通过Java中的套接字进行全双工通信

Question

Can I use two different threads to read and write from the same socket without any synchronization in java?

My code is -

public class Server {

    public static void main(String[] args) {
        Serve s = new Serve();
    }
}
class Serve {

ServerSocket sS;
String serverAddress;
int port;

public Serve() {
    serverAddress = "127.0.0.1";
    port = 8091;
    try  {
        sS = new ServerSocket(port);
        System.out.println("Server listening on port " + port+ " ...");
        Socket incomming = sS.accept();
        System.out.println("Connected to Client.");
        Runnable r = new Read(incomming);
        Runnable w = new Write(incomming);
        Thread read = new Thread(r);
        Thread write = new Thread(w);
        read.start();
        write.start();
        incomming.close();
        sS.close();
    }
    catch(IOException e) {
        e.printStackTrace();    
    }
}
}
class Read implements Runnable {

Socket readSocket;

public Read(Socket readSocket) {
    this.readSocket = readSocket;
}

@Override
public void run() {
    try {
        InputStream inStream = readSocket.getInputStream();
        try (Scanner in = new Scanner(inStream)) {
            boolean done = false;
            PrintWriter out = new PrintWriter(System.out, true);
            while(!done && in.hasNextLine()) {
                String line = in.nextLine();
                out.println("Client>" + line);
                if(line.trim().equals("BYE")) done = true;
            }
        }
    }
    catch(Exception e) {
        e.printStackTrace();
    }


}

}

class Write implements Runnable {

Socket writeSocket;

public Write(Socket writeSocket) {
    this.writeSocket = writeSocket;
    System.out.println("This is printed on the client terminal");
}

@Override
public void run() {
    try {
        OutputStream outStream = writeSocket.getOutputStream();
        try (Scanner in = new Scanner(System.in)) {
            boolean done = false;
            PrintWriter out = new PrintWriter(outStream, true);
            while(!done && in.hasNextLine()) {
                System.out.print("Server>");
                String line = in.nextLine();
                out.println(line);
                if(line.trim().equals("BYE")) done = true;
            }
         }
    }
    catch(Exception e) {
        System.out.println("Exception thrown here");
        e.printStackTrace();
    }


}

}

Two Problems -

1. The string in the Write constructor is getting printed on the client terminal. Why is this happening?

2. Why is the exception being thrown in the Write run() method?

Answer 1

Can I use two different threads to read and write from the same socket without any synchronization in java?

You can't avoid synchronization because the implementation for Socket is already synchronized.

You can avoid additional synchronization if you only have one thread reading and another thread writing.

The string in the Write constructor is getting printed on the client terminal. Why is this happening?

Most likely because you are running that code on the client as well.

Why is the exception being thrown in the Write run() method?

You have an error, which would be easier to diagnose if you read it and told us what it was (including the stack trace)

Note: Threads take time to start and run. If you close the connection immediately, the threads might not even get a chance to read the connection before you have closed it.

    read.start();
    write.start();
    incomming.close(); // == kill the connection

Instead of using

 while(!done ...) {

    if (condition)
        done = true;
 }

You can use

 while(...) {

    if (condition)
       break;
 }