Apache Struts Interview Questions & Answers

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Exceptions in Struts are handled in two ways:

► Programmatic exception handling :
Explicit try/catch blocks in any code that can throw exception. It works well when custom value (i.e., of variable) needed when error occurs.

► Declarative exception handling :You can either define <global-exceptions> handling tags in your struts-config.xml or define the exception handling tags within <action></action> tag. It works well when custom page needed when error occurs. This approach applies only to exceptions thrown by Actions.

<global-exceptions>
<exception key="some.key"
type="java.lang.NullPointerException"
path="/WEB-INF/errors/null.jsp"/>
</global-exceptions>

or

<exception key="some.key"
type="package.SomeException"
path="/WEB-INF/somepage.jsp"/>

The various Struts tag libraries are:

► HTML Tags
► Bean Tags
► Logic Tags
► Template Tags
► Nested Tags
► Tiles Tags

<html:errors/> tag displays all the errors. <html:errors/> iterates over ActionErrors request attribute.

A specialized subclass of ActionForm that allows the creation of form beans with dynamic sets of properties (configured in configuration file), without requiring the developer to create a Java class for each type of form bean.

The SwitchAction class provides a means to switch from a resource in one module to another resource in a different module. SwitchAction is useful only if you have multiple modules in your Struts application. The SwitchAction class can be used as is, without extending.

The difference between LookupDispatchAction and DispatchAction is that the actual method that gets called in LookupDispatchAction is based on a lookup of a key value instead of specifying the method name directly.

LookupDispatchAction is useful if the method name in the Action is not driven by its name in the front end, but by the Locale independent key into the resource bundle. Since the key is always the same, the LookupDispatchAction shields your application from the side effects of I18N.

The LookupDispatchAction is a subclass of DispatchAction. It does a reverse lookup on the resource bundle to get the key and then gets the method whose name is associated with the key into the Resource Bundle.

The difference is that you need to use the IncludeAction only if the action is going to be included by another action or jsp. Use ForwardAction to forward a request to another resource in your application, such as a Servlet that already does business logic processing or even another JSP page.

The IncludeAction class is useful when you want to integrate Struts into an application that uses Servlets. Use the IncludeAction class to include another resource in the response to the request being processed.

The ForwardAction class is useful when you’re trying to integrate Struts into an existing application that uses Servlets to perform business logic functions. You can use this class to take advantage of the Struts controller and its functionality, without having to rewrite the existing Servlets. Use ForwardAction to forward a request to another resource in your application, such as a Servlet that already does business logic processing or even another JSP page. By using this predefined action, you don’t have to write your own Action class. You just have to set up the struts-config file properly to use ForwardAction.

To use the DispatchAction, follow these steps :

► Create a class that extends DispatchAction (instead of Action)
► In a new class, add a method for every function you need to perform on the service – The method has the same signature as the execute() method of an Action class.
► Do not override execute() method – Because DispatchAction class itself provides execute() method.
► Add an entry to struts-config.xml

The DispatchAction class is used to group related actions into one class. Using this class, you can have a method for each logical action compared than a single execute method. The DispatchAction dispatches to one of the logical actions represented by the methods. It picks a method to invoke based on an incoming request parameter. The value of the incoming parameter is the name of the method that the DispatchAction will invoke.

The different kinds of actions in Struts are:

► ForwardAction
► IncludeAction
► DispatchAction
► LookupDispatchAction
► SwitchAction

when the scope is request,the values of formbean would be available for the current request.
when the scope is session,the values of formbean would be available throughout the session.

Yes, we can have more than one struts-config.xml for a single Struts application. They can be configured as follows:

<servlet>
<servlet-name>action</servlet-name>
<servlet-class>
org.apache.struts.action.ActionServlet
</servlet-class>
<init-param>
<param-name>config</param-name>
<param-value>
/WEB-INF/struts-config.xml,
/WEB-INF/struts-admin.xml,
/WEB-INF/struts-config-forms.xml
</param-value>
</init-param>
.....
<servlet>

Struts is based on model 2 MVC (Model-View-Controller) architecture. Struts controller uses the command design pattern and the action classes use the adapter design pattern. The process() method of the RequestProcessor uses the template method design pattern. Struts also implement the following J2EE design patterns.

► Service to Worker
► Dispatcher View
► Composite View (Struts Tiles)
► Front Controller
► View Helper
► Synchronizer Token

In the execute() method of Action class the business logic is executed.

public ActionForward execute(
ActionMapping mapping,
ActionForm form,
HttpServletRequest request,
HttpServletResponse response)
throws Exception ;


execute() method of Action class:

► Perform the processing required to deal with this request
► Update the server-side objects (Scope variables) that will be used to create the next page of the user interface
► Return an appropriate ActionForward object

An Action Class performs a role of an adapter between the contents of an incoming HTTP request and the corresponding business logic that should be executed to process this request.

We can specify the action mapping in the configuration file called struts-config.xml. Struts framework creates ActionMapping object from <ActionMapping> configuration element of struts-config.xml file

<action-mappings>
<action path="/submit"
type="submit.SubmitAction"
name="submitForm"
input="/submit.jsp"
scope="request"
validate="true">
<forward name="success" path="/success.jsp"/>
<forward name="failure" path="/error.jsp"/>
</action>
</action-mappings>

Action mapping contains all the deployment information for a particular Action bean. This class is to determine where the results of the Action will be sent once its processing is complete.

validate() : Used to validate properties after they have been populated; Called before FormBean is handed to Action. Returns a collection of ActionError as ActionErrors. Following is the method signature for the validate() method.

reset(): reset() method is called by Struts Framework with each request that uses the defined ActionForm. The purpose of this method is to reset all of the ActionForm's data members prior to the new request values being set.

The important methods of ActionForm are : validate() & reset().

ActionForm is javabean which represents the form inputs containing the request parameters from the View referencing the Action bean.

ActionServlet performs the role of Controller:

► Process user requests
► Determine what the user is trying to achieve according to the request
► Pull data from the model (if necessary) to be given to the appropriate view,
► Select the proper view to respond to the user
► Delegates most of this grunt work to Action classes
► Is responsible for initialization and clean-up of resources