JNI Java 使用以对象为参数的 DLL 函数

Question

I will need to use an external, unknown DLL and build a java wrapper around it. I do not have the DLL nor the header file at the moment, and maybe will not even get the header-file, but I'd like to prepare myself. (I have no experience with C++ )

Following situation:

Let's say this DLL has a function, which contains one or more C++ classes as method signature. Then how could I call this function with JNI, as in my java project those custom classes in the DLL are non-existent? Is there a option to "clone" or "port" the C++ class to java? Is there anything I could do with a tool like Dependency Walker to resolve this problem?

What would be the best / most simple approach to accomplish that?

Here is some code which I already tried, to find out how it behaves:

Java Class with main

public class FourthJNI {

    public static native int returnAgeOfHuman(int zuQuadrierendeZahl);

    public static void main(String[] args) {
//      /* This message will help you determine whether
//        LD_LIBRARY_PATH is correctly set
//       */
//      System.out.println("library: "
//              + System.getProperty("java.library.path"));

        Human testHuman = new Human("abcde", 23, "M");

        /* Call to shared library */
        int ageOfHuman = FourthJNI.returnAgeOfHuman(5);
        System.out.println(testHuman.toString());
        System.out.println("Age: " + ageOfHuman);
    }
} 

generatd h-file

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class FourthJNI */

#ifndef _Included_FourthJNI
#define _Included_FourthJNI
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     FourthJNI
 * Method:    returnAgeOfHuman
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_FourthJNI_returnAgeOfHuman
  (JNIEnv *, jclass, jint);

#ifdef __cplusplus
}
#endif
#endif

Answer 1

The best way here is to use adapter pattern ( https://en.wikipedia.org/wiki/Adapter_pattern ). Inside your JNI code you have to call DLL by creating all the objects, as expected by C++ API.

You can find sample here: http://jnicookbook.owsiak.org/recipe-No-021/ and here https://github.com/mkowsiak/jnicookbook/tree/master/recipes/recipeNo025

You can also take a look at the code where one shared library ( JNI ) calls the code from another one: http://jnicookbook.owsiak.org/recipe-No-023/

Basically, what you have to do is to create JNI based wrapper code that translates Java calls to native methods into C++ calls, and vice versa - the code that translates return values into something that is expected by Java.