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Apache Struts 2 Documentation
Unknown Handlers
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Unknown Handler stacks are available from Struts 2.1 on.

## Unkown Handlers

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:

## Stacking Unknown Handlers

Multiple unknown handlers can be defined, using the unknown-handler-stack tag:

When multiple unknown handlers are stacked like above, they will be called in the specified order, in the same cases when a single unknown handler would be called (when an unknown action, result, or method are executed), until one of them handles the requested action.

## Uknown Handlers Manager

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`