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16.12 A Debugging Web Server

423

Figure 16 26 In Internet Explorer, repeatedly pressing the TAB key cycles the input 

focus among the first, third, and second text fields, in that order (as dictated by 

TABINDEX). In Netscape, the input focus would cycle among the first, second, and 

third fields, in that order (based on the order the elements appear on the page).

16.12 A Debugging Web Server

This section presents a mini  Web server  that is useful when you are trying

to understand the behavior of HTML forms. I used it for many of the exam 

ples earlier in the chapter. It simply reads all the HTTP data sent to it by the

browser, then returns a Web page with those lines embedded within a 

PRE

element. This server is also extremely useful for debugging servlets. When

something goes wrong, the first task is to determine if the problem lies in the

way in which you collect data or the way in which you process it. Starting the

EchoServer

 on, say, port 8088 of your local machine, then changing your

forms to specify 

http://localhost:8088/

 lets you see if the data being col 

lected is in the format you expect.

EchoServer

Listing 16.9 presents the top level server code. You typically run it from the

command line, specifying a port to listen on or accepting the default of 8088.

It then accepts repeated HTTP requests from clients, packaging all HTTP

data sent to it inside a Web page that is returned to the client. In most cases,

the server reads until it gets a blank line, indicating the end of 

GET

, 

HEAD

, or

most other types of HTTP requests. In the case of 

POST

, however, the server

checks the 

Content Length

 request header and reads that many bytes

beyond the blank line. 

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

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