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by Keld H. Hansen
Introduction
This article is about Jakarta Struts 1.1, the most popular
framework for building Java servlet applications. One of the
new, useful features in 1.1 is the possibility to specify
exception handling in the Struts config file. It's called
"declarative exception handling", and it's one of the
topics for this article.
You use exceptions when the application has encountered a
serious error. The error could be an SQL statement that fails,
or a connection to another computer that couldn't be
established. As a programmer, you should first of all record the
situation as precisely as you can. For example, writing
information to a log file. You might also have to fix up a few
things to keep the program state stable, but eventually you'll
also have to inform the user of the situation. So to move forward,
one issue in exception handling is how to handle messages,
and I'll therefore start by explaining how you generally should
handle messages for the end user in Struts. A special class of
messages are "validation messages", which are sent out
as the result of the validation of a form. If you're interested
in how to set up forms validation you may want to read my
article "Stepping through the Struts 1.1 Validator".
After having discussed message handling in Struts, we'll see how
Struts handles exceptions and start to investigate what the
benefits of declarative exception handling could be.
Handling of Messages
Let's set the scene: we're in the Action class and want
to send a message to the user at the browser. It could be
anything from a kind message like "Your data has been
stored successfully in the data base", or a warning about
some incorrect data entered in a form, to the real bad news like
"Database failure. Please contact support". Since the
message is going to be displayed in a browser, it will have to be
inserted in some way in a jsp-page.
What if we had to invent a message handling scheme ourselves? A
sensible solution could be to put our messages in some kind of
Java Collection, maybe a LinkedList:
Listing 1: Using a List to hold messages
List messages = new LinkedList();
messages.add("Data saved successfully");
messages.add("Please continue");
request.setAttribute("messages", messages);
In the jsp-page we could then easily show the messages using Struts tags:
Listing 2: Displaying the messages in a jsp-page
<logic:present name="messages">
<h3>Messages:</h3>
<logic:iterate id="msg" name="messages">
<bean:write name="msg"/><br>
</logic:iterate>
When the jsp-page displays we'll see this:
Messages:
Data saved successfully
Please continue
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No rocket science in this. However, Struts has a more powerful
mechanism for storing and handling messages based upon the
ActionMessage and ActionMessages
classes. If you're already a Struts user you may know the
ActionErrors and ActionError classes. They're
almost identical to the Message classes, and most of what
I'll explain below for the Message classes works equally
well for the Error classes. How we cope with having two
different set of classes I'll return to later.
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