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Sockets

Sockets. Clients and servers. Computers can communicate with one another in many ways The best approach is the client-server model One computer acts as a server The other computers (clients) connect to it

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Sockets

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  1. Sockets

  2. Clients and servers • Computers can communicate with one another in many ways • The best approach is the client-server model • One computer acts as a server • The other computers (clients) connect to it • Think of it as a telephone call--client(s) have to know the phone number (IP address) of the server • Once the connection is made, there is no necessary difference between client and server

  3. Creating the client socket • import java.net.*; • Socket s; // This is a "client" socket • int PORT = 5654; • can be any number between 1024 and 9999 • s = new Socket("localhost", PORT); • or • s = new Socket("153.104.220.117", PORT); • use ipconfig to get the IP address of your computer

  4. Aside: Java I/O • Java I/O is very flexible but very complicated • Everything is done as a "stream" (a flow of data) • It's tricky to figure out how to set up a stream to do what you want • Once you have it figured out, it's "cookbook"--you can just do it the same way every time • I wanted to connect sockets, and to read and write lines

  5. Using the client socket • BufferedReader input = new BufferedReader( new InputStreamReader( s.getInputStream( ) ) ); • PrintStream output = new PrintStream(s.getOutputStream( ) ); • output.println("Hello, Server!"); • String response = input.readLine( );

  6. Creating the server socket • import java.net.*; • int PORT = 5654; // Same number as before • ServerSocket ss = new ServerSocket(PORT); • Socket s = ss.accept( );

  7. Using the server socket • BufferedReader input = new BufferedReader( new InputStreamReader( s.getInputStream( ) ) ); • PrintStream output = new PrintStream(s.getOutputStream( ) ); • String message = input.readLine( ); • output.println("Hello, Client!"); • This is just like using the client socket!

  8. Complications I • You have to do things in the proper order • The server must be started before the client • ss.accept( ) (to accept a call from a client) and input.readLine( ) (to get data) both block • You must be very careful to write only when the other program is reading • Proper use of Threads can solve some of these problems

  9. Complications II • There are many possible Exceptions to be caught • For simple programs, it's OK to just exit when an Exception occurs • Remember to close all your streams • The server socket must be closed, or it may persist "forever" • This will mess up later runs of the programs • The finally clause of a try statement is the best place to close the server socket

  10. Summary • Network communication is complicated, but it's a lot simpler in Java than in other languages • You have to be extremely careful to do things in the correct order (Threads can help) • Chapter 8 of your textbook explains all this in the context of a High Score Server • The textbook uses deprecated methods in setting up streams--use mine instead

  11. The End

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