How to expose Java enum to native code through JNI

Question

I'm using the enum pattern to define a singleton in my application. I need to access methods of this singleton, but I can't seem to pass the singleton correctly.

I've tried passing the enum and creating a jobject as a globalRef out of it like this:

class FrameElapsedListener : public gkEngine::Listener{
public:
JNIEnv* env;
jobject entityManager;

FrameElapsedListener(JNIEnv* env, jobject entityManager) :
    env(env),
    entityManager(env->NewGlobalRef(entityManager)){}
~FrameElapsedListener(){
    env->DeleteGlobalRef(entityManager);
}
void tick(gkScalar rate);
};

but I got a crash when using it that the "local ref" was out of scope like this:

jclass entityManagerClass = env->FindClass(entityManagerClassPath.c_str());
jmethodID entityManagerFrameElapsedMethodId = env->GetMethodID(entityManagerClass, "frameElapsed", "(F)V");
env->CallVoidMethod(entityManager, entityManagerFrameElapsedMethodId, rate); //crashes here

Now I'm trying to just get the singleton out of the enum, but I'm not sure how to grab enum values like that. Any advice?

Some error checking in general would help. But, in particular, what is the return value of GetMethodID and did it throw a Java exception (check with `ExceptionCheck` and debug with `ExceptionDescribe`). — Tom Blodget, Jun 20 '13 at 19:26

score 0 · Answer 1 · answered Jun 20 '13 at 22:32

0

You can't save a JNIEnv pointer for use in a different thread. You have to get a new one in the thread, by calling AttachCurrentThread().

answered Jun 20 '13 at 22:32

user207421

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I am using the same thread. The second section is in tick, which is called from the same thread I grab the JNIEnv* from. – mpellegr Jun 21 '13 at 14:35
The evidence and your comment suggest otherwise. Who calls `tick()`? You also need to be aware that a `jclass` is-a `jobject` and may also need a global-reference regime. – user207421 Jun 22 '13 at 09:56

score 0 · Accepted Answer · answered Jun 24 '13 at 18:19

I was able to solve my problem by changing my method of ensuring a singleton from an enum with a single enum value, to a class with a private constructor, private instance, and a getInstance() method. I had read in other places that the JNI can not use Java enums natively, but I thought since Enum is a class, I could just treat it as an object. Apparently, that's not possible.

score 0 · Answer 3 · answered Jan 03 '14 at 11:36

0

I have wrote a library which using Java Enum in C++ native code,I wrote a helper class which is simliar to yours,here it is,https://github.com/bugparty/jni_wifi_manager

answered Jan 03 '14 at 11:36

bowman han

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How to expose Java enum to native code through JNI

3 Answers3