Java Socket Programming

Java Socket programming is used for communication between the applications running on different JRE.

Java Socket programming can be connection-oriented or connection-less.

Socket and ServerSocket classes are used for connection-oriented socket programming and DatagramSocket and DatagramPacket classes are used for connection-less socket programming.

The client in socket programming must know two information:

1. IP Address of Server, and
2. Port number.

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Socket class

A socket is simply an endpoint for communications between the machines. The Socket class can be used to create a socket.

Important methods

Method	Description
1) public InputStream getInputStream()	returns the InputStream attached with this socket.
2) public OutputStream getOutputStream()	returns the OutputStream attached with this socket.
3) public synchronized void close()	closes this socket

ServerSocket class

The ServerSocket class can be used to create a server socket. This object is used to establish communication with the clients.

Important methods

Method	Description
1) public Socket accept()	returns the socket and establish a connection between server and client.
2) public synchronized void close()	closes the server socket.

Example of Java Socket Programming

Let's see a simple of java socket programming in which client sends a text and server receives it.

File: MyServer.java

1. import java.io.*;  
2. import java.net.*;  
3. public class MyServer {  
4. public static void main(String[] args){  
5. try{  
6. ServerSocket ss=new ServerSocket(6666);  
7. Socket s=ss.accept();//establishes connection   
8. DataInputStream dis=new DataInputStream(s.getInputStream());  
9. String  str=(String)dis.readUTF();  
10. System.out.println("message= "+str);  
11. ss.close();  
12. }catch(Exception e){System.out.println(e);}  
13. }  
14. }  

File: MyClient.java

1. import java.io.*;  
2. import java.net.*;  
3. public class MyClient {  
4. public static void main(String[] args) {  
5. try{      
6. Socket s=new Socket("localhost",6666);  
7. DataOutputStream dout=new DataOutputStream(s.getOutputStream());  
8. dout.writeUTF("Hello Server");  
9. dout.flush();  
10. dout.close();  
11. s.close();  
12. }catch(Exception e){System.out.println(e);}  
13. }  
14. }  

To execute this program open two command prompts and execute each program at each command prompt as displayed in the below figure.

After running the client application, a message will be displayed on the server console.

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Example of Java Socket Programming (Read-Write both side)

In this example, client will write first to the server then server will receive and print the text. Then server will write to the client and client will receive and print the text. The step goes on.

File: MyServer.java

1. import java.net.*;  
2. import java.io.*;  
3. class MyServer{  
4. public static void main(String args[])throws Exception{  
5. ServerSocket ss=new ServerSocket(3333);  
6. Socket s=ss.accept();  
7. DataInputStream din=new DataInputStream(s.getInputStream());  
8. DataOutputStream dout=new DataOutputStream(s.getOutputStream());  
9. BufferedReader br=new BufferedReader(new InputStreamReader(System.in));  
10.   
11. String str="",str2="";  
12. while(!str.equals("stop")){  
13. str=din.readUTF();  
14. System.out.println("client says: "+str);  
15. str2=br.readLine();  
16. dout.writeUTF(str2);  
17. dout.flush();  
18. }  
19. din.close();  
20. s.close();  
21. ss.close();  
22. }}  

File: MyClient.java

1. import java.net.*;  
2. import java.io.*;  
3. class MyClient{  
4. public static void main(String args[])throws Exception{  
5. Socket s=new Socket("localhost",3333);  
6. DataInputStream din=new DataInputStream(s.getInputStream());  
7. DataOutputStream dout=new DataOutputStream(s.getOutputStream());  
8. BufferedReader br=new BufferedReader(new InputStreamReader(System.in));  
9.   
10. String str="",str2="";  
11. while(!str.equals("stop")){  
12. str=br.readLine();  
13. dout.writeUTF(str);  
14. dout.flush();  
15. str2=din.readUTF();  
16. System.out.println("Server says: "+str2);  
17. }  
18.   
19. dout.close();  
20. s.close();  
21. }}