UML Weekend Crash Course™ - To Parent Directory

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Sunday Afternoon
and time. A JSP page is composed of your Web content, such as HTML, with Java code intermixed
to insert the dynamic content. Comparing the Java servlet in Listing 27-2 and the
JSP page in Listing 27-3, you can probably see why JSP code is usually far easier to write
and develop than a servlet for producing Web content. A JSP page generally has less complicated
Java logic, is usually easier to read, and is much easier to maintain. A JSP page also
doesn’t need to be compiled by the programmer and may be easier to deploy into the Web
server, which makes the development and maintenance process a bit simpler. Other template
page technologies like ASP, PHP, and Cold Fusion work in a similar way, although each has
its own specific features, advantages, and disadvantages.
It is worth noting that JavaServer Pages turn into servlets. The JSP container class automatically
writes a Java servlet much like the one in Listing 27-2 (although significantly
harder to read) that has the same functionality as the JSP page that you wrote. All requests
for your JSP page will actually be handled by the Java servlet that represents it. Thus, JSP
is simply an easy way to write a Java servlet without having to write as much Java code.
The UML state diagram in Figure 27-4 explains this. The programmer writes a JSP. At or
before the first request for that JSP, the JSP container automatically writes the servlet that
represents that JSP and compiles it. From this point on, the lifecycle of a JSP is the same as
a servlet. When a request comes in, the servlet will be instantiated and the same instance of
the servlet will be used for all requests until the JSP container decides that the servlet
should be unloaded, usually due to the container shutting down or a long period with no
requests for that JSP. If the programmer changes the JSP, that servlet will be permanently
unloaded and the lifecycle starts over with the new version of the JSP being translated.
JSP Written By Programmer
JSP Page
JSP Container Translates JSP
Servlet Class
(Not Instantiated)
Final
State
JSP Removed Or Modified
Servlet Class (Instantiated
and initialized)
HTTP Request Received
JSP Container Unloads JSP
HTTP Request Received/Request Serviced
Figure 27-4 UML state diagram, JSP Lifecycle
You may think that writing only JSPs, as opposed to writing servlets, would be easier,
but that isn’t actually the case. If you need to write a servlet or JSP that primarily generates
Web content, then it will almost always be easier to write it as a JSP. If you need to
write a servlet or JSP that has a lot of logic and generates very little or no content, then it
will usually be easier to write it as a servlet.