Call C# method from Java using JNI

Question

I've tried to follow the examples which I found here, but I am unable to get it to work. This is my Java class

package jniTester;
public class JNITester {
    static {
        System.load("D:\\\\VisualStudio_Cpp_2017\\SkriptumTeil5\\Debug\\HelloWorldJNI.dll");
    }

    public static native String welcome(String name);
}

From this I've created with javah the jniTester.h file

This is my C# class

namespace HelloWorldJNI
{

    public static class HelloWorldJNI
    {
        public static String Welcome(String name)
        {
            return "Hello " + name + "! This is your C# buddy.";
        }
    }
}

From this I've created HelloWorldJNI.netmodule

Here is my cpp class

#include "stdafx.h"
#include <jni.h>
#include <string>
#include "jniTester.h"
#using "D:\VisualStudio_C#_2017\SkriptumTeil5\HelloWorldJNI\HelloWorldJNI.netmodule"


using namespace std;

JNIEXPORT jstring JNICALL Java_jniTester_JNITester_welcome(JNIEnv *env, jclass thisclass, jstring inJNIStr) {
    // Step 1: Convert the JNI String (jstring) into C-String (char*)
    const char *inCStr = env->GetStringUTFChars(inJNIStr, NULL);
    if (NULL == inCStr) return NULL;

    // Step 2: Convert the C++ string to C-string, then to JNI String (jstring) and return
    //string outCppStr = "Hello " + std::string(inCStr) + ". Greetings from your C++ buddy";
    //env->ReleaseStringUTFChars(inJNIStr, inCStr);  // release resources
    //return env->NewStringUTF(outCppStr.c_str());

    //// Alternate Step 2:
    System::String^ outStr = HelloWorldJNI::HelloWorldJNI::Welcome(gcnew System::String(inCStr));
    env->ReleaseStringUTFChars(inJNIStr, inCStr);  // release resources
    char* converted = static_cast<char*>((System::Runtime::InteropServices::Marshal::StringToHGlobalAnsi(outStr)).ToPointer());
    return env->NewStringUTF(converted);
 }

The code under Step 2 works. However this is not calling my C# method. The implementation under Alternate Step 2 fails with

# A fatal error has been detected by the Java Runtime Environment:
#
#  Internal Error (0xe0434352), pid=37224, tid=0x00003350

I am no cpp expert, so I am totally in the dark. What is wrong here?

Add export "C" { /* my JNI functions */ } around of your JNI functions implementations. Since you are using C++ to wrap the C#. Also check - how to call managed code from the native code — Victor Gubin
– Victor Gubin, Commented May 15, 2018 at 9:28
thanks for the quick answer. However, I am unable to use the export statement. It produces syntax errors already in Visual Studio — Walter Taus
– Walter Taus, Commented May 15, 2018 at 10:02
Forget the export "C" stuff. First it is extern not export. Second it must be in the header already, otherwise you wouldn't be able to call the function at all. Have you tried calling a C# function without arguments? Have you tried in a standalone C++ executable? This is unlikely a JNI problem. — user2543253
– user2543253, Commented May 15, 2018 at 12:21

Victor Gubin · Accepted Answer · 2018-05-21 11:50:47Z

First of all you need to create assembly DLL in C# with the COM interface.

Then you can make a native (unmanaged) DLL wrapper library, which you can use with JNI.

You can follow this guide for Calling Managed NET COM Objects from Unmanaged C code

Also you can check the JNI manual

And a short example:

The C# code Managed.cs

using System;
using System.Reflection;
using System.Runtime.InteropServices;

[assembly: ComVisible(true)] 
[assembly: AssemblyDelaySign(false)] 
[assembly:AssemblyKeyFileAttribute("Managed.snk")]
[assembly: AssemblyVersion("1.0.0.0")]
namespace ManagedClassLibrary {

    [Guid("1CE4ECCB-EB78-4A50-8781-444052C7AEAE")]
    [ComVisible(true)]
    public interface IArithmetic {
        int sum(int lsh, int rhs);
        int subtract(int lsh, int rhs);
    }
    [Guid("30F078F9-F161-4112-B61A-B3BD6B63CB4C"), 
        ClassInterface(ClassInterfaceType.None), 
        ComSourceInterfaces(typeof(IArithmetic))
    ]
    [ComVisible(true)]
    public class ArithmeticImpl:IArithmetic {

        public int sum(int lsh, int rhs)
        {
             return lsh + rhs;
        } 
        public int subtract(int lsh, int rhs)
        {
             return lsh - rhs;
        }
    }

}

The C++ dll wrapper code dotnet_arithmetic.cpp

#include <windows.h>
#include <Objbase.h>

#include <jni.h>

#import "Managed.tlb" named_guids raw_interfaces_only

using namespace Managed;

class DNArithmetic 
{
    DNArithmetic(const DNArithmetic&) = delete;
    DNArithmetic& operator=(const DNArithmetic&) = delete;
private:
    DNArithmetic() noexcept
    {
     mi_.CreateInstance(CLSID_ArithmeticImpl);
    }
public:
    const DNArithmetic* instanse() {
        if(!_co_init) { 
         ::CoInitialize(NULL);
         _co_init = true;
        }
        static DNArithmetic _ret;            
        return &_ret;
    }
    int sum(int lsh, int rhs) const {
        int ret;
        mi_->sum(lsh, rhs, &ret);
        return ret;
    }

    int subtract(int lsh, int rhs) const {
        int ret;
        mi_->subtract(lsh, rhs, &ret);
        return ret;
    }

    ~DNArithmetic() noexcept
    { 
      ::CoUninitialize();           
    }
private:
    IArithmeticPtr mi_;
    static bool _co_init;
};

bool DNArithmetic::_co_init = false;

extern "C" {

    JNIEXPORT jint JNICALL Java_Arithmetic_dotnet_1sum(JNIEnv *evn, jclass clazz, jint lsh, jint rhs)
    {
        return DNArithmetic::instance()->sum(lsh, rhs);
    }

    JNIEXPORT jint JNICALL Java_Arithmetic_dotnet_1subtract(JNIEnv *evn, jclass clazz, jint lsh, jint rhs)
    {
        return DNArithmetic::instance()->subtract(lsh, rhs);
    }

    BOOL WINAPI DllMain(::HMODULE hprocess,::DWORD fdwReason,::LPVOID lpvReserved)
    {
        switch (fdwReason) {
        case DLL_PROCESS_ATTACH:           
            break;
        case DLL_PROCESS_DETACH:                
            break;
        case DLL_THREAD_ATTACH:
           break;
        case DLL_THREAD_DETACH:
           break;
        }
        return TRUE; // successful
    }

}

The Java Code Arithmetic.java

public class Arithmetic {

    private static native int dotnet_sum(int lsh, int rhs);

    private static native int dotnet_subtract(int lsh, int rhs);

    private Arithmetic() {
        try {
            System.loadLibrary("dotnet_arithmetic.dll");
        } catch (UnsatisfiedLinkError e) {
            throw new IllegalStateException(e);
        }
    }

    private int sum(int lsh, int rhs) {
        return  dotnet_sum(lsh, rhs);
    }

    private int subtract(int lsh, int rhs) {
        return dotnet_subtract(lsh,rhs);
    }

    public static void main(String[] args) {
        System.setProperty("java.library.path", System.getProperty("user.dir") );
        System.out.println("About to call C# functions from Java over the MS COM");
        Arithmetic instance = new Arithmetic();
        System.out.println("C# 1 + 2 = " + instance.sum(1,2) );
        System.out.println("C# 2 - 1 = " + instance.subtract(2,1) );
    }
}

And a build file build.cmd (simple bat file for demo proposes)

@echo off
rem make sure you have JAVA_HOME system variable set
rem Please set your VS root dir, should be  somewhere in C:\Program Files 
SET VS_HOME= 
echo %VS_HOME%

echo Take Visual studio command line tools
    call %VS_HOME%\VC\Auxiliary\Build\vcvars64.bat

echo Build C# class library
    sn /k Managed.snk
    csc /optimize /target:library /out:Managed.DLL Managed.cs
    RegAsm  Managed.DLL /tlb:Managed.tlb
    gacutil /i Managed.DLL 

echo Build C++ wrapper JNI DLL
    cl /c /nologo /GL /Zl /std:c++latest /I%JAVA_HOME%\include /I%JAVA_HOME%\include\win32 /Fodotnet_arithmetic.obj dotnet_arithmetic.cpp
    link /DLL /LTCG /LIBPATH:%JAVA_HOME%\lib /OUT:dotnet_arithmetic.dll msvcrt.lib kernel32.lib Ole32.lib jvm.lib dotnet_arithmetic.obj

echo Build Java

javac Arithmetic.java

echo Running the Demo

java Arithmetic

pause

NOTE: This solution for Windows OS only. And should not work for Mono on Unix.

Thanks a lot, Victor, for this detailed example. However, I have a compile error in the .cpp file with IArithmeticPtr and CLSID_ArithmeticImpl (unknown datatype).

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