[Apache Struts Wiki] New: ActionErrorsAndActionMessages

[de...@struts.apache.org]

   Date: 2004-06-23T06:54:23
   Editor: 68.79.151.2 <>
   Wiki: Apache Struts Wiki
   Page: ActionErrorsAndActionMessages
   URL: http://wiki.apache.org/struts/ActionErrorsAndActionMessages

   Attempt to clarify differences between *errors and *messages

New Page:

With the many [http://jakarta.apache.org/struts/api/deprecated-list.html deprecations] related to the ActionErrors and ActionError classes, there are a variety of questions relating to:
 * the relationship between [http://jakarta.apache.org/struts/api/org/apache/struts/action/ActionMessages.html ActionMessages] and [http://jakarta.apache.org/struts/api/org/apache/struts/action/ActionErrors.html ActionErrors] and between [http://jakarta.apache.org/struts/api/org/apache/struts/action/ActionMessage.html ActionMessage] and [http://jakarta.apache.org/struts/api/org/apache/struts/action/ActionError.html ActionError]
 * the differences between [http://jakarta.apache.org/struts/api/org/apache/struts/action/Action.html#saveErrors(javax.servlet.http.HttpServletRequest,%20org.apache.struts.action.ActionMessages) Action.saveErrors(...)] and [http://jakarta.apache.org/struts/api/org/apache/struts/action/Action.html#saveMessages(javax.servlet.http.HttpServletRequest,%20org.apache.struts.action.ActionMessages) Action.saveMessages(...)]
 * the differences between the [http://jakarta.apache.org/struts/userGuide/struts-html.html#errors html:errors] JSP tag and the [http://jakarta.apache.org/struts/userGuide/struts-html.html#messages html:messages] JSP tag.

To summarize, the difference between the classes ActionErrors/ActionError/ActionMessages/ActionMessage has '''absolutely nothing''' to do with the difference in behavior in Action.saveErrors(...) and Action.saveMessages(...)  

The difference between the classes is zero -- all behavior in ActionErrors was pushed up into ActionMessages and all behavior in ActionError was pushed up into ActionMessage.  This was done in the attempt to clearly signal that these classes can be used to pass any kind of messages from the controller to the view -- errors being only one kind of message.

The difference between saveErrors(...) and saveMessages(...) is simply the attribute name under which the ActionMessages object is stored, providing two convenient default locations for storing controller messages for use by the view.  If you look more closely at the html:errors and html:messages tags, you can actually use them to get an ActionMessages object from any arbitrary attribute name in any scope.

While we're clarifying, the difference between html:errors and html:messages is purely in syntax and model -- both tags *default* to look for an ActionMessages object under [http://jakarta.apache.org/struts/api/org/apache/struts/Globals.html#ERROR_KEY Globals.ERROR_KEY] despite the difference in names.  I wasn't part of the  history, but I'm assuming that around the same time that people were realizing that there's more than one kind of message to pass, they also realized that sometimes you want more flexibility in displaying them.  html:messages provides more flexibility at the cost of more typing.

I hope this helps to clarify things.  I would strongly encourage people to have a look inside the Struts source code, as it's really quite clear when you look under the hood.  You can see what happens in validation by examining the "processValidate" method in RequestProcessor:
http://cvs.apache.org/viewcvs.cgi/jakarta-struts/src/share/org/apache/struts/action/RequestProcessor.java?view=markup

You can see what happens with saveErrors and saveMessages by examining those methods in Action
http://cvs.apache.org/viewcvs.cgi/jakarta-struts/src/share/org/apache/struts/action/Action.java?view=markup

You can see what the tags do by looking at their respective source files:
http://cvs.apache.org/viewcvs.cgi/jakarta-struts/src/share/org/apache/struts/taglib/html/MessagesTag.java?view=markup
http://cvs.apache.org/viewcvs.cgi/jakarta-struts/src/share/org/apache/struts/taglib/html/ErrorsTag.java?view=markup


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