Re: JNI accessing a class that instantiates another class

From:
Danno <danmwall-google@yahoo.com>
Newsgroups:
comp.lang.java.programmer
Date:
Mon, 15 Aug 2011 12:28:22 -0700 (PDT)
Message-ID:
<391b55eb-b8cc-413e-9c58-21ce7e81b934@v7g2000vbk.googlegroups.com>
On Aug 15, 1:06 pm, Florian Weimer <f...@deneb.enyo.de> wrote:

* Danno:

I'm taking my first stab at JNI. I have a main Java class that
instantiates another class. In my C++ code, I am using the FindClass
function for the main class and calling the method in the main class
using GetStaticMethodId/CallStaticIntMethod. The C++ code is
successfully finding the Java class and calling the method that I
specified in the GetStaticMethoId function. The only problem is, whe=

n

the Java class attempts to create an instance of this second, inner
class, it dies. I don't get an error, it just doesn't work.


Please post code demonstrating the problem. Have you checked if
-Xcheck:jni reports anything useful?


Thank you for taking the time to reply to my post. I have attached my
code below. I have not used -Xcheck:jni before. I may not fully
understand how to use it, but the way I understand it, it is a command
line argument. The java program, however, is not being called from a
command line. I have a MicroFocus COBOL program that is calling a C++
dll that starts a JVM and invokes a Java method via JNI. Is there a
way to run -Xcheck:jni under that scenario?

Here is my code...

C++:
#include <jni.h>
#include <windows.h>
#include <string>
#include <stdio.h>
#include <stdlib.h>
#include "IGVIEWER_JNI.H"

using namespace std;

bool initJNI() {
        JNILoaded = false;
        JavaVMOption options[1];
        JavaVMInitArgs vm_args;
        long status;

        options[0].optionString = "-Djava.class.path=.";
        memset(&vm_args, 0, sizeof(vm_args));
        vm_args.version = JNI_VERSION_1_4;
        vm_args.nOptions = 1;
        vm_args.options = options;
        status = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);

        if (status == 0) {
                cls = env->FindClass("IgViewerJNI");
                if(cls ==0) {
                        jvm->DestroyJavaVM();
                        return false;
                } else {
                        JNILoaded = true;
                        return true;
                }
        }
        else {
                return false;
        }

}

extern ULONG WINAPI IMGINIT_JNI () {
        jmethodID mid;
        bool bResults;

        if (!JNILoaded) {
                bResults = initJNI();
                if (!bResults) {
                        return 8;
                }
        }

        mid = env->GetStaticMethodID(cls, "initializeViewer", "()I");
        if(mid !=0) {
                return env->CallStaticIntMethod(cls, mid);
        } else {
                return 8;
        }

}

Java:

import javax.swing.JOptionPane;

public class IgViewerJNI {

        public static int initializeViewer(){

                try {
                        // ** THIS IS WHERE IT DIES - I didn't include
the IgViewerInit class code since it seemed unnecessary.
                        IgViewerInit viewInit = new IgViewerInit();
                        return viewInit.initializeViewer();
                } catch (Exception e) {
                        JOptionPane.showMessageDialog(null, "Error - "
+ e.getMessage());
                        return 8;
                }
        }

        public static void main(String args[]) {

        }

}

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