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22

Chapter 2 First Servlets

Listing 2.1 ServletTemplate.java 

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class ServletTemplate extends HttpServlet {

  public void doGet(HttpServletRequest request,

                    HttpServletResponse response)

      throws ServletException, IOException {

    // Use "request" to read incoming HTTP headers

    // (e.g. cookies) and HTML form data (e.g. data the user

    // entered and submitted).

    // Use "response" to specify the HTTP response status

    // code and headers (e.g. the content type, cookies).

    PrintWriter out = response.getWriter();

    // Use "out" to send content to browser

  }

}

Both of these methods take two arguments: an 

HttpServletRequest

 and

an 

HttpServletResponse

. The 

HttpServletRequest

 has methods by

which you can find out about incoming information such as form data, HTTP

request headers, and the client's hostname. The 

HttpServletResponse

 lets

you specify outgoing information such as HTTP status codes (200, 404, etc.),

response headers (

Content Type

, 

Set Cookie

, etc.), and, most importantly,

lets you obtain a 

PrintWriter

 used to send the document content back to the

client. For simple servlets, most of the effort is spent in 

println

 statements

that generate the desired page. Form data, HTTP request headers, HTTP

responses, and cookies will all be discussed in detail in the following chapters.

Since 

doGet

 and 

doPost

 throw two exceptions, you are required to

include them in the declaration. Finally, you have to import classes in

java.io

 (for 

PrintWriter

, etc.), 

javax.servlet

 (for 

HttpServlet

, etc.),

and 

javax.servlet.http

 (for 

HttpServletRequest

 and 

HttpServlet 

Response

). 

Strictly speaking, 

HttpServlet

 is not the only starting point for servlets, since

servlets could, in principle, extend mail, FTP, or other types of servers. Servlets

for these environments would extend a custom class derived from 

Generic 

Servlet

, the parent class of 

HttpServlet

. In practice, however, servlets are

used almost exclusively for servers that communicate via HTTP (i.e., Web and

application servers), and the discussion in this book will be limited to this usage.

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.