Tutorials : Handling Messages, Errors and Exceptions in Struts 1.1 :

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:

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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