Java Servlet Programming, 2nd Edition

Chapter 4. Retrieving Information

To build a successful web application, you often need to know a lot about the environment in which it is running. You may need to find out about the server that is executing your servlets or the specifics of the client that is sending requests. And no matter what kind of environment the application is running in, you most certainly need information about the requests that the application is handling.

A number of methods provide servlets access to this information. For the most part, each method returns one specific result. Compared this to the way environment variables are used to pass a CGI program its information, the servlet approach has several advantages:

• Stronger type checking. Servlets get more help from the compiler in catching errors. A CGI program uses one function to retrieve its environment variables. Many errors cannot be found until they cause runtime problems. Let’s look at how a CGI program and a servlet find the port on which its server is running.

  A CGI script written in Perl calls:

  $port = $ENV{'SERVER_PORT'};

  where $port is an untyped variable. A CGI program written in C calls:

  char *port = getenv("SERVER_PORT");

  where port is a pointer to a character string. The chance for accidental errors is high. The environment variable name could be misspelled (it happens often enough) or the datatype might not match what the environment variable returns.

  A servlet, on the other hand, calls:

  int port = req.getServerPort()

  This eliminates a lot ...