Programming Jakarta Struts, Second Edition

Using ActionErrors

Earlier in this chapter in Example 7-2, we saw that the validate() method returns an ActionErrors object. The ActionErrors class encapsulates one or more errors that have been discovered by the application. Each problem discovered is represented by an instance of org.apache.struts.action.ActionError.

Tip

It should be pointed out that in Example 7-2, instances of the ActionMessage class and not ActionError were added to the ActionErrors collection. ActionMessage is actually the parent of ActionError. We will talk more about the usage of the ActionMessage class in the next section. For now, when you see ActionError mentioned, just know that you can substitute ActionMessage in most cases.

An ActionErrors object has request scope. Once an instance is created and populated by the validate( ) method, it is stored into the request. Later, the JSP page can retrieve the object from the request and use the ActionError objects contained within it to display error messages to the user.

Tip

The Struts framework includes several JSP custom tags that make retrieving and displaying the messages very easy. Two of them, ErrorsTag and MessagesTag, are discussed in Chapter 8.

An instance of ActionErrors can be instantiated in the validate() method and populated by adding instances of the ActionMessage class to it. The LoginForm from Example 7-2 demonstrated this and is illustrated again here for convenience:

public ActionErrors validate(ActionMapping mapping, HttpServletRequest request){ ...