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Trail: Custom Networking
Lesson: All About Sockets
All About Sockets
What Is a Socket?
Reading from and Writing to a Socket
Writing the Server Side of a Socket
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Reading from and Writing to a Socket

> Let's look at a simple example that illustrates how a program can establish a connection to a server program using the `Socket` class and then, how the client can send data to and receive data from the server through the socket.

> The example program implements a client, `EchoClient`, that connects to the Echo server. The Echo server simply receives data from its client and echoes it back. The Echo server is a well-known service that clients can rendezvous with on port 7.

> `EchoClient` creates a socket thereby getting a connection to the Echo server. It reads input from the user on the standard input stream, and then forwards that text to the Echo server by writing the text to the socket. The server echoes the input back through the socket to the client. The client program reads and displays the data passed back to it from the server:

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

> >     public class EchoClient {
> >         public static void main(String[] args) throws IOException {

> >             Socket echoSocket = null;
> >             PrintWriter out = null;
> >             BufferedReader in = null;

> >             try {
> >                 echoSocket = new Socket("taranis", 7);
> >                 out = new PrintWriter(echoSocket.getOutputStream(), true);
> >                 in = new BufferedReader(new InputStreamReader(
> >                                             echoSocket.getInputStream()));
> >             } catch (UnknownHostException e) {
> >                 System.err.println("Don't know about host: taranis.");
> >                 System.exit(1);
> >             } catch (IOException e) {
> >                 System.err.println("Couldn't get I/O for "
> >                                    + "the connection to: taranis.");
> >                 System.exit(1);
> >             }

> >     	BufferedReader stdIn = new BufferedReader(
> >                                        new InputStreamReader(System.in));
> >     	String userInput;

> >     	while ((userInput = stdIn.readLine()) != null) {
> >     	    out.println(userInput);
> >     	    System.out.println("echo: " + in.readLine());
> >     	}

> >     	out.close();
> >     	in.close();
> >     	stdIn.close();
> >     	echoSocket.close();
> >         }
> >     }

> Note that `EchoClient` both writes to and reads from its socket, thereby sending data to and receiving data from the Echo server.

> Let's walk through the program and investigate the interesting parts. The three statements in the `try` block of the `main` method are critical. These lines establish the socket connection between the client and the server and open a `PrintWriter` and a `BufferedReader` on the socket:

> >     echoSocket = new Socket("taranis", 7);
> >     out = new PrintWriter(echoSocket.getOutputStream(), true);
> >     in = new BufferedReader(new InputStreamReader(
> >                                  echoSocket.getInputStream()));

> The first statement in this sequence creates a new `Socket` object and names it `echoSocket`. The `Socket` constructor used here requires the name of the machine and the port number to which you want to connect. The example program uses the host name `taranis`. This is the name of a hypothetical machine on our local network. When you type in and run this program on your machine, change the host name to the name of a machine on your network. Make sure that the name you use is the fully qualified IP name of the machine to which you want to connect. The second argument is the port number. Port number 7 is the port on which the Echo server listens.

> The second statement gets the socket's output stream and opens a `PrintWriter` on it. Similarly, the third statement gets the socket's input stream and opens a `BufferedReader` on it. The example uses readers and writers so that it can write Unicode characters over the socket.

> To send data through the socket to the server, `EchoClient` simply needs to write to the `PrintWriter`. To get the server's response, `EchoClient` reads from the `BufferedReader`. The rest of the program achieves this. If you are not yet familiar with the Java platform's I/O classes, you may wish to read Basic I/O.

> The next interesting part of the program is the `while` loop. The loop reads a line at a time from the standard input stream and immediately sends it to the server by writing it to the `PrintWriter` connected to the socket:

> >     String userInput;

> >     while ((userInput = stdIn.readLine()) != null) {
> >         out.println(userInput);
> >         System.out.println("echo: " + in.readLine());
> >     }

> The last statement in the `while` loop reads a line of information from the `BufferedReader` connected to the socket. The `readLine` method waits until the server echoes the information back to `EchoClient`. When `readline` returns, `EchoClient` prints the information to the standard output.

> The `while` loop continues until the user types an end-of-input character. That is, `EchoClient` reads input from the user, sends it to the Echo server, gets a response from the server, and displays it, until it reaches the end-of-input. The `while` loop then terminates and the program continues, executing the next four lines of code:

> >     out.close();
> >     in.close();
> >     stdIn.close();
> >     echoSocket.close();

> These lines of code fall into the category of housekeeping. A well-behaved program always cleans up after itself, and this program is well-behaved. These statements close the readers and writers connected to the socket and to the standard input stream, and close the socket connection to the server. The order here is important. You should close any streams connected to a socket before you close the socket itself.

> This client program is straightforward and simple because the Echo server implements a simple protocol. The client sends text to the server, and the server echoes it back. When your client programs are talking to a more complicated server such as an HTTP server, your client program will also be more complicated. However, the basics are much the same as they are in this program:

> 1. Open a socket.
> 2. Open an input stream and output stream to the socket.
> 3. Read from and write to the stream according to the server's protocol.
> 4. Close the streams.
> 5. Close the socket.
> Only step 3 differs from client to client, depending on the server. The other steps remain largely the same.

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