How can I close the socket in a proper way? [duplicate]

Question

This is a simple TCP server. How can i close the socket when the program is terminated? I have using try/finally and try to close the socket. But it doesn't run the finally block when I exit the program.

Anyone can have idea on how to close the socket in a proper way?

try {
        socket = new ServerSocket(port);
        System.out.println("Server is starting on port " + port + " ...");
    }catch (IOException e){
        System.out.println("Error on socket creation!");
    }

    Socket connectionSocket = null;
    try{
        while(true){            
            try{
                connectionSocket = socket.accept();
                Thread t =  new Thread(new ClientConnection(connectionSocket));
                t.start();
            }catch (IOException e) {
                System.out.println("Error on accept socket!");
            }
        }
    }finally{
        this.socket.close();
        System.out.println("The server is shut down!");
    }

Answer 1

After creating your ServerSocket, you could add a ShutdownHook to close it on JVM termination, something like this:

Runtime.getRuntime().addShutdownHook(new Thread(){public void run(){
    try {
        socket.close();
        System.out.println("The server is shut down!");
    } catch (IOException e) { /* failed */ }
}});

Invoking ServerSocket#close will terminate the blocking ServerSocket.accept call, causing it to throw a SocketException. However, note that your current handling of IOException in the while loop means you will then re-enter the while loop to attempt accept on a closed socket. The JVM will still terminate, but it's a bit untidy.

Shutdown hooks do not run if you terminate a console application in Eclipse (on Windows at least). But they do run if you CTRL-C Java in a normal console. For them to run, you need the JVM to be terminated normally, e.g. SIGINT or SIGTERM rather than SIGKILL (kill -9).

A simple program which you can execute in Eclipse or a console will demonstrate this.

public class Test implements Runnable {

  public static void main(String[] args) throws InterruptedException {
    final Test test = new Test();
    Runtime.getRuntime().addShutdownHook(new Thread(){public void run(){
      test.shutdown();
    }});
    Thread t = new Thread(test);
    t.start();
  }

  public void run() {
    synchronized(this) {
      try {
        System.err.println("running");
        wait();
      } catch (InterruptedException e) {}
    }
  }

  public void shutdown() {
    System.err.println("shutdown");
  }
}

Answer 2

No need in your particular case, the operating system will close all the TCP sockets for you when the program exits.