Question

I am having a problem with JNI, calling a method from C++ to Java.

I am trying to call a void method that takes a boolean. My java code is the following:

public void setStatus(boolean bool) {
    // Do stuff...
}

public native void initialize(int defaultPort);

In my C++ code, I am making a struct to hold the env and object and pass it to a thread:

JNIEXPORT void JNICALL Java_com_device_client_HostConnection_initialize
  (JNIEnv * env, jobject obj, jint port)
{
    struct javaInfo* data = (struct javaInfo*) malloc(sizeof(struct javaInfo));
    data->env = env;
    data->javaObjHost = obj;

    pthread_t pth;
    pthread_create(&pth, NULL, startServer, (void *) data);

    free(data);
}

In the actual function, I am trying to obtain the class and then the MethodID and then call the void method, as follows:

void *startServer(void* arg) {
    struct javaInfo* data = (struct javaInfo*) arg; 
    JNIEnv* env = data->env;
    jobject javaObjHost = data->javaObjHost;

    cls = env->GetObjectClass(javaObjHost);
    mid = env->GetMethodID(cls, "setStatus", "(Z)V");
    if (mid == 0) {
        exit(-1);
    }
    env->CallVoidMethod(javaObjHost, mid, true);
}

It is hard for me to debug with JNI. I have tried putting a breakpoint in Eclipse in setStatus() but it never gets called. exit() is not called as well. The programs stomps for a second or two, then continues. I am not sure what is going on.

Could anyone please help me?

Thank you very much.

Answer 1

You cannot pass env pointers to other threads. You need to join the thread to the JVM.

In the original thread, called GetJavaVM to obtain a JavaVM pointer:

JavaVM *vm = 0;
env->GetJavaVM(&vm);

Then in the other thread, attach the VM to that thread and get a new env pointer:

vm->AttachCurrentThread(&env, 0);