struts2学习笔记(3)--截拦器(Interceptors)

• 简介

Many Actions share common concerns. Some Actions need input validated. Other Actions may need a file upload to be pre-processed. Another Action might need protection from a double submit. Many Actions need drop-down lists and other controls pre-populated before the page displays.

The framework makes it easy to share solutions to these concerns using an "Interceptor" strategy. When you request a resource that maps to an "action", the framework invokes the Action object. But, before the Action is executed, the invocation can be intercepted by another object. After the Action executes, the invocation could be intercepted again. Unsurprisingly, we call these objects  "Interceptors."

• Understanding Interceptors

Interceptors can execute code before and after an Action is invoked. Most of the framework's core functionality is implemented as Interceptors. Features like double-submit guards, type conversion, object population, validation, file upload, page preparation, and more, are all implemented with the help of Interceptors. Each and every Interceptor is pluggable, so you can decide exactly which features an Action needs to support.

Interceptors can be configured on a per-action basis. Your own custom Interceptors can be mixed-and-matched with the Interceptors bundled with the framework. Interceptors "set the stage" for the Action classes, doing much of the "heavy lifting" before the Action executes.

Action Lifecyle 
  [img]http://wishlife.iteye.com/upload/picture/pic/15449/bcaed8a7-b471-3b0e-8223-b008bdabd917.png [/img]

In some cases, an Interceptor might keep an Action from firing, because of a double-submit or because validation failed. Interceptors can also change the state of an Action before it executes.

The Interceptors are defined in a stack that specifies the execution order. In some cases, the order of the Interceptors on the stack can be very important.

• 常用截拦器介绍

1. alias :

Converts similar parameters that may be named differently between requests.



2. chain :

Makes the previous Action's properties available to the current Action. Commonly used together with <result type="chain"> (in the previous Action).



3. checkbox :

Adds automatic checkbox handling code that detect an unchecked checkbox and add it as a parameter with a default (usually 'false') value. Uses a specially named hidden field to detect unsubmitted checkboxes. The default unchecked value is overridable for non-boolean value'd checkboxes.



4. cookie :

Inject cookie with a certain configurable name / value into action. (Since 2.0.7.)



5. conversionError :

Adds conversion errors from the ActionContext to the Action's field errors



6. createSession :

Create an HttpSession automatically, useful with certain Interceptors that require a HttpSession to work properly (like the TokenInterceptor)



7. debugging :

Provides several different debugging screens to provide insight into the data behind the page.



8. execAndWait :

Executes the Action in the background and then sends the user off to an intermediate waiting page.



9. exception :

Maps exceptions to a result.



10. fileUpload :

An Interceptor that adds easy access to file upload support.



11. i18n :

Remembers the locale selected for a user's session.



12. logger :

Outputs the name of the Action.



13. store :

Store and retrieve action messages / errors / field errors for action that implements ValidationAware interface into session.



14. modelDriven :

If the Action implements ModelDriven, pushes the getModel Result onto the Value Stack.



15. scopedModelDriven :

If the Action implements ScopedModelDriven, the interceptor retrieves and stores the model from a scope and sets it on the action calling setModel.



16. params :

Sets the request parameters onto the Action.



17. prepare :

If the Action implements Preparable, calls its prepare method.



18. scope :

Simple mechanism for storing Action state in the session or application scope.



19. servletConfig :

Provide access to Maps representing HttpServletRequest and HttpServletResponse.



20. staticParams :

Sets the struts.xml defined parameters onto the action. These are the <param> tags that are direct children of the <action> tag.



21. roles :

Action will only be executed if the user has the correct JAAS role.



22. timer :

Outputs how long the Action takes to execute (including nested Interceptors and View)



23. token :

Checks for valid token presence in Action, prevents duplicate form submission.



24. tokenSession :

Same as Token Interceptor, but stores the submitted data in session when handed an invalid token



25. validation :

Performs validation using the validators defined in action-validation.xml



26. workflow :

Calls the validate method in your Action class. If Action errors are created then it returns the INPUT view.



27. N/A :

Removes parameters from the list of those available to Actions .



28. profiling :

Activate profiling through parameter .