Thread safe IO?

Mark Space <>
Tue, 30 Jan 2007 02:29:58 GMT
Hi all. I'm trying to write some basic network communication in Java,
and I've got a problem I can't resolve. The code below compiles and
works correctly (in so far as I'm able to test), but I fear there may be
some hideous issues lying just beneath the surface.

The basic idea below is a chat server. This illustrates my issue well
enough, although later I might be doing stuff beside just using a
PrintWriter for output. The class Main starts by initializing a list of
connections to null, and then waiting for ServerSocket connections on
port 24674. When it gets one, it creates a new instance of itself (this
initializes the "client" variable) and then spawns an new thread.

Once in the new thread (in "run") I create the readers and writers I
need, then I add the PrintWriter to the linked list "connections."
There's only one linked list, it's a static variable shared by all
threads. Then the thread waits for input, and when it gets a line from
the socket, it iterates over the linked list "connections," sending it's
out to each PrintWriter there.

This works fine. I can create multiple connections to this application
with telnet, and see any input I type echoed in each window.

What I'm concerned about is that this doesn't really seem like regular
use of the socket. Normally I think one thread would first read, then
write to a socket, or perhaps vice-versa. Here, for each socket, most
times one thread will be blocked on the input, while another thread is
stuffing bytes down it's output. Plus if a thread isn't blocked (is
actively collecting bytes or something) and some other thread is writing
bytes one the same socket, well, I have no idea what'll happen.

I can't find any documentation on this. Socket objects, and their IO
objects, aren't listed as being inherently thread safe. I'm not
accessing a single object twice, but there could be stuff going on under
the hood that makes this not safe.

Any one got some docs where thread safety might be touched on in greater
detail for Socket based IO? Any help is much appreciated.

Cut code below this line:

package chattest;

import java.util.Collections;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

public class Main implements Runnable
     private static List connections;
     Socket client;

     public Main( Socket client ) {
    this.client = client;

     public static void main(String[] args) throws IOException
    connections = Collections.synchronizedList( new LinkedList() );
    ServerSocket ss = new ServerSocket( 24674 );
    while( true )
        new Thread( new Main( ss.accept())).start();

     public void run() {
    try {
        BufferedReader in = new BufferedReader( new InputStreamReader(
        OutputStream out = client.getOutputStream();
        PrintWriter pout = new PrintWriter( new OutputStreamWriter( out),
true );
        connections.add( pout );
        while( true ) {
        String chat = in.readLine();
        synchronized( connections ) {
            PrintWriter temp_pout;
            Iterator itor = connections.iterator();
            while( itor.hasNext() ) {
            temp_pout = (PrintWriter);
            temp_pout.println( chat );
    catch (IOException ex) {

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