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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."
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.
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.
With most web applications, we find ourselves wanting to apply the same set of Interceptors over and over again. Rather than reiterate the same list of Interceptors, we can bundle these Interceptors together using an Interceptor Stack.
struts-default.xml, we can see how it's done.
struts-default.xml is included in the application's configuration by default, all of the predefined interceptors and stacks are available "out of the box".
Interceptor classes are also defined using a key-value pair specified in the Struts configuration file. The names specified below come specified in struts-default.xml. If you extend the
struts-default package, then you can use the names below. Otherwise, they must be defined in your package with a name-class pair specified in the <interceptors> tag.
Converts similar parameters that may be named differently between requests.
Makes the previous Action's properties available to the current Action. Commonly used together with <result type="chain"> (in the previous Action).
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.
Inject cookie with a certain configurable name / value into action. (Since 2.0.7.)
Transfer cookies from action to response (Since 2.3.15.)
Adds conversion errors from the ActionContext to the Action's field errors
Create an HttpSession automatically, useful with certain Interceptors that require a HttpSession to work properly (like the TokenInterceptor)
Provides several different debugging screens to provide insight into the data behind the page.
When devMode is on, logs all unknown or deprecated settings
Executes the Action in the background and then sends the user off to an intermediate waiting page.
Maps exceptions to a result.
An Interceptor that adds easy access to file upload support.
Remembers the locale selected for a user's session.
Outputs the name of the Action.
Store and retrieve action messages / errors / field errors for action that implements ValidationAware interface into session.
If the Action implements ModelDriven, pushes the
If the Action implements ScopedModelDriven, the interceptor retrieves and stores the model from a scope and sets it on the action calling
Sets the request parameters onto the Action.
If the Action implements Preparable, calls its
Simple mechanism for storing Action state in the session or application scope.
Provide access to Maps representing HttpServletRequest and HttpServletResponse.
Action will only be executed if the user has the correct JAAS role.
Outputs how long the Action takes to execute (including nested Interceptors and View)
Checks for valid token presence in Action, prevents duplicate form submission.
Same as Token Interceptor, but stores the submitted data in session when handed an invalid token
Performs validation using the validators defined in action-validation.xml
Removes parameters from the list of those available to Actions
Activate profiling through parameter
Like the checkbox interceptor detects that no value was selected for a field with multiple values (like a select) and adds an empty parameter
MethodFilterInterceptor is an abstract
Interceptor used as
a base class for interceptors that will filter execution based on method
names according to specified included/excluded method lists.
Settable parameters are as follows:
NOTE: If method name are available in both includeMethods and excludeMethods, it will be considered as an included method: includeMethods takes precedence over excludeMethods.
Interceptors that extends this capability include:
Interceptor's parameter could be overridden through the following ways :
In the first method, the whole default stack is copied and the parameter then changed accordingly.
In the second method, the
interceptor-ref refers to an existing interceptor-stack, namely
defaultStack in this example, and override the
excludeMethods attribute. Note that in the
param tag, the name attribute contains a dot (.) the word before the dot(.) specifies the interceptor name whose parameter is to be overridden and the word after the dot (.) specifies the parameter itself. The syntax is as follows:
Note also that in this case the
interceptor-ref name attribute is used to indicate an interceptor stack which makes sense as if it is referring to the interceptor itself it would be just using Method 1 describe above.
Parameters override are not inherited in interceptors, meaning that the last set of overridden parameters will be used. For example, if a stack overrides the parameter "defaultBlock" for the "postPrepareParameterFilter" interceptor as:
and an action overrides the "allowed" for "postPrepareParameterFilter":
Then, only "allowed" will be overridden for the "postPrepareParameterFilter" interceptor in that action, the other params will be null.
Interceptors provide an excellent means to wrap before/after processing. The concept reduces code duplication (think AOP).
Note that some Interceptors will interrupt the stack/chain/flow ... so the order is very important.
com.opensymphony.xwork2.interceptor.PreResultListener will run after the Action executes but before the Result executes.