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Patrick c01.tex V3 - 09/18/2009 12:15pm Page 12

Chapter 1: Building Web Applications in WebLogic

When the buffer ﬁlls, the response is committed, and the ﬁrst chunk of information is sent

to the browser. Once this commit occurs, the server will no longer honor

jsp:forward

HTTP header changes (such as redirects), or additional cookies. The server will generate an

IllegalStateException

if any of these operations is attempted after the buffer ﬁlls and the

response is committed.

The default size of the JSP output buffer is 8KB in WebLogic Server, which you can control using

the

page

directiveineachJSPpage:

<%@ page buffer="32kb" %>

Output buffering may also be turned off using this directive by specifying

none

for a size, but this

practice is not recommended.

Output buffers should be set to at least 32KB in most applications to avoid ﬁlling the buffer and

committing the response before the page is complete. The minor additional memory requirement

(32KB times the number of threads) is a small price to pay for correct error page handling and the

ability to add cookies and response headers at any point in large pages.

Best Practice

Always use output buffering in JSP pages. Increase the size of the buffer to at least

32KB to avoid redirect, cookie,

jsp:forward

, and error page problems.

JSP Pages Have Unique Capabilities

Unique capabilities are available in JSP pages that are not present in servlets. The most important of

these is the ability to embed custom XML tags within the JSP page.

Custom tags provide a mechanism to interact with a custom developed Java class that encapsulates

business logic, presentation logic, or both. Custom tag elements are placed in the JSP page by the

developer and then parsed and preprocessed by the application server during the conversion from

JSP to servlet. The tag elements are converted by the server to the Java code required to interact

with the tag class and perform the desired function. Later in this chapter we discuss custom tags

and commonly used tag libraries in more detail and present best practices for their use in WebLogic

Server.

To summarize, JavaServer Pages technology is a scripting language used to create HTML responses.

JSP pages are converted to pure Java servlets by the application server during processing, and they

can perform nearly any task a pure Java servlet can perform. JSP pages also have unique directives,

features, and customization capabilities unavailable to servlets.

Why not use JSP for everything and forget servlets completely? Although it is possible to do

so, servlets often provide a better mechanism for implementing presentation-tier business

logic. Chapter 2 addresses this issue in detail and provides guidance for the proper use of each

technology.