从Java调用c ++，然后从相同的c ++方法调用Java方法

Question

The following code can invoke a C++ method from Java.

JNITest.java

public class JNITest  {
    static {
        System.load("D:\\gagan docs\\TestJava\\Example.dll");
    }

    public native int intMethod(int n);

    public static void main(String[] args) {
        JNITest jnitest = new JNITest();
        int square=jnitest.intMethod(5);
        System.out.println("HelloWorld");
        System.out.println("From init method"+square);
    }

}

Example.cpp

#include "JNITest.h"
#include<string.h>
JNIEXPORT jint JNICALL Java_JNITest_intMethod(JNIEnv *env, jobject obj, jint num)
{
    return num*num;
}

But when I try to call a java method from the above CPP using the following, the JVM crashes.

I have also tried with createJavaVM method to create JVM, but it crashed.

Example.cpp

#include "JNITest.h"
#include<string.h>
static JavaVM *jvm = NULL
JNIEXPORT jint JNICALL Java_JNITest_intMethod(JNIEnv *env, jobject obj, jint num) 
{
    JNIEnv* jenv;
    int res = jvm->AttachCurrentThread((void **)&jenv, NULL);
    return num*num;
}

I am using Linux(64 bit)machine and JDK 64 bit(Java 1.6).

I want to make a complete flow from Java -> C++ -> Java. Any information on this issue will be very useful.

Answer 1

Calling Java from C++ is a (relatively) simple matter of:

JNIEXPORT jint JNICALL Java_JNITest_intMethod(JNIEnv *env, jobject obj, jint num)
{
    jint x = (env)->CallIntMethod(obj, method_id);
    // or, (env)->CallVoidMethod(obj, voidMethod, ...), etc.
}

This assumes the class of your jobject has the method identified by jmethodID . Obtaining the jmethodID is simple too:

jclass class_inst = (env)->FindClass("pkg/to/my/stuff/Class");
jmethodID method_id = (env)->GetMethodId(class_inst, "methodName", "()I");

Getting the method you want to call for a particular class is probably your most challenging task, but not at all hard once you have a good grasp of fundamentals.

For a nice example (disclaimer: I'm the example's author), see inotify-interface.cpp .

Note also, you probably aren't interested in calling AttachCurrentThread !