Lesson 1: Socket Communications
Java Programming Language Basics, Part 1, finished with a simple network communications example using the Remote Method Invocation (RMI) application programming interface (API). The RMI example allows multiple client programs to communicate with the same server program without any explicit code to do this because the RMI API is built on sockets and threads.
This lesson presents a simple sockets-based program to introduce the concepts of sockets and multi-threaded programming. A multi-threaded program performs multiple tasks at one time such as fielding simultaneous requests from many client programs.
What are Sockets and Threads?
A socket is a software endpoint that establishes bidirectional communication between a server program and one or more client programs. The socket associates the server program with a specific hardware port on the machine where it runs so any client program anywhere in the network with a socket associated with that same port can communicate with the server program.
A server program typically provides resources to a network of client programs. Client programs send requests to the server program, and the server program responds to the request.
One way to handle requests from more than one client is to make the server program multi-threaded. A multi-threaded server creates a thread for each communication it accepts from a client. A thread is a sequence of instructions that run independently of the program and of any other threads.
Using threads, a multi-threaded server program can accept a connection from a client, start a thread for that communication, and continue listening for requests from other clients.
About the Examples
The examples for this lesson consist of two versions of the client and server program pair adapted from the FileIO.java application presented in Part 1, Lesson 6: File Access and Permissions.
Example 1 sets up a client and server communication between one server program and one client program. The server program is not multi-threaded and cannot handle requests from more than one client.
Example 2 converts the server program to a multi-threaded version so it can handle requests from more than one client.
Example 1: Client-Side Behavior
The client program presents a simple user interface and prompts for text input. When you click the Click Me button, the text is sent to the server program. The client program expects an echo from the server and prints the echo it receives on its standard output.
Example 1: Server-Side Behavior
The server program presents a simple user interface, and when you click the Click Me button, the text received from the client is displayed. The server echoes the text it receives whether or not you click the Click Me button.
Example 1: Compile and Run
To run the example programs, start the server program first. If you do not, the client program cannot establish the socket connection. Here are the compiler and interpreter commands to compile and run the example.
javac SocketServer.java javac SocketClient.java java SocketServer java SocketClient Example 1: Server-Side Program
The server program establishes a socket connection on Port 4321 in its listenSocket method. It reads data sent to it and sends that same data back to the server in its actionPerformed method.
listenSocket Method
The listenSocket method creates a ServerSocket object with the port number on which the server program is going to listen for client communications. The port number must be an available port, which means the number cannot be reserved or already in use. For example, Unix systems reserve ports 1 through 1023 for administrative functions leaving port numbers greater than 1024 available for use.
public void listenSocket() < try< server = new ServerSocket(4321); >catch (IOException e)
listenSocket Socket server.accept Socket
listenSocket BufferedReader client PrintWriter