Interceptors are conceptually the same as servlet filters or the JDKs Proxy class. Interceptors allow for crosscutting functionality to be implemented separately from the action as well as the framework. You can achieve the following using interceptors:

• Providing preprocessing logic before the action is called.
• Providing postprocessing logic after the action is called.
• Catching exceptions so that alternate processing can be performed.

Many of the features provided in the Struts2 framework are implemented using interceptors;

Examples include exception handling, file uploading, lifecycle callbacks, etc. In fact, as Struts2 emphasizes much of its functionality on interceptors, it is not likely to have 7 or 8 interceptors assigned per action.

Struts 2 Framework Interceptors
Struts 2 framework provides a good list of out-of-the-box interceptors that come preconfigured and ready to use. Few of the important interceptors are listed below:

S.No	Interceptor & Description
1	alias Allows parameters to have different name aliases across requests.
2	checkbox Assists in managing check boxes by adding a parameter value of false for check boxes that are not checked.
3	conversionError Places error information from converting strings to parameter types into the action's field errors.
4	createSession Automatically creates an HTTP session if one does not already exist.
5	debugging Provides several different debugging screens to the developer.
6	execAndWait Sends the user to an intermediary waiting page while the action executes in the background.
7	exception Maps exceptions that are thrown from an action to a result, allowing automatic exception handling via redirection.
8	fileUpload Facilitates easy file uploading.
9	i18n Keeps track of the selected locale during a user's session.
10	logger Provides simple logging by outputting the name of the action being executed.
11	params Sets the request parameters on the action.
12	prepare This is typically used to do pre-processing work, such as setup database connections.
13	profile Allows simple profiling information to be logged for actions.
14	scope Stores and retrieves the action's state in the session or application scope.
15	ServletConfig Provides the action with access to various servlet-based information.
16	timer Provides simple profiling information in the form of how long the action takes to execute.
17	token Checks the action for a valid token to prevent duplicate form submission.
18	validation Provides validation support for actions

Please look into Struts 2 documentation for complete detail on the above-mentioned interceptors. But I will show you how to use an interceptor in general in your Struts application.

How to Use Interceptors?
Let us see how to use an already existing interceptor to our "Hello World" program. We will use the timer interceptor whose purpose is to measure how long it took to execute an action method. At the same time, I'm using params interceptor whose purpose is to send the request parameters to the action. You can try your example without using this interceptor and you will find that name property is not being set because parameter is not able to reach to the action.

We will keep HelloWorldAction.java, web.xml, HelloWorld.jsp and index.jsp files as they have been created in Examples chapter but let us modify the struts.xml file to add an interceptor as follows


Right click on the project name and click Export > WAR File to create a War file. Then deploy this WAR in the Tomcat's webapps directory. Finally, start Tomcat server and try to access URL http://localhost:8080/HelloWorldStruts2/index.jsp. This will produce the following screen:

Now enter any word in the given text box and click Say Hello button to execute the defined action. Now if you will check the log generated, you will find the following text:


Here bottom line is being generated because of timer interceptor which is telling that action took total 109ms to be executed.

Create Custom Interceptors
Using custom interceptors in your application is an elegant way to provide cross-cutting application features. Creating a custom interceptor is easy; the interface that needs to be extended is the following Interceptor interface:


As the names suggest, the init() method provides a way to initialize the interceptor, and the destroy() method provides a facility for interceptor cleanup. Unlike actions, interceptors are reused across requests and need to be thread-safe, especially the intercept() method.
The ActionInvocation object provides access to the runtime environment. It allows access to the action itself and methods to invoke the action and determine whether the action has already been invoked.
If you have no need for initialization or cleanup code, the AbstractInterceptor class can be extended. This provides a default no-operation implementation of the init() and destroy() methods.

Create Interceptor Class
Let us create the following MyInterceptor.java in Java Resources > src folder:


As you notice, actual action will be executed using the interceptor by invocation.invoke()call. So you can do some pre-processing and some post-processing based on your requirement.
The framework itself starts the process by making the first call to the ActionInvocation object's invoke(). Each time invoke() is called, ActionInvocation consults its state and executes whichever interceptor comes next. When all of the configured interceptors have been invoked, the invoke() method will cause the action itself to be executed.

The following diagram shows the same concept through a request flow:

Create Action Class
Let us create a java file HelloWorldAction.java under Java Resources > src with a package name com.jtc.struts2 with the contents given below.


This is a same class which we have seen in previous examples. We have standard getters and setter methods for the "name" property and an execute method that returns the string "success".

Create a View
Let us create the below jsp file HelloWorld.jsp in the WebContent folder in your eclipse project.


Create Main Page
We also need to create index.jsp in the WebContent folder. This file will serve as the initial action URL where a user can click to tell the Struts 2 framework to call the a defined method of the HelloWorldAction class and render the HelloWorld.jsp view.


The hello action defined in the above view file will be mapped to the HelloWorldAction class and its execute method using struts.xml file.

Configuration Files
Now, we need to register our interceptor and then call it as we had called default interceptor in previous example. To register a newly defined interceptor, the <interceptors>...</interceptors> tags are placed directly under the <package> tag insstruts.xml file. You can skip this step for a default interceptors as we did in our previous example. But here let us register and use it as follows:


It should be noted that you can register more than one interceptors inside <package> tag and same time you can call more than one interceptors inside the <action> tag. You can call same interceptor with the different actions.
The web.xml file needs to be created under the WEB-INF folder under WebContent as follows:


Right click on the project name and click Export > WAR File to create a War file. Then deploy this WAR in the Tomcat's webapps directory. Finally, start Tomcat server and try to access URL http://localhost:8080/HelloWorldStruts2/index.jsp. This will produce the following screen:

Now enter any word in the given text box and click Say Hello button to execute the defined action. Now if you will check the log generated, you will find the following text at the bottom:


Stacking Multiple Interceptors
As you can imagine, having to configure multiple interceptor for each action would quickly become extremely unmanageable. For this reason, interceptors are managed with interceptor stacks. Here is an example, directly from the struts-default.xml file:


The above stake is called basicStack and can be used in your configuration as shown below. This configuration node is placed under the <package .../> node. Each <interceptor-ref .../> tag references either an interceptor or an interceptor stack that has been configured before the current interceptor stack. It is therefore very important to ensure that the name is unique across all interceptor and interceptor stack configurations when configuring the initial interceptors and interceptor stacks.
We have already seen how to apply interceptor to the action, applying interceptor stacks is no different. In fact, we use exactly the same tag:


The above registration of "basicStack" will register complete stake of all the six interceptors with hello action. This should be noted that interceptors are executed in the order, in which they have been configured. For example, in the above case, exception will be executed first, second would be servlet-config and so on.

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