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Chapter

ervlets are great when your application requires a lot of real program 

ming to accomplish its task. As you've seen elsewhere in the book,

S

servlets can manipulate HTTP status codes and headers, use cookies,

track sessions, save information between requests, compress pages, access

databases, generate GIF images on the fly, and perform many other tasks

flexibly and efficiently. But, generating HTML with servlets can be tedious

and can yield a result that is hard to modify. That's where JSP comes in; it lets

you separate much of the presentation from the dynamic content. That way,

you can write the HTML in the normal manner, even using HTML specific

tools and putting your Web content developers to work on your JSP docu 

ments. JSP expressions, scriptlets, and declarations let you insert simple Java

code into the servlet that results from the JSP page, and directives let you

control the overall layout of the page. For more complex requirements, you

can wrap up Java code inside beans or define your own JSP tags.

Great. We have everything we need, right? Well, no, not quite. The

assumption behind a JSP document is that it provides a single overall presen 

tation. What if you want to give totally different results depending on the data

that you receive? Beans and custom tags, although extremely powerful and

flexible, don't overcome the limitation that the JSP page defines a relatively

fixed top level page appearance. The solution is to use both servlets and Jav 

aServer Pages. If you have a complicated application that may require several

substantially different presentations, a servlet can handle the initial request,

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