Socket communication between Java and C# application

Question

I have 02 applications, one is written in java and the another one is written in C#. I need to exchange data (Strings) between them thus I use a socket connection.

Currently the C# application acts as server and java application acts as client. The C# application will continuously listen on port "7777" and the Java application will make a socket to "localhost:7777" as start up and use that socket to send data to C# server. I don't close the connection between them since I need to exchange data frequently.

Everything is great until my C# server application sometimes needs to send some Strings to the Java client application. I think about using another port to make my java application becomes a server and the C# application will also become a client. Yet I don't feel that it is a good solution.

P/S: Yes, socket is Bi-direction, in my current application I can send string data from my C# application back to Java application, but only when the Java application first sent a string to the C# application and now it is waiting for the respond data from the C# application.

Thus I want to ask if there is another better solution ? Is it possible if we only use 1 port in total ? Thank you very much.

Answer 1

Your socket works both ways (bi-directional), so you won't need to create another. You can just get the output stream and input stream of the socket, on both sides of the connection, and use those for sending/receiving.

To send a string, the easiest way would probably be to use a DataOutputStream as you can write UTF-8 strings with a simple function.

Java side:

String blah = "hey";
DataOutputStream dataOs = new DataOutputStream(socket.getOutputStream());
dataOs.writeUTF(blah);

The C# side is slightly trickier as you need to account for the fact that the first two bytes sent from the java function writeUTF() will actually be the length (in bytes) of the string that follows. You can grab those 2 bytes first and then throw them into an int, shifting the bits as you go along. Then you can use that int as the size of the buffer when you request the string from the socket. Should look something like this.

C# side:

int length = 0;
byte[] sizeBuffer = new byte[2];
socket.Receive(sizeBuffer);
for (int i = 0; i < sizeBuffer.Length; i += 1)
{
    length = length << 8;
    length += sizeBuffer[i];
}

byte[] stringBuffer = new byte[length];
socket.Receive(stringBuffer);

string myString = Encoding.UTF8.GetString(stringBuffer, 0, stringBuffer.Length);