Tutorials :
: A very simple JSP-architecture Part 2
A very simple JSP-architecture Part 2
by Keld H. Hansen
In my last article
the architecture and code for a simple "customer-lookup" application was presented.
The main idea was to build on the MVC-architecture and to implement this using only JSP-pages and
JavaBeans. Furthermore each window of the application should be handled by a controller/view-pair of
JSP-pages, where the controller was always invoked with a "command", that told the controller which
function to handle.
In the current article I'll show how simple it is to add a couple of new functions to this page,
but inspired by a few flaws in the code presented in the first article I'll also discuss briefly the
important topic "portability of JSP-pages". Also, while we're into the application error arena, I'll
give a few tips on how to ease debugging
JSP-pages.
Are JSP-pages portable?
Since Java is designed to be a portable language (you know--"Write once, run anywhere") you might
think that JSP-pages are also portable. Since a JSP-page eventually is being translated into a servlet,
which is Java class-file, it will be just as easy to port it to other computers and operating system
as any other class file. But the conversion from a JSP-file to a servlet may be handled somewhat
differently by today's servlet engines. I tested my code on an older version of JRun, and after the
article was published I had the opportunity to test the code against two other servlet engines:
WebSphere and SUN's JSWDK. This revealed two flaws in my code:
I have observed minor differences like this many times before when working with the JSP-technology,
and even if the two actual cases are rather simple to debug and correct, you should always be prepared
to do modifications in a JSP-application when moving to another servlet engine.
By the way, you may have wondered why I do not use upper case letters in filenames--for example
ctrlCustomerInfo.jsp--to make them more readable. The reason is that servlet engines--like the Java
language itself--are case-sensitive. But when writing an URL in the browser's address field you
normally prefer to write in lower case letters and most users do not expect an URL to be case
sensitive.
Debugging JSP-pages
The major problem with using JSP-pages in a web-application is actually how to locate some of the
syntax errors that always will occur when you're coding--no matter how hard you try to avoid them.
Below I'll give you a few tips to make life easier for you.
- Write some small test JSP-pages where you deliberately make syntax errors. Now look at the reaction
of the servlet engine. Note that most of the errors are reasonably well diagnosed, but some of them
result in absolutely useless error messages. Take your time to experiment, and learn your servlet
engine's behavior.
The reason for the poor error messages, is that they relate to the servlet that has been generated.
Line numbers for example will therefore point to the servlet code, not to your JSP-code.
Here are a couple of test cases to try which will challenge most engines:
- omit the ending quote of a text
- omit an ending curly parenthesis
- omit an ending "%>"
-
Use editors that can color-code the elements of your JSP-pages. With this feature enabled an
unfinished text string, for example, will show clearly.
-
Test your pages often while you are coding them. Don't code more than 10 lines before you test
them. If you do this and suddenly get an error in your page, and you can not spot the error right
away, you simply remove the new statements one by one (by using your editors "Undo" tool if available)
until the error disappears.
- If an error is really nasty to find, then you might look into the servlet code that has been
generated by the servlet engine. If you have a line number to look for you might be able to find the
error.
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