Can I Call method on one JVM from another through a dll?

Question

Let me explain.
I have this java jar file that I can only have one instance of running at any given time. I'm using a shared data segment in a dll to store a bool indicating whether the program is already running or not. If it's already running, I have to not run the second instance and give focus to the current running instance.
The jar file calls a native method "canInstantiate()" on a dll to see if there's already an app running. If there isn't, the env and obj are stored in the shared data segment of the dll and we return true. If there is already an instance of the program running, I want canInstantiate call a function on the current instance of the jar (like a callback) to tell it to request focus. It's not working. Can someone tell me if my code is right?
The .h file
#include "stdafx.h"
#include <jni.h>
#include "CardServer.h"
#pragma data_seg("SHARED") // Begin the shared data segment.
static volatile bool instanceExists = false;
static JavaVM *theJavaVM = NULL;
static JNIEnv* theJavaEnv= NULL;
static jobject instanceObject = NULL;
static jmethodID mid = NULL;
static jclass cls = NULL;
#pragma data_seg()
#pragma comment(linker, "/section:SHARED,RWS")
jdouble canInstantiate(JNIEnv *env, jobject obj);
jdouble instantiate(JNIEnv *env, jobject obj);
jdouble uninstantiate(JNIEnv *env, jobject obj);
void grabFocus();
</code>
The .cpp file:
<code>
#include "MyFunctions.h"
#include <string.h>
#include <stdlib.h>
#include "stdafx.h"
#include <iostream.h>
jdouble canInstantiate(JNIEnv *env, jobject obj)
printf("In canInstantiate!!");
if (!instanceExists)
printf("No instance exists!!");
return (jdouble)0.0;
else
printf("An instance already exists!!");
grabFocus();
return (jdouble)1.0;
jdouble instantiate(JNIEnv *env, jobject obj)
printf("**In CPP: Instantiate!!\n");
cout << "At start, env is: " << env << endl;
cout << "At start, obj is: " << obj << endl;
if (instanceExists == false)
instanceExists = true;
theJavaEnv = env;
instanceObject = obj;
theJavaEnv->GetJavaVM(&theJavaVM);
cls = (theJavaEnv)->FindClass("TheMainClassOfTheJar");
if (cls == 0) {
fprintf(stderr, "Can't find Prog class\n");
exit(1);
mid = (theJavaEnv)->GetMethodID(cls, "grabFocusInJava", "(I)I");
if (mid == 0) {
fprintf(stderr, "Can't find grabFocusInJava\n");
exit(1);
printf("About to call grabFocusInJava\n");
grabFocus();
printf("CPP: After the grab focus command in instantiate!!\n");
cout << "At end, env is: " << env << endl;
cout << "At end, obj is: " << obj << endl;
return 0.0;
else
printf("CPP: Finished Instantiate!!\n");
return 1.0;
jdouble uninstantiate(JNIEnv *env, jobject obj)
printf("CPP: In uninstantiate!!\n");
if (instanceExists == true)
instanceExists = false;
theJavaVM = NULL;
instanceObject = NULL;
printf("CPP: Finishing uninstantiate!!\n");
return 0.0;
else
printf("CPP: Finishing uninstantiate!!\n");
return 1.0;
void grabFocus()
printf("In CPP::GrabFocus!!\n");
instanceObject = theJavaEnv->NewGlobalRef(instanceObject);
cls = (theJavaEnv)->FindClass("CardFormatter");
if (cls == 0) {
fprintf(stderr, "Can't find Prog class\n");
exit(1);
printf("Got the cls id again!!\n");
if (cls == 0)
printf("IT'S INVALID!!\n");
mid = (theJavaEnv)->GetMethodID(cls, "grabFocusInJava", "(I)I");
if (mid == 0) {
fprintf(stderr, "Can't find grabFocusInJava\n");
exit(1);
theJavaEnv->CallIntMethod(instanceObject, mid, 2);
printf("Called grabFocusInJava\n");
</code>
thanks in advance

Answer 1

Can I Call method on one JVM from another through a dll
...The rest of your question merely expands on your title.
And the answer to that question is no.
When you call a method you are executing a "thread of execution." A thread of execution exists only in a single process. It can not exist in another process.
If the dll is doing some interesting things then you could call a method that sets a flag. Data can move between instances. But you would then have to have a thread in that different process monitoring that flag. And sharing data in a dll is not a normal process, so it would have to be coded appropriately.
If all you want to do is set the current focus to the existing application, then that can be done with existing windows functionality. You don't need to do anything special in your dll. You can probably search these forums to find the exact code. If not there are countless examples in windows repositories (like MSDN) on how to do that.