Question

I have a DLL written in C++ that I can not change. It has the following exposed function

// c++ 
DllExport unsigned int ProcessMessage( char * in_message, USHORT in_message_length, char * connectionString, bool ( SendMessage)( char * connectionString, BYTE * payload, USHORT iPayloadSize )  );

I have a Java application that needs to call this DLL function. I am currently using a java library com.sun.jna

// Java 
public class main {
    public interface CBlargAPI extends Library {
        interface sendMessage_t extends Callback {
            boolean invoke(String connectionString, Pointer payload, short iPayloadSize );
        }
        int ProcessMessage( byte[] in_message, short in_message_length, String connectionString, sendMessage_t SendMessage ) ; 
    }
    public static void main(String[] args) throws Exception 
    {
        // Override function thingy (#A) 
        CBlarg.sendMessage_t sendMessage_fn  = new CBlarg.sendMessage_t() {
            @Override
            public boolean invoke(String connectionString, Pointer payload, short iPayloadSize) {
                System.out.println("sendMessage_t: " )  ;
                return false; 
                }
            };         
        }
        CBlargAPI.INSTANCE.ProcessMessage( receivePacket.getData(), (short) receivePacket.getLength(), connectionString, sendMessage_fn ); 
    }

    // static member function (#B) 
    public static boolean SendUDPMessage( String connectionString, Pointer payload, short length )  {    
        // ToDo: I want to use this one. 
    }
}

Currently this is working with the override function thingy (#A) but I want to use the static member function (#B) instead. I have tried a few things without success such as

// Errors with "cannot find symbol, symbol: class SendUDPMessage, location: class main" 
CBACnetAPI.sendMessage_t sendMessage_fn  = new main.SendUDPMessage();

I am primary a c++ programmer and rarely touch java

My question is:

• How do I call the static member function SendUDPMessage() as the callback instead of the Override function thingy (#A)?

Answer 1

You have to make the override function thingy (#A) call the static member function(#B). Java does not have function pointers, so you need an interface object to serve that purpose.

Why do you need to do #B over #A?