JNI：从JVM到C ++的回调无法运行

Question

I start JVM from C++ program.

C++ code:

JNIEXPORT jobject JNICALL com_javelin_JavelinMarketData_callBackIntoNative(JNIEnv* env, jobject obj, jlong ptr)
{
        std::cout << "com_javelin_JavelinMarketData_callBackIntoNative called" << std::endl;
}

int main()
{
       JavaVM* jvm;
       JNIEnv* env;
       ...
       JNI_CreateJavaVM(&jvm, (void **)&env, &args);
       jmethodID mainMethod =  env->GetStaticMethodID(helloWorldClass, "main", "([Ljava/lang/String;)V");
       env->CallStaticVoidMethod(helloWorldClass, mainMethod, ...);
}

then I need my C++ function called back from java:

Java code:

native void callBackIntoNative(long ptr);

public void onResponse(long param)
{
    System.out.println("Calling callBackIntoNative()");
    callBackIntoNative(param); 
}

When I run my C++ program, JVM starts correctly, but after it prints "Calling callBackIntoNative()" the following acception appears:

Exception during callBackIntoNative(): 'com.javelin.JavelinMarketData.callBackIntoNative(J)V'
java.lang.UnsatisfiedLinkError: com.javelin.JavelinMarketData.callBackIntoNative(J)V

Can you please help?

Answer 1

Thanks, but I found a solution: I should have registered my C++ function as a native method using the code:

const JNINativeMethod methods[] = { { "callBackIntoNative", "(J)V", (void*)&com_javelin_JavelinMarketData_callBackIntoNative } };

const int methods_size = sizeof(methods) / sizeof(methods[0]);

jclass jClass = env->FindClass("com/javelin/JavelinMarketData");

env->RegisterNatives(jClass, methods, methods_size);

Now it works fine.

Answer 2

I would compile your callback apart into a dynamic library (.dll, .so, whatever your OS) and put it accesible to your java program. Then you just load your library by using JNI and call from your java classes whatever functionality you have in the library.