Introducing Threads in Socket Programming in Java

08 Feb, 2018
Prerequisites : Socket Programming in Java
This article assumes that you have basic knowledge of socket programming in java and the basic details of client-server model used in communication.
Why to use threads in network programming?
The reason is simple, we don’t want only a single client to connect to server at a particular time but many clients simultaneously. We want our architecture to support multiple clients at the same time. For this reason, we must use threads on server side so that whenever a client request comes, a separate thread can be assigned for handling each request.
Let us take an example, suppose a Date-Time server is located at a place, say X. Being a generic server, it does not serve any particular client, rather to a whole set of generic clients. Also suppose at a particular time, two requests arrives at the server. With our basic server-client program, the request which comes even a nano-second first would be able to connect to the server and the other request would be rejected as no mechanism is provided for handling multiple requests simultaneously. To overcome this problem, we use threading in network programming.The following article will focus on creating a simple Date-Time server for handling multiple client requests at the same time.
Quick Overview
As normal, we will create two java files,Server.java and Client.java. Server file contains two classes namely Server (public class for creating server) and ClientHandler (for handling any client using multithreading). Client file contain only one public class Client (for creating a client). Below is the flow diagram of how these three classes interact with each other.
Server Side Programming(Server.java)
Server class : The steps involved on 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 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.
Establishing the Connection: Server socket object is initialized and inside a while loop a socket object continuously accepts 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.
Establishing the Connection: Server socket object is initialized and inside a while loop a socket object continuously accepts 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.
ClientHandler class : As we will be using separate threads for each request, lets understand the working and implementation of the ClientHandler class extending Threads. An object of this class will be instantiated each time a request comes.First of all this class extends Thread so that its objects assumes all properties of Threads.Secondly, the constructor of this class takes three parameters, which can uniquely identify any incoming request, i.e. a Socket, a DataInputStream to read from and a DataOutputStream to write to. Whenever we receive any request of client, the server extracts its port number, the DataInputStream object and DataOutputStream object and creates a new thread object of this class and invokes start() method on it.Note : Every request will always have a triplet of socket, input stream and output stream. This ensures that each object of this class writes on one specific stream rather than on multiple streams.Inside the run() method of this class, it performs three operations: request the user to specify whether time or date needed, read the answer from input stream object and accordingly write the output on the output stream object.
First of all this class extends Thread so that its objects assumes all properties of Threads.Secondly, the constructor of this class takes three parameters, which can uniquely identify any incoming request, i.e. a Socket, a DataInputStream to read from and a DataOutputStream to write to. Whenever we receive any request of client, the server extracts its port number, the DataInputStream object and DataOutputStream object and creates a new thread object of this class and invokes start() method on it.Note : Every request will always have a triplet of socket, input stream and output stream. This ensures that each object of this class writes on one specific stream rather than on multiple streams.Inside the run() method of this class, it performs three operations: request the user to specify whether time or date needed, read the answer from input stream object and accordingly write the output on the output stream object.
First of all this class extends Thread so that its objects assumes all properties of Threads.
Secondly, the constructor of this class takes three parameters, which can uniquely identify any incoming request, i.e. a Socket, a DataInputStream to read from and a DataOutputStream to write to. Whenever we receive any request of client, the server extracts its port number, the DataInputStream object and DataOutputStream object and creates a new thread object of this class and invokes start() method on it.Note : Every request will always have a triplet of socket, input stream and output stream. This ensures that each object of this class writes on one specific stream rather than on multiple streams.
Inside the run() method of this class, it performs three operations: request the user to specify whether time or date needed, read the answer from input stream object and accordingly write the output on the output stream object.
// Java implementation of  Server side// It contains two classes : Server and ClientHandler// Save file as Server.java  import java.io.*;import java.text.*;import java.util.*;import java.net.*;  // Server classpublic class Server {    public static void main(String[] args) throws IOException     {        // server is listening on port 5056        ServerSocket ss = new ServerSocket(5056);                  // running infinite loop for getting        // client request        while (true)         {            Socket s = null;                          try             {                // socket object to receive incoming client requests                s = ss.accept();                                  System.out.println("A new client is connected : " + s);                                  // obtaining input and out streams                DataInputStream dis = new DataInputStream(s.getInputStream());                DataOutputStream dos = new DataOutputStream(s.getOutputStream());                                  System.out.println("Assigning new thread for this client");                  // create a new thread object                Thread t = new ClientHandler(s, dis, dos);                  // Invoking the start() method                t.start();                              }            catch (Exception e){                s.close();                e.printStackTrace();            }        }    }}  // ClientHandler classclass ClientHandler extends Thread {    DateFormat fordate = new SimpleDateFormat("yyyy/MM/dd");    DateFormat fortime = new SimpleDateFormat("hh:mm:ss");    final DataInputStream dis;    final DataOutputStream dos;    final Socket s;            // Constructor    public ClientHandler(Socket s, DataInputStream dis, DataOutputStream dos)     {        this.s = s;        this.dis = dis;        this.dos = dos;    }      @Override    public void run()     {        String received;        String toreturn;        while (true)         {            try {                  // Ask user what he wants                dos.writeUTF("What do you want?[Date | Time]..\n"+                            "Type Exit to terminate connection.");                                  // receive the answer from client                received = dis.readUTF();                                  if(received.equals("Exit"))                {                     System.out.println("Client " + this.s + " sends exit...");                    System.out.println("Closing this connection.");                    this.s.close();                    System.out.println("Connection closed");                    break;                }                                  // creating Date object                Date date = new Date();                                  // write on output stream based on the                // answer from the client                switch (received) {                                      case "Date" :                        toreturn = fordate.format(date);                        dos.writeUTF(toreturn);                        break;                                              case "Time" :                        toreturn = fortime.format(date);                        dos.writeUTF(toreturn);                        break;                                              default:                        dos.writeUTF("Invalid input");                        break;                }            } catch (IOException e) {                e.printStackTrace();            }        }                  try        {            // closing resources            this.dis.close();            this.dos.close();                      }catch(IOException e){            e.printStackTrace();        }    }}
Output
A new client is connected : Socket[addr=/127.0.0.1,port=60536,localport=5056]
Assigning new thread for this client
Client Socket[addr=/127.0.0.1,port=60536,localport=5056] sends exit...
Closing this connection.
Connection closed

Client Side Programming (Client.java)
Client side programming is similar as in general socket programming program with the following steps-
Establish a Socket ConnectionCommunication
Establish a Socket Connection
Communication
// Java implementation for a client// Save file as Client.java  import java.io.*;import java.net.*;import java.util.Scanner;  // Client classpublic class Client {    public static void main(String[] args) throws IOException     {        try        {            Scanner scn = new Scanner(System.in);                          // getting localhost ip            InetAddress ip = InetAddress.getByName("localhost");                  // establish the connection with server port 5056            Socket s = new Socket(ip, 5056);                  // obtaining input and out streams            DataInputStream dis = new DataInputStream(s.getInputStream());            DataOutputStream dos = new DataOutputStream(s.getOutputStream());                  // the following loop performs the exchange of            // information between client and client handler            while (true)             {                System.out.println(dis.readUTF());                String tosend = scn.nextLine();                dos.writeUTF(tosend);                                  // If client sends exit,close this connection                 // and then break from the while loop                if(tosend.equals("Exit"))                {                    System.out.println("Closing this connection : " + s);                    s.close();                    System.out.println("Connection closed");                    break;                }                                  // printing date or time as requested by client                String received = dis.readUTF();                System.out.println(received);            }                          // closing resources            scn.close();            dis.close();            dos.close();        }catch(Exception e){            e.printStackTrace();        }    }}
Output :
What do you want?[Date | Time]..
Type Exit to terminate connection.
Date
2017/06/16
What do you want?[Date | Time]..
Type Exit to terminate connection.
Time
05:35:28
What do you want?[Date | Time]..
Type Exit to terminate connection.
Geeks
Invalid input
What do you want?[Date | Time]..
Type Exit to terminate connection.
Exit
Closing this connection : Socket[addr=localhost/127.0.0.1,port=5056,localport=60536]
Connection closed

How these programs works together?
When a client, say client1 sends a request to connect to server, the server assigns a new thread to handle this request. The newly assigned thread is given the access to streams for communicating with the client.After assigning the new thread, the server via its while loop, again comes into accepting state.When a second request comes while first is still in process, the server accepts this requests and again assigns a new thread for processing it. In this way, multiple requests can be handled even when some requests are in process.
When a client, say client1 sends a request to connect to server, the server assigns a new thread to handle this request. The newly assigned thread is given the access to streams for communicating with the client.
After assigning the new thread, the server via its while loop, again comes into accepting state.
When a second request comes while first is still in process, the server accepts this requests and again assigns a new thread for processing it. In this way, multiple requests can be handled even when some requests are in process.
How to test the above program on your system?
Save the two programs in same package or anywhere. Then first run the Server.java followed by the Client.java. You can either copy the client program in two three separate files and run them individually, or if you have an IDE like eclipse, run multiple instances from the same program. The output shown above is from a single client program, the similar results will be achieved if multiple clients are used. Next: Multi-threaded chat Application : Server Side Programming , Client Side Programming 
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