HTTP Servlet Basics In this chapter: HTTP
HTTP Servlet Basics In this chapter: HTTP Basics The Servlet API Page Generation Server-Side Includes Servlet Chaining and Filters JavaServer Pages Moving On This chapter provides a quick introduction to some of the things an HTTP servlet can do. For example, an HTTP servlet can generate an HTML page, either when the servlet is accessed explicitly by name, by following a hypertext link, or as the result of a form submission. An HTTP servlet can also be embedded inside an HTML page, where it functions as a server-side include. Servlets can be chained together to produce complex effects one common use of this technique is for filtering content. Finally, snippets of servlet code can be embedded directly in HTML pages using a new technique called JavaServer Pages. Although the code for each of the examples in this chapter is available for down-load (as described in the Preface), we would suggest that for these first examples you deny yourself the convenience of the Internet and type in the examples. It should help the concepts seep into your brain. Don’t be alarmed if we seem to skim lightly over some topics in this chapter. Servlets are powerful and, at times, complicated. The point here is to give you a general overview of how things work, before jumping in and overwhelming you with all of the details. By the end of this jsp blog, we promise that you’ll be able to write servlets that do everything but make tea. HTTP Basics Before we can even show you a simple HTTP servlet, we need to make sure that you have a basic understanding of how the protocol behind the Web, HTTP, works. If you’re an experienced CGI programmer (or if you’ve done any serious server-side web programming), you can safely skip this section. Better yet, you might skim it to refresh your memory about the finer points of the GET and POST methods. If you are new to the world of server-side web programming, however, you should read this material carefully, as the rest of the book is going to assume that you understand HTTP. For a more thorough discussion of HTTP and its methods, see Web Client Programming by Clinton Wong (O’Reilly). Requests, Responses, and Headers HTTP is a simple, stateless protocol. A client, such as a web browser, makes a request, the web server responds, and the transaction is done. When the client sends a request, the first thing it specifies is an HTTP command, called a method, that tells the server the type of action it wants performed. This first line of the request also specifies the address of a document (a URL) and the version of the HTTP protocol it is using. For example:
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