Java Examples in a Nutshell, 3rd Edition

A Hello World Servlet

Example 20-1 is a listing of HelloNet.java , which implements a simple “Hello world” servlet, illustrated in Figure 20-1. The HelloNet servlet inherits from javax.servlet.http.HttpServlet and overrides the doGet( ) method to provide output in response to an HTTP GET request. It also overrides the doPost( ) method so it can respond to POST requests in the same way. The doGet( ) method outputs a string of HTML text. By default, this string is “Hello World”.

Figure 20-1. The output of the HelloNet servlet

If the HelloNet servlet can determine a username, however, it greets the user by name. The servlet looks for a username in two places, starting in the HTTP request (an HttpServletRequest object), as the value of a parameter named username. If the servlet cannot find a request parameter with this name, it looks for an HttpSession object associated with the request and sees if that object has an attribute named username. Servlet-enabled web servers (i.e., servlet containers) provide a session-tracking layer on top of the stateless HTTP protocol. The HttpSession object allows a servlet to set and query named attributes with a single client session. Later in the chapter, we’ll see an example that takes advantage of the session-tracking ability of this HelloNet servlet.

Example 20-1. HelloNet.java

package je3.servlet; import javax.servlet.*; // Basic servlet ...