Advanced Java - VIII - Java Networking

Jalaz Kumar · September 17, 2020

Java

Networking is a concept of connecting 2 or more computing devices together for resource sharing.

Network Terminologies:

  1. IP Address
    • A unique number assigned to a node of a network
    • A logical address that can be changed
  2. Protocol
    • A set of rules for communication between devices.
    • Example: TCP, UDP, Telnet etc.
  3. Port Number
    • For Uniquely identifying different applications
    • Acts as a communication endpoint between 2 applications.
  4. MAC Address
    • A unique identifier of NIC (Network Interface Controller)
    • A network node can have multiple NIC with each having unique MAC.
  5. Connection-oriented and connection-less protocol
    • In connection-oriented protocol, acknowledgement is sent by the receiver. So it is reliable but slow. The example of connection-oriented protocol is TCP.
  • In connection-less protocol, acknowledgement is not sent by the receiver. So it is not reliable but fast. The example of connection-less protocol is UDP.
  1. Socket
    • An endpoint between two way communication.

java.net package provides many classes to deal with networking applications in Java.

Connection-Oriented Socket Programming

  • Socket class
    • Used to create a socket.
    • Important methods:
      • public InputStream getInputStream()
      • public OutputStream getOutputStream()
      • public synchronized void close()
  • ServerSocket class
    • Used to create a server socket.
    • Used to establish communication with the clients.
    • Important methods:
      • public Socket accept()
      • public synchronized void close()

Client.java

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

public class Client {  
   public static void main(String[] args) {  
       try {      
         Socket s = new Socket("localhost", 6666);  
         DataOutputStream dout = new DataOutputStream(s.getOutputStream());  
         dout.writeUTF("Hello Server");  
         dout.flush();  
         dout.close();  
         s.close();  
       } catch(Exception e) { System.out.println(e); }  
   }  
}

Server.java

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

public class Server {  
   public static void main(String[] args) {  
       try {  
         ServerSocket ss = new ServerSocket(6666);  
         Socket s = ss.accept();                       //establishes connection   
         DataInputStream dis = new DataInputStream(s.getInputStream());  
         String  str = (String)dis.readUTF();  
         System.out.println("message= "+str);  
         ss.close();  
       } catch(Exception e) { System.out.println(e); }  
   }  
}

Socket-API

Important Networking Classes:

  • URL

    • Java URL class represents an URL.

    • URL is an acronym for Uniform Resource Locator which points to a resource on the World Wide Web.

       import java.net.*;  

   public class URLExample {  
       public static void main(String[] args) {  
           try {  
           URL url = new URL("https://jaykay12.github.io/tech/java-basics.md");  

           System.out.println("Protocol: " + url.getProtocol());         // https
           System.out.println("Host Name: " + url.getHost());            // jaykay12.github.io
           System.out.println("Port Number: " + url.getPort());          // -1
           System.out.println("File Name: " + url.getFile());            // /tech/java-basics.md

           } catch(Exception e) { System.out.println(e); }  
       }  
   }
  • URLConnection

    • Represents a communication link between the URL and the application.

    • Used to read and write data to the specified resource referred by the URL.

    • openConnection() method of URL class returns the object of URLConnection class, which can then be used to download the source-code of url.

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

   public class URLConnectionExample {  
       public static void main(String[] args) {  
           try{  
               URL url = new URL("https://jaykay12.github.io/tech/java-basics.md");  
               URLConnection urlcon = url.openConnection();  
               InputStream stream = urlcon.getInputStream();  
               int i;  
               while((i=stream.read())!=-1){  
                  System.out.print((char)i);  
               }  
           }catch(Exception e){System.out.println(e);}  
       }  
   }
  • HttpUrlConnection

    • Represents http specific URLConnection. The java.net.HttpURLConnection is subclass of URLConnection class.

    • It works for HTTP protocol only.

    • We can information of any HTTP URL such as header information, status code, response code etc. using this.

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

   public class HttpURLConnectionExample {    
       public static void main(String[] args) {    
           try {    
               URL url = new URL("https://jaykay12.github.io/tech/java-basics.md");    
               HttpURLConnection huc = (HttpURLConnection)url.openConnection();  
               for(int i=1;i<=8;i++) {
                  System.out.println(huc.getHeaderFieldKey(i)+" = "+huc.getHeaderField(i));  
               }
               huc.disconnect();
           } catch(Exception e) { System.out.println(e); }    
       }    
   }
  • InetAddress

    • Represents an IP address.

    • java.net.InetAddress class provides methods to get the IP of any host name for example www.jaykay12.github.io

    • InetAddress has a cache mechanism to store successful and unsuccessful host name resolutions.

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

   public class InetExample {  
       public static void main(String[] args){  
           try{  
               InetAddress ip = InetAddress.getByName("www.jaykay12.github.io");  

               System.out.println("Host Name: " + ip.getHostName());         // jaykay12.github.io
               System.out.println("IP Address: " + ip.getHostAddress());     // 35.206.76.226
           } catch(Exception e) { System.out.println(e); }  
       }  
   }

Connection-less Socket Programming

  • DatagramSocket class

    • Represents connection-less socket for sending and receiving datagram packets.

    • Most commonly used constructor: DatagramSocket(int port, InetAddress address) throws SocketException

    Creates a datagram socket and binds it with the specified port number and host address.

  • DatagramPacket class

    • Datagram is an information but there is no guarantee of its content, arrival or arrival time.

    • Java DatagramPacket is a message that can be sent or received.

    • If multiple packet are sent, it may arrive in any order. Additionally, packet delivery is not guaranteed.

    • DatagramPacket(byte[] barr, int length) : Used to receive the packets after creation.

    • DatagramPacket(byte[] barr, int length, InetAddress address, int port) : Used to send the packets after creation.

DGSender.java

import java.net.*;

public class DGSender {  
    public static void main(String[] args) throws Exception {  
        DatagramSocket ds = new DatagramSocket();  
        String str = "Welcome java";  
        InetAddress ip = InetAddress.getByName("127.0.0.1");  

        DatagramPacket dp = new DatagramPacket(str.getBytes(), str.length(), ip, 3000);  
        ds.send(dp);  
        ds.close();  
    }  
}

DGReceiver.java

import java.net.*;

public class DGReceiver {  
    public static void main(String[] args) throws Exception {  
        DatagramSocket ds = new DatagramSocket(3000);  
        byte[] buf = new byte[1024];  
        DatagramPacket dp = new DatagramPacket(buf, 1024);  
        ds.receive(dp);  
        String str = new String(dp.getData(), 0, dp.getLength());  
        System.out.println(str);  
        ds.close();  
    }  
}

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