Calling Java from C++ using JNI

Question

I have been looking into JNI, and I ran sample where the Java calls Native c/c++ methods. Now to do it the other way, having C/C++ to call Java method by passing argument and getting the results back, I think I need to follow the example in Chapter 5 sun, which is creating JVM in the native code, accessing the Java class and method, and then finally calling the Java method.
First I like to know if I'm right above, and second if I need to pass a C++ class as the argument to the Java method and then the result from the method is Java class, what would be the best way to do? I know I need some to convert the class type some how.
Thanks,
Eli

Answer 1

Are you also wanting to launch Java from C++? Unzip JAVA_HOME/src.zip and look at the source code for java.exe
If you have native called by a Java object then you can call any non native method in the Java object from the C/C++ side. Example Java method, reportCurrentStagePosition :
    public void reportCurrentStagePosition (int fault, double x, double y, double z, double φ)
        if (fault == 0)
            reportCurrentStagePosition (x, y, z, φ);
        else
            faultMessage = "Stage Positioning Fault: X="+eFormat(x)+", Y="+eFormat(y)+", Z="+eFormat(z)+", phi="+eFormat(φ);
            System.out.println ("***Aero Fault Message: "+faultMessage);
    public native long moveLinearXYZPhiAxes() throws PositionException;Then from the native side of moveLinearXYZPhiAxes the obj is the calling object and has the reportCurrentStagePosition method
JNIEXPORT jlong JNICALL Java_com_hypernex_goniometer_AerotechWrapperN_moveLinearXYZPhiAxes (JNIEnv *env, jobject obj)
     jmethodID jReportPositions = env->GetMethodID(env->GetObjectClass(obj), "reportCurrentStagePosition", "(IDDDD)V");
     if (jReportPositions == NULL)
          printf ("***ERROR*** about to call jReportPositions but it is NULL ***\n");
     else
         env->CallVoidMethod (obj, jReportPositions, 0, xFeedBack, yFeedBack, zFeedBack, phiFeedBack);
}Since reportCurrentStagePosition returns a void you use the CallVoidMethod of env. The "(IDDDD)V" is the signature of the method. If it is there GetMethodID returns a non null.