Calling C++ dll from Java

Question

I'm using Java for a small app. It's a rewrite of an existing MFC project. There is an existing dll that I need to change to enable access from Java using JNI. All of this Java stuff is new to me, so I'm having a little trouble and feeling rather dense when I read other forum posts. In the existing dll I have a function like this:

extern "C" __declspec(dllexport) bool Create()
{
     return TRUE;
}

Dumb question time. How do I properly set it up to be called by Java?

I tried this:

JNIEXPORT jboolean JNICALL Create()
{
     return TRUE;
}

I'm including jni.h and everything compiles fine. However, when I call it from Java I get UnsatisfiedLinkError. I'm calling it from Java using this:

public static native boolean CreateSession();

System.load("D:\\JavaCallTest.dll");
Create();

Could someone kindly push me in the proper direction? I sincerely appreciate any help.

Thanks,

Nick

Answer 1

You need to include the Java class name and path in your native code, for example if your native method was declared in Java as:

public class NativeCode {
    public static native boolean CreateSession();
}

and the class path was (for example) com.example.NativeCode you would declare your method in native as follows:

extern "C"
JNIEXPORT jboolean JNICALL Java_com_example_NativeCode_CreateSession(JniEnv* env, jclass clazz)
{
    return JNI_TRUE;
}

All JNI methods have a JNIEnv pointer and class as their first two parameters.

Answer 2

A static native method still needs at least two parameters:

JNIEnv *env
jclass clazz

The function name also has to correspond the the java package structure.

JNIEXPORT jboolean JNICALL Java_com_example_CreateSession(JNIEnv *env, jclass clazz)

Ideally, you would use the javah tool to create a header file from the java class declaring the native method and then implement the declared function prototypes.

Answer 3

I had a similar problem - an existing C-Codebase which i needed to access from Java. It paid off for me, to get familiar with SWIG , a tool to generate an intermediate C++ DLL (which calls the C-Code), plus Java-Code that is calling into the C++ DLL.

If you have more than just 1 function of the DLL to wrap, it might pay off to check out this tool, otherwise you'd have to get familiar with JNI...

EDIT :

It seems like your DLL is not found by the System.load() call. You might want to try System.loadLibrary() , but note that your DLL must then be located in the path denoted by the Java system property java.library.path . Also dont pass the full filename in this case, but just the filename without extension.