Question

I am not sure what obj is in MonitorEnter JNI function. Is it obj passed as parameter in native function or shared variable which I want to synchronize?
I have a variable called buffer which is shared by two threads.
This is my code.

JNIEXPORT void JNICALL Java_company_com_HelloActivity_setBuffer(JNIEnv *env, jobject obj, jstring jstr)
{
    char buf[256];

    int len = (*env)->GetStringLength(env, jstr);       
    (*env)->GetStringUTFRegion(env, jstr, 0, len, buf);
    (*env)->MonitorEnter(env, obj); // I don't think this is correct.
    strcat(buffer, buf); // buffer is declared as global char buffer[256];
    (*env)->MonitorExit(env, obj);
}

EDIT:
How about this? syncobj is defined in Activity as static Object and shared with another thread.

JNIEXPORT void JNICALL Java_company_com_HelloActivity_setBuffer(JNIEnv *env, jobject obj, jstring jstr, jobject syncobj)
{
    char buf[256];

    int len = (*env)->GetStringLength(env, jstr);       
    (*env)->GetStringUTFRegion(env, jstr, 0, len, buf);
    (*env)->MonitorEnter(env, syncobj);
    strcat(buffer, buf);
    (*env)->MonitorExit(env, syncobj);
}

Answer 1

It is exactly the same as in this Java code:

synchronized (syncobj) // = MonitorEnter(env, syncobj)
{
  // ...
} // = MonitorExit(env, syncobj)