Question

I’m diving into Java’s OpenJDK’s source code and I was in hopes of learning a bit about CallStaticVoidMethod. But I’m having a hardtime doing so.

This is where I got stomped:

00535     void (JNICALL *CallStaticVoidMethod)
00536       (JNIEnv *env, jclass cls, jmethodID methodID, ...);

If my rough C knowledge is right, this seems like a function call(to where?). Maybe it could be that’s making a call to some alread compiled library?

(JNICALL is just a calling convention)
being that JNICALL seems to be __stdcall ( What is __stdcall? )

Here is the file’s full source code:
http://xdprof.sourceforge.net/doxygen/jni_8h-source.html

Thanks

Answer 1

Maybe it could be that's making a call to some alread compiled library?

Yeah – I think you need some background on what the JNI is. Let me try and provide that background quickly, as I think it will clear up your confusion, or at least set you on the right track.

Java runtimes can’t run on Java – they are implemented as native executables.

The JNI (the Java Native Interface) is, essentially, a native interface for asking a Java runtime to do various things.

Amongst a ton of other things, you can use the JNI to invoke methods implemented in Java.

The JNI has a bunch of different helper methods for invoking different types of methods.

The method you are asking about, in particular, CallStaticVoidMethod would be used to invoke a Java function such as the one in this example:
public static void DoSomething() { ... }

In order for the runtime to invoke that method, it needs to know a few things – such as: information about the current runtime/context/environment (this is the JNIEnv * env parameter), the class the static method is declared in (this is the jclass cls parameter), the method to invoke (this is the jmethodID methodID parameter).

EDIT:

Followup to your reply:

I found it without much trouble in the OpenJDK code.

http://hg.openjdk.java.net/

cvmi/cvmi/jdk Common VM Interface

http://hg.openjdk.java.net/cvmi/cvmi/jdk/archive/tip.zip

Under:

./src/share/javavm/export/jni.h

void CallStaticVoidMethod(jclass cls, jmethodID methodID, ...) {
    va_list args;
    va_start(args,methodID);
    functions->CallStaticVoidMethodV(this,cls,methodID,args);
    va_end(args);
}

I don’t know where this is assigned: functions->CallStaticVoidMethodV but I’m sure if you go through the trouble of downloading all the source for the various components you’ll find a struct with that member and/or an assignment to that function pointer – and you can go from there.

Because it’s supposed to be a standard/common interface for multiple runtimes I wouldn’t be surprised if there was some layer of indirection between the actual implementation and the way it’s exposed through the JNI.