Java - How to Create a Simple Server Using Java Socket

Source here.

What Will I Learn?

In this tutorial, I will guide you the fundamentals of creating a simple server using Java socket. We will try connecting to the server using telnet to address: 127.0.0.1, and port 234.

• Java Socket Connection
• Java Concurrency by Using Thread

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Requirements

• Any current JDK.
• IntelliJ IDEA
• If you are comfortable using the Windows platform, make sure to do this following:
  • Open Control Panel,
  • Select Programs, then choose Turn Windows features on or off,
  • Finally, find and checked Telnet Client.
    

Difficulty

• Basic

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Tutorial Contents

• Creation of Server
  We are still using the previous project (Sample) to expand this tutorial. Open IntelliJ IDEA, right click on Sample/src/main/java, select New > Java Class, then on opened pop-up dialog, type: com.murez.branch.net.Server. Server class just have two actions, i.e.:

  • Start, to start this server, and

  public void start() {
     . . .
  }
  

  • Close, to stop this server.

  public void close() {
     . . .
  }
  

  And this is the source code of Server,

  
     public class Server implements AutoCloseable {
         private ServerSocket server;
         private Runner runner;
         private int port;
     
         public Server(int port) {
             if((this.port = port) < 0 || port > 0xFFFF)
                 throw new IllegalArgumentException("Port value out of range: " + port);
         }
     
         public void start() throws IOException {
             if(server == null) {
                 server = new ServerSocket();
                 server.bind(new InetSocketAddress("localhost", port));
                 runner = new Runner(server);
                 runner.start();
                 System.out.println("Server's starting");
             } else
                 throw new IllegalArgumentException("Server's already started");
         }
     
         public void close() throws Exception {
             if(!server.isClosed()) {
                 server.close();
                 server = null;
                 System.out.println("Server has closed");
             } else
                 throw new IllegalArgumentException("Server's already closed");
         }
     }
     

  

  Next, we need to define the Runner. Why is the Runner?
  The Runner is a separate process that will monitor all of incoming connections from client via telnet.
  Imagine if we do not define it, next statement below will never be executed forever since there's no client attempts to reach the server
  

  So the solution is to separate it (action when the server accepts the client connection) from the normal flow using Thread like we did at first.

  As mentioned in the Java API, server.accept() listens for a connection to be made to this socket and accepts it. The method blocks until a connection is made and return the new socket.

  References:

  • https://docs.oracle.com/javase/7/docs/api/, select java.net in the Packages menu (top of left-side navigation), then find and hit ServerSocket in the Classes (below of Packages).
  • https://developer.android.com/reference/java/net/ServerSocket.html#accept()

  Here's the complete implementation of Runner, we solve it with Runner as a static inner class.
  Next, Runner will also create a new Thread, which each of them will send a short message to the client. We can send a message to the client via the client.getOutputStream() method.
  To see the result clearly, we only add delay for 3 seconds before connection to client closed.

  
     private static class Runner implements Runnable, AutoCloseable {
         private final Object LOCK = new Object();
         private final ServerSocket SERVER;
         private boolean up;
         private Thread t;
     
         private Runner(ServerSocket server) {
             SERVER = server;
         }
     
         public void run() {
             for(;;) {
                 try {
                     Socket client = SERVER.accept();
                     new Thread(() -> {
                         SocketAddress address = client.getRemoteSocketAddress();
                         System.out.println("New client: " + address);
                         try(OutputStream out = client.getOutputStream();) {
                             out.write((address + " > Hello from Server!").getBytes());
                             // Going to close after 10 seconds
                             Thread.sleep(10000);
                             out.write((System.lineSeparator() + "Say good bye from Server!").getBytes());
                             out.flush();
                         }
                         catch(Exception e) {
                             e.printStackTrace();
                         }
                     }).start();
                 }
                 catch(IOException e) { break; }
                 synchronized(LOCK) {
                     if(!up) break;
                 }
             }
         }
     
         public void start() {
             synchronized(LOCK) {
                 if(!up) {
                     up = !up;
                     (t = new Thread(this)).start();
                 }
             }
         }
     
         public void close() {
             synchronized(LOCK) {
                 if(up) {
                     t.interrupt();
                     t = null;
                     up = false;
                 }
             }
         }
     }
     

  

• Creation of Launcher

  Just defined the main method, create an instance of Server and pass 234 as port number. Finally, start it.
  As seen below,

  
     package com.murez.branch.net;
     
     import java.io.IOException;
     
     public class Main {
         public static void main(String[] args) {
             Server server = new Server(234);
             try { server.start(); }
             catch(IOException e) { e.printStackTrace(); }
         }
     }
     

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Test

• We just need two clients to demonstrate. So, open two Command Prompts, type: telnet localhost 234, then hit Enter at all.
  <sub>Result
  
  Server's Log</sub>
  

Congratulations! We have successfully created a simple server using Java socket.

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Thank you!

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Curriculum

• Java (Spring Framework) - First App with Connection to MySQL Database

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Posted on Utopian.io - Rewarding Open Source Contributors

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