access java synchronized method from native code

Question

I have a java class that has some (private static) synchronized methods which I want to call from native code as well. with some example code it becomes more clear what I mean

public class SomeClass {
   private static synchronized void method() {
     //do something that needs synchronization
   }
}

and the associated native code (C++)

void someFunction(JNIEnv * env) {
   jclass someClass = env->findClass("SomeClass");
   jmethodID methodId = env->GetStaticMethodID(jclass, "method", "()V");
   env->MonitorEnter(jclass); // <--- IS THIS NEEDED/ALLOWED
   env->CallStaticVoidMethod(jclass, methodId);
   env->MonitorExit(jclass); // <--- IS THIS NEEDED/ALLOWED
}

So what I am wondering is if I need to call MonitorEnter/MonitorExit, or if the method synchronization is enforced already by the synchronized attribute on SomeClass.method(). I am not so much interested in rewriting the code. I can think of a few solutions to work around this, but I am interested in what the behaviour is, given a synchronized method that is called from native code.

Answer 1

Section 8.4.3.6 synchronized Methods of the Java Language Specification says that declaring the method synchronized has the same effect as adding a synchronized block within the method.

Answer 2

No, explicit MonitorEnter / MonitorExit are not needed. According to The JNI guide,

...it is preferable to express synchronization constructs in the Java programming language. If, for example, a static native method needs to enter the monitor associated with its defining class, you should define a static synchronized native method as opposed to performing JNI-level monitor synchronization in native code.

Even though your case of calling a Java method from the native code (and not vice versa) isn't directly discussed in the spec, the opposite is not stated either, so I would assume that it works similarly.