Page History

Introduction

So far our tutorials have not delved into the inner workings of the Struts 2 framework. But in this tutorial we'll introduce a key set of classes the Struts 2 framework relies upon to do most of the work whenever an Action is executed. In this tutorial's example project there is a register link that is mapped in the Struts XML configuration file (struts.xml) to the execute method of class Register. Before that execute method is called much work is done behind the scenes by the Struts 2 framework. For example:

...

1. Handling any exceptions generated
2. Converting the Register class's instance fields to String values for display in the view page
3. Forwarding to the correct view page depending on the result String returned by the execute method

The benefit of using Struts 2 is all this work happens automatically. You can focus on the logic of the controller (the Struts 2 ActionSupport class), the Service layer, the data access layer, your domain models, etc.

Introducing Interceptors

The tasks that are done by the Struts 2 framework before and after an Action is executed are done by Struts 2 interceptors. Interceptors are standard Java classes included in the Struts 2 core jar which are executed in a specific order.

...

Sometime the Struts 2 default stack of interceptors are not exactly what you need for a particular action. You may want to use interceptors that are not part of the Struts 2 default stack. For an individual Action or for the entire package of Actions, you can specify a different stack of interceptors that the Action or package should use. Below is how you would specify that the register Action should use both the logger and timer interceptors in addition to the interceptors provided by the default stack.

<action name="register" class="org.apache.struts.register.action.Register" method="execute">
	<interceptor-ref name="timer" />
	<interceptor-ref name="logger" />
	<interceptor-ref name="defaultStack">
		<param name="exception.logEnabled">true</param>
		<param name="exception.logLevel">ERROR</param>
	</interceptor-ref>
	<result name="success">thankyou.jsp</result>
	<result name="input">register.jsp</result>
</action>

The logger interceptor logs the start and end of the execution of an Action. The timer interceptor logs the amount of time (in milliseconds) for execution of the Action. These two interceptors used together can provide developers useful feedback.

...

How did I know to use the value of timer for the name attribute and even that there is a timer interceptor? On the Interceptors web page in the Struts 2 documentation are a list of interceptors that come with the Struts 2 framework and what the name value is for each interceptor.

...

Note the param nodes. These nodes are used to provide a value to the setLogEnabled and setLogLevel methods of the exception interceptor Exception Interceptor. Providing the values of true and ERROR will cause the Struts 2 framework to log any exceptions not caught by the application's code and to log those exceptions at the ERROR level.

...

Nov 20, 2010 9:55:48 AM com.opensymphony.xwork2.util.logging.jdk.JdkLogger info

INFO: Executed action //register!execute took 177 ms.

If you wanted to have the logger and timer interceptors executed for all Actions in a package you would use the following in struts.xml:

<package name="basicstruts2" extends="struts-default" > 


       <interceptors> 

         <interceptor-stack name="appDefault"> 

            <interceptor-ref name="timer" /> 

            <interceptor-ref name="logger" /> 

            <interceptor-ref name="defaultStack" /> 

         </interceptor-stack> 

        </interceptors> 
          

        <default-interceptor-ref name="appDefault" /> 

       <!-- rest of package omitted --> 


</package> 

In the code above we use the interceptors node to define a new stack of interceptors that includes the timer, logger, and defaultStack interceptors. We give this new interceptor stack a name of appDefault. Then we use the default-interceptor-ref node to specify that for all Actions defined inside this package node the appDefault stack of interceptors are to be used. Thus the timer and logger interceptor will be executed for each Action in this package.

...

In addition to specifying your own stack of interceptors, you can also write your own new interceptor and add it to the stack that is executed. The Struts Writing Interceptors guide explains how to do this. For example, you could create your own interceptor to handle authentication and authorization.

...

For more information about interceptors consult the Struts 2 Interceptor documentation.