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2.9 Debugging Servlets

51

your servlet, your first task is to discover exactly how far the 

servlet got before it failed and to gather some information about 

the key data structures during the time period just before it 

failed. Simple 

println

 statements are surprisingly effective for 

this purpose. If you are running your servlets on a server that 

you cannot easily halt and restart, then do your debugging with 

the JSWDK, Tomcat, or the Java Web Server on your personal 

machine, and save deployment to the real server for later.

4. Use the log file.

The 

HttpServlet

 class has a method called 

log

 that lets you 

write information into a logging file on the server. Reading debug 

ging messages from the log file is a bit less convenient than watch 

ing them directly from a window as with the previous approach, 

but using the log file does not require stopping and restarting the 

server. There are two variations of this method: one that takes a 

String

, and the other that takes a 

String

 and a 

Throwable

 (an 

ancestor class of 

Exception

). The exact location of the log file is 

server specific, but it is generally clearly documented or can be 

found in subdirectories of the server installation directory.

5. Look at the request data separately.

Servlets read data from the HTTP request, construct a response, 

and send it back to the client. If something in the process goes 

wrong, you want to discover if it is because the client is sending 

the wrong data or because the servlet is processing it incorrectly. 

The 

EchoServer

 class, shown in Section 16.12 (A Debugging 

Web Server), lets you submit HTML forms and get a result that 

shows you exactly how the data arrived at the server. 

6. Look at the response data separately.

Once you look at the request data separately, you'll want to do 

the same for the response data. The 

WebClient

 class, presented 

next in Section 2.10 (WebClient: Talking to Web Servers Inter 

actively), permits you to connect to the server interactively, 

send custom HTTP request data, and see everything that comes 

back, HTTP response headers and all.

7. Stop and restart the server.

Most full blown Web servers that support servlets have a desig 

nated location for servlets that are under development. Servlets in 

this location (e.g., the 

servlets

 directory for the Java Web 

Server) are supposed to be automatically reloaded when their 

associated class file changes. At times, however, some servers can 

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

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