Server and Java Applet: Connecting Socket - sockets

Question

I have a java applet recently stopped working after the server is updated, more specifically:
1. The server is updated from Sun, running Solaris 9, 32 bit. (installed in 2005) to CentOS 5, (linux) on 64 bit.
2. The applet has two major classes 1) collect.class: collects data from a canvas 2) server.class: listens to collect.class through a PORT and acts accordingly;
but the applet got stuck and I check the start_server.sh (which produces a report nohup.out) there is a line
Exception creating server socket: java.net.BindException: Address already in use
This is weird, because PORT = 9999 which collect.class uses with no problem. How comes the problem happens only in server.class (who listens to collet.class).
Please help!
ADDITIONAL INFORMATION:
I.IN COLLECT.JAVA:
There is a canvas with grid on it, the user draw some area on the grid and click "Submit".
-> The MineCanvas.submit() is triggered -> The value of the area is computed by MineCanvas.ComputeGridValue() -> then Collect.cleintSend (stuck here)
import java.applet.*;
import java.awt.*;
import java.net.*;
import java.io.*;
import java.util.*;
public class Collect extends Applet {
...
public static final int PORT = 8888;
...
public boolean action(Event e, Object arg) {
...
if (arg.equals("Submit")) {
if (action(null, "Update Grid")) {
minecanvas.Submit();
} else {
return true;
}
}
return true;
}
...
public void clientSend(){
s = new Socket(this.getCodeBase().getHost(), PORT);
in = new DataInputStream(s.getInputStream());}
out = new DataOutputStream(s.getOutputStream());
listener = new SolutionListener(in, minecanvas);}
minecanvas.mode = MineCanvas.SUBMITTING;
minecanvas.repaint();
int n = 1;
out.writeBytes(minecanvas.gridh + "\n" + minecanvas.gridw + "\n");
for (int h = 0; h < minecanvas.gridh; h++) {
for (int w = 0; w < minecanvas.gridw; w++) {
out.writeBytes(n + " " + minecanvas.AllCells[w][h].net + "\n");
n++;
}
}
out.writeBytes("done\n");
s = null;
in = null;
out = null;
}
}
class MineCanvas extends Canvas {
...
public int gridw = 0; // number of grid squares width-ly
public int gridh = 0; // number of grid squares height-ly
public GridCell[][] AllCells; // array of grid cells comprising the grid
...
// compute values for minecanvas
public void ComputeGridValue() {...}
public void Submit() {
ComputeGridValue();
parent.clientSend();
}
...
}
...
}
II. SERVER.JAVA
import java.io.*;
import java.net.*;
public class Server extends Thread {
private OPM_Server opm; // this is the corresponding server for collect
...
public Server() {
...
opm = new OPM_Server();
}
public static void main(String[] args) {
new Server();
}
}
...
// OPM: correspond to Collect
class OPM_Server extends Thread {
public final static int DEFAULT_PORT = 8888;
protected int port;
protected ServerSocket listen_socket;
public static void fail(Exception e, String msg) {
System.err.println(msg + ": " + e);
System.exit(1);
}
public OPM_Server() {
this.port = DEFAULT_PORT;
try { listen_socket = new ServerSocket(port); }
catch (IOException e){ fail(e, "Exception creating server socket");}
System.out.println("Server: listening on port " + port);
this.start();
}
public void run() {
try {
while(true) {
System.out.println("I got to before ServerSocket");
Socket client_socket = listen_socket.accept();
OPM_Connection c = new OPM_Connection(client_socket);
}
}
catch (IOException e) {fail(e, "Exception while listening for connections");}
}
}
...
class OPM_Connection extends Thread {
protected Socket client;
protected BufferedReader in;
protected DataOutputStream out;
File mine_data = new File("mine_data"); // output file data
FileOutputStream file_stream;
DataOutputStream file_out;
public OPM_Connection(Socket client_socket) {
client = client_socket;
try {
in = new BufferedReader(new InputStreamReader(client.getInputStream()));
out = new DataOutputStream(client.getOutputStream());
} catch (IOException e) {
try {
client.close();
} catch (IOException e2) {
}
;
System.err.println("Exception while getting socket stream: "
+ e.toString());
return;
}
this.start();
}
public void run() {
...
file_stream = new FileOutputStream(mine_data);
file_out = new DataOutputStream(file_stream);
...// write to mine data
file_out = null;
if (inputGood == true) {
System.out.println(pid + "> ---Got all info from client");
Runtime r = Runtime.getRuntime();
Process Aproc = null;
Process Bproc = null;
int returnVal = -1;
try {
Aproc = r.exec("runOPM");
} catch (IOException e) {
inputGood = false;
System.out.println(pid + "> runOPM didn't exec");
}
try {
returnVal = Aproc.waitFor();
} catch (InterruptedException e) {
inputGood = false;
System.out.println(pid + "> runOPM didn't return");
}
System.out.println(pid + "> ---All execing done");
File report = new File("mine_report");
FileInputStream report_stream = null;
...
// create a mine report
System.out.println(pid + "> ---Done sending data back to client");
}
try {
client.close();
} catch (IOException e2) {
}
;
System.out.println(pid + "> EXITING THREAD");
}
}

Answer 1

Exception creating server socket: java.net.BindException: Address
already in use
This exception means that the port number the socket is trying to bind to (the port number your socket is trying to use in the local-end of the connection) is already in use by some other program. To fix it, you either need to find out what other software is using the port and see if you can safely change it, or change the port your program is using.
Edit: It might be worth trying to look for rarely used port(s), to lessen the chance of using yet another port that is known to be used by some common software, here's Wikipedias list of typical TCP and UDP ports in use by common programs and services.