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Unknown Handlers are classes that implement the com.opensymphony.xwork2.UnknownHandler interface, and are called by the framework, when an unknown action, result, or method are executed. To define an unknown handler, create a class implementing the mentioned interface, and add a bean definition to your struts.xml:

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The default class that handles unknown handler stacks is com.opensymphony.xwork2.DefaultUnknownHandlerManager, a custom implementation of this class can be provided by implementing the interface com.opensymphony.xwork2.UnknownHandlerManager and setting the property struts.unknownHandlerManager