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See the Interceptors page for an overview of how interceptors work.
Interceptors must implement the com.opensymphony.xwork2.interceptor.Interceptor interface.
The init method is called the after interceptor is instantiated and before calling intercept. This is the place to allocate any resources used by the interceptor.
The intercept method is where the interceptor code is written. Just like an action method, intercept returns a result used by Struts to forward the request to another web resource. Calling invoke on the parameter of type ActionInvocation will execute the action (if this is the last interceptor on the stack) or another interceptor.
Keep in mind that invoke will return after the result has been called (eg. after you JSP has been rendered), making it perfect for things like open-session-in-view patterns. If you want to do something before the result gets called, you should implement a PreResultListener.
Overwrite destroy to release resources on application shutdown.
Interceptors must be thread-safe!
A Struts 2 Action instance is created for every request and do not need to be thread-safe. Conversely, Interceptors are shared between requests and must be thread-safe.
The AbstractInterceptor class provides an empty implementation of init and destroy, and can be used if these methods are not going to be implemented.
Interceptors are declared using the interceptor element, nested inside the interceptors element. Example from struts-default.xml:
Assuming there is an action of type "MyAction", with a setDate(Date) method, this simple interceptor will set the date of the action to the current date: