JNI, How to access the current Java thread in JNI

Question

is there a way to get Java thread(ID, name) from JNI. I am not talking about passing Thread.currentThread().getId() from java to JNI. Does JNI provide API to access currently running thread?

Answer 1

You can (as mentioned by Alex) resort to java.lang.Thread.

// First, we have to find Thread class
jclass cls = (*env)->FindClass(env, "java/lang/Thread");

// Then, we can look for it's static method 'currentThread'
/* Remember that you can always get method signature using javap tool
        > javap -s -p java.lang.Thread | grep -A 1 currentThread
            public static native java.lang.Thread currentThread();
            descriptor: ()Ljava/lang/Thread; */
jmethodID mid = (*env)->GetStaticMethodID(
    env,
    cls,
    "currentThread",
    "()Ljava/lang/Thread;"
);

// Once you have method, you can call it. Remember that result is
// a jobject
jobject thread = (*env)->CallStaticObjectMethod(env, cls, mid);
if( thread == NULL ) {
    printf("Error while calling static method: currentThread\n");
}

// Now, we have to find another method - 'getId'
/* Remember that you can always get method signature using javap tool
        > javap -s -p java.lang.Thread | grep -A 1 getId
            public long getId();
            descriptor: ()Jjavap -s -p java.lang.Thread | grep -A 1 currentThread */
jmethodID mid_getid = (*env)->GetMethodID(env, cls, "getId", "()J");
if( mid_getid == NULL ) {
    printf("Error while calling GetMethodID for: getId\n");
}

// This time, we are calling instance method, note the difference
// in Call... method
jlong tid = (*env)->CallLongMethod(env, thread, mid_getid);

// Finally, let's call 'getName' of Thread object
/* Remember that you can always get method signature using javap tool
        > javap -s -p java.lang.Thread | grep -A 1 getName
            public final java.lang.String getName();
            descriptor: ()Ljava/lang/String;
*/
jmethodID mid_getname = (*env)->GetMethodID(
    env,
    cls,
    "getName",
    "()Ljava/lang/String;"
);

if( mid_getname == NULL ) {
    printf("Error while calling GetMethodID for: getName\n");
}

// As above, we are calling instance method
jobject tname = (*env)->CallObjectMethod(env, thread, mid_getname);

// Remember to retrieve characters from String object
const char *c_str;
c_str = (*env)->GetStringUTFChars(env, tname, NULL);
if(c_str == NULL) {
    return;
}

// display message from JNI
printf("[C   ] name: %s id: %ld\n", c_str, tid);

// and make sure to release allocated memory before leaving JNI
(*env)->ReleaseStringUTFChars(env, tname, c_str);

You can find full sample here.