How to create a multithreaded server in Java

Problem Description

How to create a multithreaded server?

Solution

Following example demonstrates how to create a multithreaded server by using ssock.accept() method of Socket class and MultiThreadServer(socketname) method of ServerSocket class.

import java.io.IOException;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;

public class MultiThreadServer implements Runnable {
   Socket csocket;
   MultiThreadServer(Socket csocket) {
      this.csocket = csocket;
   }
   public static void main(String args[]) throws Exception { 
      ServerSocket ssock = new ServerSocket(1234);
      System.out.println("Listening");
      
      while (true) {
         Socket sock = ssock.accept();
         System.out.println("Connected");
         new Thread(new MultiThreadServer(sock)).start();
      }
   }
   public void run() {
      try {
         PrintStream pstream = new PrintStream(csocket.getOutputStream());
         for (int i = 100; i >= 0; i--) {
            pstream.println(i + " bottles of beer on the wall");
         }
         pstream.close();
         csocket.close();
      } catch (IOException e) {
         System.out.println(e);
      }
   }
}

Result

The above code sample will produce the following result.

Listening
Connected

Following is an another example of how to create a multithreaded server.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;

import java.net.ServerSocket;
import java.net.Socket;
import java.text.SimpleDateFormat;
import java.util.Calendar;
 
public class NewClass {
   ServerSocket myServerSocket;
   boolean ServerOn = true;
   public NewClass() { 
      try {
         myServerSocket = new ServerSocket(8888);
      } catch(IOException ioe) { 
         System.out.println("Could not create server socket on port 8888. Quitting.");
         System.exit(-1);
      } 
		
      Calendar now = Calendar.getInstance();
      SimpleDateFormat formatter = new SimpleDateFormat(
         "E yyyy.MM.dd 'at' hh:mm:ss a zzz");
      System.out.println("It is now : " + formatter.format(now.getTime()));
      
      while(ServerOn) { 
         try { 
            Socket clientSocket = myServerSocket.accept();
            ClientServiceThread cliThread = new ClientServiceThread(clientSocket);
            cliThread.start(); 
         } catch(IOException ioe) { 
            System.out.println("Exception found on accept. Ignoring. Stack Trace :"); 
            ioe.printStackTrace(); 
         }  
      } 
      try { 
         myServerSocket.close(); 
         System.out.println("Server Stopped"); 
      } catch(Exception ioe) { 
         System.out.println("Error Found stopping server socket"); 
         System.exit(-1); 
      } 
   }
	
   public static void main (String[] args) { 
      new NewClass();        
   } 
	
   class ClientServiceThread extends Thread { 
      Socket myClientSocket;
      boolean m_bRunThread = true; 
      public ClientServiceThread() { 
         super(); 
      } 
		
      ClientServiceThread(Socket s) { 
         myClientSocket = s; 
      } 
		
      public void run() { 
         BufferedReader in = null; 
         PrintWriter out = null; 
         System.out.println(
            "Accepted Client Address - " + myClientSocket.getInetAddress().getHostName());
         try { 
            in = new BufferedReader(
               new InputStreamReader(myClientSocket.getInputStream()));
            out = new PrintWriter(
               new OutputStreamWriter(myClientSocket.getOutputStream()));
            
            while(m_bRunThread) { 
               String clientCommand = in.readLine(); 
               System.out.println("Client Says :" + clientCommand);
               
               if(!ServerOn) { 
                  System.out.print("Server has already stopped"); 
                  out.println("Server has already stopped"); 
                  out.flush(); 
                  m_bRunThread = false;
               } 
               if(clientCommand.equalsIgnoreCase("quit")) {
                  m_bRunThread = false;
                  System.out.print("Stopping client thread for client : ");
               } else if(clientCommand.equalsIgnoreCase("end")) {
                  m_bRunThread = false;
                  System.out.print("Stopping client thread for client : ");
                  ServerOn = false;
               } else {
                  out.println("Server Says : " + clientCommand);
                  out.flush(); 
               } 
            } 
         } catch(Exception e) { 
            e.printStackTrace(); 
         } 
         finally { 
            try { 
               in.close(); 
               out.close(); 
               myClientSocket.close(); 
               System.out.println("...Stopped"); 
            } catch(IOException ioe) { 
               ioe.printStackTrace(); 
            } 
         } 
      } 
   } 
}