ServerSocket accept method

Question

In my application, the serversocket object is waiting forever in the accept() method. The client applications tries to connect but the server doesn?t answer. Is there any bug in JVM or a TCP/socket delay that can explain it ?

Answer 1

Here's code for a simple multi-socket echo server that uses mutiple threads. It takes an argument of a port number. Try it and see if you can connect to it, if not then your problem isn't your code or the Java system. This code works in JDK 1.3, 1.4 and 1.5.
If this doesn't work your problem is likely network related, eg. you are on a private network and trying to connect from the internet.
import java.io.*;
import java.net.*;
import java.util.*;
import java.text.*;
public class Echo
     public static boolean abortFlag = false;
     public static boolean running = false;
     private void start(int port)
          running = true;
          System.out.println("Listening on port="+port);
          try
               ServerSocket ss = new ServerSocket(port);     // listen for connections
               while(running)new EchoUser(ss.accept()).start();     // start a client connection     
          catch(IOException ie){ie.printStackTrace(); System.exit(0);}
     static public void main(String [] args)
          int port = 5050;
          for(int i = 0; i<args.length; i++)
               if(args.equals("-a"))abortFlag = true;
               else
                    try{port = new Integer(args[i]).intValue();}
                    catch(NumberFormatException exc)
                         System.out.println("invalid port"+args[i]);
                         System.exit(0);
          new Echo().start(port);
     private class EchoUser
          private Socket sock = null;
          private InputStream in = null;
          private OutputStream out = null;
          private Thread recvThread;
          private Thread sendThread;
          private String name="";
          private Queue outQ= new Queue();
          private boolean running = false;          
          private long millis;
          private long bytes;
          public EchoUser(Socket sock){this.sock = sock;}
          public int start()
               name = ""+sock.getInetAddress()+":"+sock.getPort();
               System.out.println("Connection received from "+name);
               millis = System.currentTimeMillis();
               try
                    in = sock.getInputStream();
                    out = sock.getOutputStream();
               catch(Exception e){return fail(e);}
               recvThread = new Thread(new Runnable()
               {public void run(){runRecv();}},"Recv."+name);
               sendThread = new Thread(new Runnable()
               {public void run(){runSend();}},"Send."+name );
               running = true;
               sendThread.start();
               recvThread.start();
               return 0;
          private int runRecv()
               byte [] buffer = new byte[8192];
               int length;
               while (running && Echo.running)
                    try{length = in.read(buffer);}
                    catch(Exception e){return fail(e);}
                    if(length < 0)
                         System.out.println("EOF received from "+name);
                         outQ.push(null);
                         return fail(null);
                    else
                         System.out.println("received "+length+" bytes from user="+name);
                         byte [] b = new byte[length];
                         System.arraycopy(buffer,0,b,0,length);                         
                         outQ.push(b);
               return 0;
          private int runSend()
               while(running && Echo.running)
                    Object d = outQ.pop();
                    if(d == null)return 0;
                    byte[] bv = (byte[])d;
                    try
                         out.write(bv);
                         out.flush();
                         bytes+=bv.length;
                    catch(Exception e){return fail(e);}
               return 0;
          private int fail(Exception e)
               int v = 0;
               if(e != null)
                    e.printStackTrace();
                    v = 1;
                    if(Echo.abortFlag)
                         Echo.running = false;
                         System.exit(0);
               if(running)
                    double dur = (System.currentTimeMillis()-millis)/1000.0;
                    double rate = (bytes*8)/dur;
                    DecimalFormat f = (DecimalFormat)DecimalFormat.getInstance();
                    f.applyPattern("###0.00");
                    System.out.println("Closing "+name+": "+bytes+" bytes transferred in "
                         dur"seconds rate="+f.format(rate)+"bps");
                    try{sock.close();}catch(Exception ce){ce.printStackTrace();}
                    running = false;
               return v;
     // a simple queue class between two threads
     private class Queue
          LinkedList q = new LinkedList();
          public synchronized void push(Object obj)
               q.add(obj);
               this.notify();     // tell pop to run with it
          public synchronized Object pop()
               while(q.isEmpty())
                    try{this.wait();}
                    catch (InterruptedException e){}
               return q.remove(0);