19 at 13:56

106

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

edited Oct 28 '19 at 13:56

Cromax

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answered Oct 15 '12 at 17:35

main--

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Note that the second argument to `AttachCurrentThread` can be NULL if you don't need any special settings, and you should be sure to call `DetachCurrentThread` when you're finished if you weren't attached to begin with (otherwise you'll accumulate useless `Thread` objects that can't ever be GC'd). – technomage Oct 15 '12 at 18:30
the definition of AttachCurrentThread function changes in the NDK r9. here is the document link. http://docs.oracle.com/javase/1.5.0/docs/guide/jni/spec/invocation.html – Zephyr Jun 06 '14 at 21:29
Shouldn't `JNIEnv->GetJavaVM` accept `env` as the first parameter? – Denys Kniazhev-Support Ukraine Jul 07 '14 at 12:59
@DenisKniazhev `env` basically **is** the first parameter because `GetJavaVM` is invoked on the `env` pointer. – main-- Jul 07 '14 at 13:19
@main-- should it work in NDK while using pure C? I get "request for member 'GetJavaVM' in something not a structure or union" unless I call it like `(*env)->GetJavaVM(env, &jvm);` – Denys Kniazhev-Support Ukraine Jul 07 '14 at 13:45
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@DenisKniazhev Correct. C doesn't have classes, so you can't invoke a method on a pointer. In C++, JNI provides wrapper classes that automatically pass the env pointer, but in C you have to pass it manually. – main-- Jul 07 '14 at 20:29

Adam · Accepted Answer · 2020-12-15T17:08:02.133

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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();
}

edited Dec 15 '20 at 17:08

answered Oct 15 '12 at 17:47

Adam

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The only pieces of this answer that are related to the question are `GetEnv`, `AttachCurrentThread` and `DetachCurrentThread` and they are not even explained. – main-- Oct 15 '12 at 18:08
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That solves my problem completely, however above explanation by main was beautiful – Akhilesh Oct 15 '12 at 18:14
g_obj = env->NewGlobalRef(obj); -------> is throwing error: Request for member 'NewGlobalRef' in something not a structure or union – Matical Jun 29 '15 at 15:00
resolved the above error myself. I saved the file as .c but was using c++ syntax. – Matical Jun 29 '15 at 15:13
Thank you! This seems to have fixed a very stubborn but intermittent problem in my app. – Alan Kinnaman Apr 05 '16 at 22:50
@Akhilesh quite nice explanation about global and local scope of references in JNI is in https://www3.ntu.edu.sg/home/ehchua/programming/java/JavaNativeInterface.html#zz-7. – pevik Apr 08 '16 at 09:16
Shouldn't `DetachCurrentThread()` be conditioned to the execution of `AttachCurrentThread()` ? – Paulo Costa Dec 06 '18 at 19:46
Where do you get the `g_vm` variable from? – Razvan Cristian Lung Oct 22 '20 at 14:47
Wonder about the origin of `g_obj`; EDIT found in **blog** reference http://adamish.com/blog/archives/327 – Sam Ginrich May 17 '21 at 18:59
This looks like a very expensive operation. Has anyone measured it? – Lothar Jan 06 '22 at 02:39
There is another use case for this solution: You want to trigger a callback using a global reference, when in the callback function by design the current environment in not available. – Sam Ginrich Jan 08 '22 at 18:18

How to obtain JNI interface pointer (JNIEnv *) for asynchronous calls

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