1. MT311 Java Application Development and Programming Languages Li Tak Sing ( 李德成 )

2. Networking There are two modes: connection oriented and connectionless. The connection oriented mode works like telephone lines. Before you can talk to another person over the phone, you need to establish a circuit connecting you and the other person. Then, you can use this circuit to transmit voice signal. The signal will always arrive in the same order as they are sent.

3. Connectionless mode The connectionless mode works like the post office. When you send a letter to another person, the post office will deliver the letter to that person. However, if you send many letters to the same person, there is no guarantee that the letter will arrive at the same order as they are sent.

4. IP address and Port Before we can communicate with a computer, we need to know how to locate that computer first. Computers are identified by its IP address in the internet. An IP address consists of four bytes and are usually expressed in the form of: xxx.xxx.xxx.xxx Since IP address is difficult to remember, we usually use host name instead.

5. Host name and domain name server Note that IP addresses and host names do not form a one to one relation. An IP address can map to zero, one or even more host names. Similarly, a host name can also map to zero, one or even more IP addresses.

6. port A computer can have many services. For example, a server would usually provide services like http, ftp, telnet etc. So when you want to talk to a computer, you need to specify which service you want to use. This is done by specifying the port number corresponding to the service.

7. port A port number is a 2 bytes integers starting from 1. Some common services use pre-defined port: ftp: 21, telnet: 23, http: 80 etc. So when two computers are connected using the connection oriented mode, the connection is identified uniquely by the ip addresses and the ports used by the two computer.

8. IP: 123.3.24.22IP: 202.40.219.235 port 3334 port 234 A connection is uniquely identified by the IP addresses and port numbers at the two ends.

9. Server The ip address and port number work like a telephone with some extension. So you want to be a server, it is similar to one who is wait for others to call. So the first thing he/she needs to do is to let others to know the telephone number and the extension number.

10. Server However, when other people want to talk to the server, there is no need to use a particular telephone and extension. You can use any one that is available.

11. Server If the server is a multithreaded server, then the server can be talking to a number of clients at a time.

12. IP: 123.3.24.22IP: 202.40.219.235 port 3334 port 234 A connection is uniquely identified by the IP addresses and port numbers at the two ends. IP: 223.33.44.103 port 8242

13. Server So, even though a server is waiting for requests at one port only, it is possible for it to entertain many requests at the same time.

14. Socket Socket is the term used to describe one end of a connection. So to create a connection between two computers, you first need to have two sockets at the two ends.

15. Sockets There are two kinds of sockets, one on the server and one on the client. As mentioned earlier, when you need to specify the port to be used on the server side. In Java, ServerSocket is used to model sockets on the server side.

16. ServerSocket the constructor public ServerSocket(int port) throws IOException can be used to create a socket on the server side with the given port. The constructor will throw IOException if there is a problem in creating the server socket.

17. Socket After the ServerSocket is created, then we can make the server listen to the port by invoking the accept method of ServerSocket: ServerSocket sSocket=new ServerSocket(4444); Socket socket=sSocket.accept(); When the accept method is invoked, the thread is blocked until there is a request from a client for this service.

18. Socket Then, a Socket will be returned. As said earlier, a socket is one end of a network connection. To start the communication, you need to get an input stream and an output stream by using the method: InputStream getInputStream() throws IOException OutputStream getOutputStream() throws IOException

19. A single threaded server So a single threaded server would do something like this: ServerSocket sSocket=new ServerSocket(11111); while (true) { Socket socket=sSocket.accept(); OutputStream outputStream=socket.getOutputStream(); InputStream inputStream=socket.getInputStream();.... // we can do something about the input and output. }

20. The client On the client side, there is no need to create a ServerSocket. Instead, we would use a constructor of Socket: public Socket(String host, int port) throws UnknownHostException, IOException Note that we need to specify the host and the port number here. To specify the host, you can either use the IP address or the host name.

21. localhost If you have two computers, you can use one computer as the server and the other as the client to test your network program. However, if you only has one computer, how can you test a network program? You can do that by using the loopback address: 127.0.0.1 which refers to your computer.

22. A simple server Let's write a simple single threaded server which accepts two integers from a client and then returns the sum of the two integers to the client.

23. public class MyServer { static public void main(String st[]) { try { java.net.ServerSocket sSocket=new java.net.ServerSocket(11111); while (true) { java.net.Socket socket=sSocket.accept(); java.io.OutputStream output=socket.getOutputStream(); java.io.InputStream input=socket.getInputStream(); java.io.DataOutputStream dout=new java.io.DataOutputStream(output); java.io.DataInputStream din=new java.io.DataInputStream(input); int a1=din.readInt();

24. int a2=din.readInt(); dout.writeInt(a1+a2); dout.close(); din.close(); socket.close(); } catch (Exception e) { }

25. public class MyClient { public static void main(String st[]) { try { java.net.Socket socket=new java.net.Socket("127.0.0.1",11111); java.io.OutputStream output=socket.getOutputStream(); java.io.InputStream input=socket.getInputStream(); java.io.DataOutputStream dout=new java.io.DataOutputStream(output); java.io.DataInputStream din=new java.io.DataInputStream(input); dout.writeInt(3); dout.writeInt(4); int result=din.readInt(); System.out.println("the result is "+result); } catch (Exception e) { }

26. A multithreaded server The previous server can only handle one request at a time. This does not seem to be a problem as each request does not last for too long. However, if a server needs to provide some computationally intensive service, there is a need to have a multithreaded server or else a client may have to wait for a long time before it is served.

27. Multithreaded server In a multithreaded server, when there is an incoming request, we would create a thread to handle the request. Now, lets rewrite the last simple server to be a multithreaded server.

28. Multithreaded server First we create a class MyThread which is used to handle a request. In our design, the constructor of MyThread has a parameter of type Socket which is the socket returned from the accept method of ServerSocket. Then the run method of MyThread would perform the necessary operations.

29. public class MyThread extends Thread { private java.net.Socket socket; /** Creates a new instance of MyThread */ public MyThread(java.net.Socket socket) { this.socket=socket; } public void run() { try { java.io.OutputStream output=socket.getOutputStream(); java.io.InputStream input=socket.getInputStream(); java.io.DataOutputStream dout=new java.io.DataOutputStream(output);

30. java.io.DataInputStream din=new java.io.DataInputStream(input); int a1=din.readInt(); int a2=din.readInt(); dout.writeInt(a1+a2); dout.close(); din.close(); socket.close(); } catch (Exception e) {} } }

31. public class MultiThreadedServer { static public void main(String st[]) { try { java.net.ServerSocket sSocket=new java.net.ServerSocket(11111); while (true) { java.net.Socket socket=sSocket.accept(); MyThread thread=new MyThread(socket); thread.start(); } catch (Exception e) { }