Multithreaded Servers in Java

Last Updated : 09 Nov, 2020

Prerequisites: Socket Programming in Java

Multithreaded Server: A server having more than one thread is known as Multithreaded Server. When a client sends the request, a thread is generated through which a user can communicate with the server. We need to generate multiple threads to accept multiple requests from multiple clients at the same time.

Advantages of Multithreaded Server:

Quick and Efficient: Multithreaded server could respond efficiently and quickly to the increasing client queries quickly.
Waiting time for users decreases: In a single-threaded server, other users had to wait until the running process gets completed but in multithreaded servers, all users can get a response at a single time so no user has to wait for other processes to finish.
Threads are independent of each other: There is no relation between any two threads. When a client is connected a new thread is generated every time.
The issue in one thread does not affect other threads: If any error occurs in any of the threads then no other thread is disturbed, all other processes keep running normally. In a single-threaded server, every other client had to wait if any problem occurs in the thread.

Disadvantages of Multithreaded Server:

Complicated Code: It is difficult to write the code of the multithreaded server. These programs can not be created easily
Debugging is difficult: Analyzing the main reason and origin of the error is difficult.

Quick Overview

We create two java files, Client.java and Server.java. Client file contains only one class Client (for creating a client). Server file has two classes, Server(creates a server) and ClientHandler(handles clients using multithreading).

Client-Side Program: A client can communicate with a server using this code. This involves

Establish a Socket Connection
Communication

Java

import java.io.*;
import java.net.*;
import java.util.*;

// Client class
class Client {
  
    // driver code
    public static void main(String[] args)
    {
        // establish a connection by providing host and port
        // number
        try (Socket socket = new Socket("localhost", 1234)) {
          
            // writing to server
            PrintWriter out = new PrintWriter(
                socket.getOutputStream(), true);

            // reading from server
            BufferedReader in
                = new BufferedReader(new InputStreamReader(
                    socket.getInputStream()));

            // object of scanner class
            Scanner sc = new Scanner(System.in);
            String line = null;

            while (!"exit".equalsIgnoreCase(line)) {
              
                // reading from user
                line = sc.nextLine();

                // sending the user input to server
                out.println(line);
                out.flush();

                // displaying server reply
                System.out.println("Server replied "
                                   + in.readLine());
            }
          
            // closing the scanner object
            sc.close();
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Server-Side Program: When a new client is connected, and he sends the message to the server.

1. Server class: The steps involved on the server side are similar to the article Socket Programming in Java with a slight change to create the thread object after obtaining the streams and port number.

Establishing the Connection: Server socket object is initialized and inside a while loop a socket object continuously accepts an incoming connection.
Obtaining the Streams: The inputstream object and outputstream object is extracted from the current requests’ socket object.
Creating a handler object: After obtaining the streams and port number, a new clientHandler object (the above class) is created with these parameters.
Invoking the start() method: The start() method is invoked on this newly created thread object.

2. ClientHandler class: As we will be using separate threads for each request, let's understand the working and implementation of the ClientHandler class implementing Runnable. An object of this class acts as a Runnable target for a new thread.

First, this class implements Runnable interface so that it can be passed as a Runnable target while creating a new Thread.
Secondly, the constructor of this class takes a parameter, which can uniquely identify any incoming request, i.e. a Socket.
Inside the run() method of this class, it reads the client's message and replies.

Java

import java.io.*;
import java.net.*;

// Server class
class Server {
    public static void main(String[] args)
    {
        ServerSocket server = null;

        try {

            // server is listening on port 1234
            server = new ServerSocket(1234);
            server.setReuseAddress(true);

            // running infinite loop for getting
            // client request
            while (true) {

                // socket object to receive incoming client
                // requests
                Socket client = server.accept();

                // Displaying that new client is connected
                // to server
                System.out.println("New client connected"
                                   + client.getInetAddress()
                                         .getHostAddress());

                // create a new thread object
                ClientHandler clientSock
                    = new ClientHandler(client);

                // This thread will handle the client
                // separately
                new Thread(clientSock).start();
            }
        }
        catch (IOException e) {
            e.printStackTrace();
        }
        finally {
            if (server != null) {
                try {
                    server.close();
                }
                catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    // ClientHandler class
    private static class ClientHandler implements Runnable {
        private final Socket clientSocket;

        // Constructor
        public ClientHandler(Socket socket)
        {
            this.clientSocket = socket;
        }

        public void run()
        {
            PrintWriter out = null;
            BufferedReader in = null;
            try {
                  
                  // get the outputstream of client
                out = new PrintWriter(
                    clientSocket.getOutputStream(), true);

                  // get the inputstream of client
                in = new BufferedReader(
                    new InputStreamReader(
                        clientSocket.getInputStream()));

                String line;
                while ((line = in.readLine()) != null) {

                    // writing the received message from
                    // client
                    System.out.printf(
                        " Sent from the client: %s\n",
                        line);
                    out.println(line);
                }
            }
            catch (IOException e) {
                e.printStackTrace();
            }
            finally {
                try {
                    if (out != null) {
                        out.close();
                    }
                    if (in != null) {
                        in.close();
                        clientSocket.close();
                    }
                }
                catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}