How to call a method in .Net DLL in a Java program

Question

I was trying to use a .NET DLL from Java code, the tsMemberFunctions.DLL is loaded successfully, but the code fails to call the actual function.

See the snippet below:

public class tsMemberFunctions {  
    public native void GetMemberJSONSample();

    static {
        System.loadLibrary("tsMemberFunctions");
        System.out.println("Loaded");
    }

    public static void main(String[] args) {
        new tsMemberFunctions().GetMemberJSONSample();

    }
}

On executing above method I am getting below error:

Loaded
Exception in thread "main" java.lang.UnsatisfiedLinkError: tsMemberFunctions.GetMemberJSONSample()V
    at tsMemberFunctions.GetMemberJSONSample(Native Method)
    at tsMemberFunctions.main(tsMemberFunctions.java:12)

Can someone please tell me if I missed anything or anything is incorrect in the code or suggest better alternative for this use case. TIA.

Answer 1

You have to be very careful with the names and exports.

Let's say you have this super simple lib

// dllmain.cpp : Defines the entry point for the DLL application.
#include "pch.h"
#include "jni.h"
#include <stdio.h>

BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
        break;
    }
    return TRUE;
}

extern "C" JNIEXPORT void JNICALL Java_recipeNo001_HelloWorld_displayMessage

(JNIEnv* env, jclass obj) {

    printf("Hello world!\n");

}

You have to make sure to build your DLL for proper architecture (it will depend on Java version you have - 32/64 bit).

Let's say you have x64 DLL and x64 JDK, you can call your lib like this

package recipeNo001;

public class HelloWorld {

    public static native void displayMessage();

    static {
        System.load("C:\\Users\\your_name\\Source\\Repos\\HelloWorld\\x64\\Debug\\HelloWorld.dll");
    }

    public static void main(String[] args) {
      HelloWorld.displayMessage();
    }
}

In your case, I bet you don't have extern "C" in your code - this is why your symbol can't be found by JVM.

When it comes to tools, I suggest Visual Studio 2019 (when it comes to creating DLL) and IntelliJ for Java code.

You can find lots of samples here: http://jnicookbook.owsiak.org/ and here: https://github.com/mkowsiak/jnicookbook