can prevent much of 

the page from being viewed. Even better, use a formal HTML 

validator on the servlet's output. See Section 2.5 (Simple 

HTML Building Utilities) for a discussion of this approach.

2. Return error pages to the client. 

Sometimes certain classes of errors can be anticipated by the serv 

let. In these cases, the servlet should build descriptive information 

about the problem and return it to the client in a regular page or 

by means of the 

sendError

 method of 

HttpServletResponse

. 

See Chapter 6 (Generating the Server Response: HTTP Status 

Codes) for details on 

sendError

. For example, you should plan 

for cases when the client forgets some of the required form data 

and send an error page detailing what was missing. Error pages 

are not always possible, however. Sometimes something unex 

pected goes wrong with your servlet, and it simply crashes. The 

remaining approaches help you in those situations.

3. Start the server from the command line.

Most Web servers execute from a background process, and this 

process is often automatically started when the system is 

booted. If you are having trouble with your servlet, you should 

consider shutting down the server and restarting it from the 

command line. After this, 

System.out.println

 or 

Sys 

tem.err.println

 calls can be easily read from the window in 

which the server was started. When something goes wrong with 

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

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