Creating a socket server which allows multiple connections via threads and Java

Question

I am trying to adapt my simple socket server so that it can have multiple TCP connections, via multithreading, but I can't seem to get it to work. My code so far is as follows, I'm not really sure where to go from here :

import java.net.*; import java.io.*;  public class DoSomethingWithInput implements Runnable {    private final Socket clientSocket; //initialize in const'r    public void run() {        BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));         String nextline;         while ((nextline = in.readLine())!=null) {            System.out.println(nextline);         } //... close socket, etc.     } }   public class Socket{    public Socket() { } @Override public void run() {   try {     ServerSocket serverSocket = null;     serverSocket = new ServerSocket(5432);     for (;;) {       ServerSocket serverSocket = null;       serverSocket = new ServerSocket(5432);       for (;;) {         Socket clientSocket = null;         clientSocket = serverSocket.accept();         //delegate to new thread         new Thread(new DoSomethingWithInput(clientSocket)).start();       }     }   }catch (IOException e) {    System.err.println("Could not listen on port: 5432.");    System.exit(1); } } } 

Would anyone be able to give me some pointers on how I could do this, and why my current implementation will not work ? I was running through the tips in the Java tutorial http://download.oracle.com/javase/tutorial/networking/sockets/examples/KKMultiServerThread.java here, but the example they give here seems to use a lot of outside sources and classes like KnockKnockProtocol etc etc.

Would anyone be able to help me out with this?

Thank you very much!

Answer 1

The problem is that currently you're accepting the connection, but then immediately performing blocking reads on it until it's closed:

// After a few changes... Socket clientSocket = serverSocket.accept(); BufferedReader in = new BufferedReader(new InputStreamReader(      clientSocket.getInputStream())); String nextLine; while ((nextLine = in.readLine()) != null) {     System.out.println(nextline); } 

That means the same thread which accepts the connection is trying to handle the connection. That won't let you use multiple connections at the same time.

Instead, create a class (e.g. ConnectionHandler) which implements Runnable, and has a constructor taking a Socket. Its run method should handle the connection. Then change your code to:

Socket clientSocket = serverSocket.accept(); Runnable connectionHandler = new ConnectionHandler(clientSocket); new Thread(connectionHandler).start(); 

That will leave your "main" thread free to wait for the next connection.

(By the way, the KnockKnockProtocol class isn't really "external" - it's part of the example. They just didn't make it very clear that the source is here...)

Answer 2

You're not multithreading. You're creating a thread which binds on a port and then reads from any client socket until the connection is closed.

You need to pass the socket to a new thread and have it read.

public class DoSomethingWithInput implements Runnable {    private final Socket clientSocket; //initialize in const'r    public void run() {          BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));         String nextline;         while ((nextline = in.readLine())!=null) {            System.out.println(nextline);         } //... close socket, etc.     } }  //... ServerSocket serverSocket = null; serverSocket = new ServerSocket(5432); for (;;) {     Socket clientSocket = null;     clientSocket = serverSocket.accept();     //delegate to new thread     new Thread(new DoSomethingWithInput(clientSocket)).start(); } //...