Call a static java method of another package from native code

Question

For example, let's say that in Android, I need to call the static method android.os.SystemClock.elapsedRealtime(), which returns a long, from a portion of native code. In the mylib.c file, I have

JNIEXPORT jlong JNICALL 
Java_com_mpackage_MyClass_nativeMethod(JNIEnv *env, jobject obj){

  jclass cls = (*env)->GetObjectClass(env, obj);
  jmethodID mid = (*env)->GetStaticMethodID(env, cls, "android.os.SystemClock.elapsedRealtime", "(V)J");

  if (mid == 0)
    return 0L;

  return CallStaticLongMethod(cls, mid);
}

In the java MyClass.class, I have among others

static {System.loadLibrary("myLib");}
native long nativeMethod();

but when I call it, I get the following error:

ERROR/AndroidRuntime(628): java.lang.NoSuchMethodError:
android.os.SystemClock.elapsedRealtime()

at the declaration of mid line. I think this is straightforward but I'm new to jni.

Can someone point out my mistake(s)?

Solution

Looks like your usage of the JNI API is not proper. First you should get the class reference of android.os.SystemClock. The obj passed as a parameter, is an object of MyClass. You should use (*env)->FindClass(env, "android/os/SystemClock") to get a jclass for the SystemClock. Then call (*env)->GetStaticMethodID(env, cls,"elapsedRealtime", "(V)J"); to get the method id. Take a look at the JNI tutorial for further details

Comments

[Tom]

I had found this myself a few minutes ago! However, I had to specify the name fo the class like (*env)->FindClass(env, "android/os/SystemClock"). Thanks!