Java Cookbook

Handling Multiple Clients

Problem

Your server needs to handle multiple clients.

Solution

Use a thread for each.

Discussion

In the C world, there are several mechanisms that allow a server to handle multiple clients. One is to use a special system call select( ) or poll( ), which notifies the server when any of a set of file/socket descriptors is ready to read, ready to write, or has an error. By including its rendezvous socket (equivalent to our ServerSocket) in this list, the C-based server can read from any of a number of clients in any order. Java does not provide this call, as it is not readily implementable on some Java platforms. Instead, Java uses the general-purpose Thread mechanism, as described in Section 24.9. Threads are, in fact, one of the other mechanisms available to the C programmer on most platforms. Each time the code accepts a new connection from the ServerSocket, it immediately constructs and starts a new thread object to process that client.^[38]

The code to implement accepting on a socket is pretty simple, apart from having to catch IOExceptions:

/** Run the main loop of the Server. */
void runServer(  ) {
    while (true) {
        try {
            Socket clntSock = sock.accept(  );
            new Handler(clntSock).start(  );
        } catch(IOException e) {
            System.err.println(e);
        }
    }
}

To use a thread, you must either subclass Thread or implement Runnable. The Handler class must be a subclass of Thread for this code to work as written; if Handler instead implemented the Runnable interface, the code would ...