How Tomcat Works

The Socket Class

A socket is an endpoint of a network connection. A socket enables an application to read from and write to the network. Two software applications residing on two different computers can communicate with each other by sending and receiving byte streams over a connection. To send a message from your application to another application, you need to know the IP address as well as the port number of the socket of the other application. In Java, a socket is represented by the java.net.Socket class.

To create a socket, you can use one of the many constructors of the Socket class. One of these constructors accepts the host name and the port number:

public Socket (java.lang.String host, int port)

where host is the remote machine name or IP address and port is the port number of the remote application. For example, to connect to yahoo.com at port 80, you would construct the following Socket object:

new Socket ("yahoo.com", 80);

Once you create an instance of the Socket class successfully, you can use it to send and receive streams of bytes. To send byte streams, you must first call the Socket class's getOutputStream method to obtain a java.io.OutputStream object. To send text to a remote application, you often want to construct a java.io.PrintWriter object from the OutputStream object returned. To receive byte streams from the other end of the connection, you call the Socket class's getInputStream method that returns a java.io.InputStream.

The following code snippet creates a socket that can communicate with a local HTTP server (127.0.0.1 denotes a local host), sends an HTTP request, and receives the response from the server. It creates a StringBuffer object to hold the response and prints it on the console.

Socket socket = new Socket("127.0.0.1", "8080");
OutputStream os = socket.getOutputStream();
boolean autoflush = true;
PrintWriter out = new PrintWriter(socket.getOutputStream(), autoflush);
BufferedReader in = new BufferedReader(new InputStreamReader( socket.getInputstream() ));
// send an HTTP request to the web server            out.println("GET /index.jsp HTTP/1.1");             out.println("Host: localhost:8080");
out.println("Connection: Close");
out.println();
// read the response
boolean loop = true;
StringBuffer sb = new StringBuffer(8096);
while (loop) {
    if ( in.ready() ) {
        int i=0;
        while (i!=-1) {
            i = in.read();
            sb.append((char) i);
        }
        loop = false; }
        Thread.currentThread().sleep(50);
    }
    // display the response to the out console
System.out.println(sb.toString());
socket.close();

Note that to get a proper response from the web server, you need to send an HTTP request that complies with the HTTP protocol. If you have read the previous section, The Hypertext Transfer Protocol (HTTP), you should be able to understand the HTTP request in the code above.

Note: You can use the com.brainysoftware.pyrmont.util.HttpSniffer class included with this book to send an HTTP request and display the response. To use this Java program, you must be connected to the Internet. Be warned, though, that it may not work if you are behind a firewall.