Special Note: Because so many different things can cause this error, it is recommended that you turn your error logging/debugging levels on your web server to a high level of verbosity to see the underlying problems in trying to instantiate the action class you have written and associated with the specified action xxxx through an action mapping in the struts-config.xml file.

Your Action class specified in the struts-config.xml file under the class attribute of the action mapping for action xxxx cannot be found for a variety of reasons, including (but not limited to):

• Failure to place compiled .class file for the action in the classpath (needs to be under WEB-INF/classes with the appropriate directory structure underneath this that matches the package your Action class belongs to).
• Package spelling or hierarchy specified in your action class itself (using the package keyword) does not match the spelling or complete package hierachy specified for your action class in the class attribute of the action in struts-config.xml.
• Action class specified in the /xxxx action mapping in the struts-config.xml file (class attribute does not extend (directly or indirectly) from the Action class. In other words, your custom Action class does not extend off the Struts-provided Action class or off of another class that eventually extends the Action class (such as DispatchAction).
• Problem in your classpath, such as web server not being able to find ApplicationResources.properties files in the WEB-INF/classes/ directory or specified subdirectory.
• Problem in struts-config.xml file with action mapping.
• Problem with data-sources.xml file.

• This error occurs typically when the specified Java .class file cannot be located in the classpath. If this occurs at runtime of a web application (error shows on browser rather than a rendered page), this typically means that specified class is not in the web server's classpath (made up primarily of /WEB-INF/classes and /WEB-INF/lib contents). Note that the NoClassDefFoundError in general typically indicates lack of the specified class in the relevant classpath. In this particular case the missing class would be ActionForm.class
• This error is sometimes seen when one or more ActionForm.class instances are actually in the classpath. This most often occurs when ActionForm.class is made available correctly by placing struts.jar in the /WEB-INF/lib directory. When this library has been correctly placed and it is verified that ActionForm.class actually is present in the struts.jar file, the problem is either that more than one copy of ActionForm.class is in the classpath or (more likely) that duplicate versions of class files other than ActionForm are in the same classpath, causing confusion. This is especially true if a class that extends ActionForm is made available twice, such as in an .ear file that encompasses a .war file as well as in the .war file's own classpath (/WEB-INF/classes). This problem can be resolved by guaranteeing that there are no redundant classes, especially those related to Struts (directly from Struts or extensions of Struts), in the web application's view.

• Either the <servlet> tags for the Struts action servlet or the <servlet-mapping> tags for the .do extension mapping or both not present in the web.xml file. I saw a case where the web.xml file had no elements other than the root element and so this error was occurring.</td>
• Typos or spelling errors in the struts-config.xml can lead to this error message. For example, missing a slash ("/") on a closing tag can have this effect.
• Another element that must be present in the web.xml file is the load-on-startup element. This can be either an empty tag or can have an integer specified that indicates the priority of executing the associated servlet. The higher the number in the load-on-startup tags, the lower its priority.
• Another possibility, related to need to use load-on-startup tag, is that precompiling JSPs using Struts can lead to this message as well.

1. The key-value pair with specified key is not in ApplicationResources.properties file
2. ApplicationResources.properties (not in WEB-INF/classes directory in specified location)

• Explicitly trying to use message resources that are not available (such as ApplicationResources.properties not available)
• Implicitly trying to use message resources that are not available (such as using empty html:options tag instead of specifying the options in its body -- this assumes options are specified in ApplicationResources.properties file)
• XML parser issues -- too many, too few, incorrect/incompatible versions

Provide Struts with Resource Bundle - http://threebit.net/tutorials/ejb/general/ XML Parser Issues - http://www.mail-archive.com/struts-user@jakarta.apache.org/msg15779.html