Interview Questions - Struts Interview Questions
Answer: The controller is responsible for intercepting and translating user input into actions to be performed by the model. The controller is responsible for selecting the next view based on user input and the outcome of model operations. The Controller receives the request from the browser, invoke a business operation and coordinating the view to return to the client.
The controller is implemented by a java servlet, this servlet is centralized point of control for the web application. In struts framework the controller responsibilities are implemented by several different components like
The ActionServlet Class
The RequestProcessor Class
The Action Class
The ActionServlet extends the javax.servlet.http.httpServlet class. The ActionServlet class is not abstract and therefore can be used as a concrete controller by your application.
The controller is implemented by the ActionServlet class. All incoming requests are mapped to the central controller in the deployment descriptor as follows.
<servlet>
<servlet-name>action</servlet-name>
<servlet-class>org.apache.struts.action.ActionServlet</servlet-class>
</servlet>
All request URIs with the pattern *.do are mapped to this servlet in the deployment descriptor as follows.
<servlet-mapping>
<servlet-name>action</servlet-name>
<url-pattern>*.do</url-pattern>
<url-pattern>*.do</url-pattern>
A request URI that matches this pattern will have the following form.
http://www.my_site_name.com/mycontext/actionName.do
The preceding mapping is called extension mapping, however, you can also specify path mapping where a pattern ends with /* as shown below.
<servlet-mapping>
<servlet-name>action</servlet-name>
<url-pattern>/do/*</url-pattern>
<url-pattern>*.do</url-pattern>
A request URI that matches this pattern will have the following form.
http://www.my_site_name.com/mycontext/do/action_Name
The class org.apache.struts.action.requestProcessor process the request from the controller. You can sublass the RequestProcessor with your own version and modify how the request is processed.
Once the controller receives a client request, it delegates the handling of the request to a helper class. This helper knows how to execute the business operation associated with the requested action. In the Struts framework this helper class is descended of org.apache.struts.action.Action class. It acts as a bridge between a client-side user action and business operation. The Action class decouples the client request from the business model. This decoupling allows for more than one-to-one mapping between the user request and an action. The Action class also can perform other functions such as authorization, logging before invoking business operation. the Struts Action class contains several methods, but most important method is the execute() method.
public ActionForward execute(ActionMapping mapping,
ActionForm form, HttpServletRequest request, HttpServletResponse response) throws Exception;
The execute() method is called by the controller when a request is received from a client. The controller creates an instance of the Action class if one doesn?t already exist. The strut framework will create only a single instance of each Action class in your application.
Action are mapped in the struts configuration file and this configuration is loaded into memory at startup and made available to the framework at runtime. Each Action element is represented in memory by an instance of the org.apache.struts.action.ActionMapping class . The ActionMapping object contains a path attribute that is matched against a portion of the URI of the incoming request.
<action>
path= "/somerequest"
type="com.somepackage.someAction"
scope="request"
name="someForm"
validate="true"
input="somejsp.jsp"
<forward name="Success" path="/action/xys" redirect="true"/>
<forward name="Failure" path="/somejsp.jsp" redirect="true"/>
</action>
Once this is done the controller should determine which view to return to the client. The execute method signature in Action class has a return type org.apache.struts.action.ActionForward class. The ActionForward class represents a destination to which the controller may send control once an action has completed. Instead of specifying an actual JSP page in the code, you can declaratively associate as action forward through out the application. The action forward are specified in the configuration file.
<action>
path= "/somerequest"
type="com.somepackage.someAction"
scope="request"
name="someForm"
validate="true"
input="somejsp.jsp"
<forward name="Success" path="/action/xys" redirect="true"/>
<forward name="Failure" path="/somejsp.jsp" redirect="true"/>
</action>
The action forward mappings also can be specified in a global section, independent of any specific action mapping.
<global-forwards>
<forward name="Success" path="/action/somejsp.jsp" />
<forward name="Failure" path="/someotherjsp.jsp" />
</global-forwards>
public interface RequestDispatcher
Defines an object that receives requests from the client and sends them to any resource (such as a servlet, HTML file, or JSP file) on the server. The servlet container creates the RequestDispatcher object, which is used as a wrapper around a server resource located at a particular path or given by a particular name.
This interface is intended to wrap servlets, but a servlet container can create RequestDispatcher objects to wrap any type of resource.
getRequestDispatcher
public RequestDispatcher getRequestDispatcher(java.lang.String path)
Returns a RequestDispatcher object that acts as a wrapper for the resource located at the given path. A RequestDispatcher object can be used to forward a request to the resource or to include the resource in a response. The resource can be dynamic or static.
The pathname must begin with a "/" and is interpreted as relative to the current context root. Use getContext to obtain a RequestDispatcher for resources in foreign contexts. This method returns null if the ServletContext cannot return a RequestDispatcher.
Parameters:
path - a String specifying the pathname to the resource
Returns:
a RequestDispatcher object that acts as a wrapper for the resource at the specified path
See Also:
RequestDispatcher, getContext(java.lang.String)
getNamedDispatcher
public RequestDispatcher getNamedDispatcher(java.lang.String name)
Returns a RequestDispatcher object that acts as a wrapper for the named servlet.
Servlets (and JSP pages also) may be given names via server administration or via a web application deployment descriptor. A servlet instance can determine its name using ServletConfig.getServletName().
This method returns null if the ServletContext cannot return a RequestDispatcher for any reason.
Parameters:
name - a String specifying the name of a servlet to wrap
Returns:
a RequestDispatcher object that acts as a wrapper for the named servlet
See Also:
RequestDispatcher, getContext(java.lang.String), ServletConfig.getServletName()
Question: Why cant we overide create method in StatelessSessionBean?
Answer: From the EJB Spec : - A Session bean's home interface defines one or morecreate(...) methods. Each create method must be named create and must match
one of the ejbCreate methods defined in the enterprise Bean class. The return type of a create method must be the enterprise Bean's remote
interface type. The home interface of a stateless session bean must have one create method
that takes no arguments.
Question: Is struts threadsafe?Give an example?
Answer: Struts is not only thread-safe but thread-dependant. The response to a
request is handled by a light-weight Action object, rather than an individual servlet. Struts instantiates each Action class once, and allows
other requests to be threaded through the original object. This core strategy conserves resources and provides the best possible throughput. A
properly-designed application will exploit this further by routing related operations through a single Action.
Question: Can we Serialize static variable?
Answer: Serialization is the process of converting a set of object
instances that contain references to each other into a linear stream of
bytes, which can then be sent through a socket, stored to a file, or
simply manipulated as a stream of data. Serialization is the mechanism
used by RMI to pass objects between JVMs, either as arguments in a
method invocation from a client to a server or as return values from a
method invocation. In the first section of this book, There are
three exceptions in which serialization doesnot necessarily read and
write to the stream. These are
1. Serialization ignores static fields, because they are not part of any
particular object's state.
2. Base class fields are only handled if the base class itself is
serializable.
3. Transient fields.
There are four basic things
you must do when you are making a class serializable. They are:
- Implement the Serializable interface.
- Make sure that instance-level, locally defined state is serialized properly.
- Make sure that superclass state is serialized properly.
- Override equals( )and hashCode(
).
it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process .... (Source: http://www.oreilly.com/catalog/javarmi/chapter/ch10.html)
Answer: tiles-def.xml is is an xml file used to configure tiles with the struts application. You can define the layout / header / footer / body content for your View. See more at http://www.roseindia.net/struts/using-tiles-defs-xml.shtml.
The resourcebundle.properties file is used to configure the message (error/ other messages) for the struts applications.
The file validation.xml is used to declare sets of validations that should be applied to Form Beans. Fpr more information please visit http://www.roseindia.net/struts/address_struts_validator.shtml.
Question: What is the difference between
perform() and execute() methods?
Answer: Perform method is the method which was deprecated in the
Struts Version 1.1. In Struts 1.x, Action.perform() is the method called by the ActionServlet. This is typically where your business logic resides, or at least the flow control to your JavaBeans and EJBs that handle your business logic. As we already mentioned, to support declarative exception handling, the method signature changed in perform. Now execute just throws Exception. Action.perform() is now deprecated; however, the Struts v1.1 ActionServlet is smart enough to know whether or not it should call perform or execute in the Action, depending on which one is available.
Question: What are the various Struts tag libraries?
Answer: Struts is very rich framework and it provides very good
and user friendly way to develop web application forms. Struts provide many
tag libraries to ease the development of web applications. These tag
libraries are:
* Bean tag library - Tags for accessing JavaBeans and their properties.
* HTML tag library - Tags to output standard HTML, including forms, text
boxes, checkboxes, radio buttons etc..
* Logic tag library - Tags for generating conditional output, iteration capabilities and flow management
* Tiles or Template tag library - For the application using tiles
* Nested tag library - For using the nested beans in the application
Question: What do you understand by
DispatchAction?
Answer: DispatchAction is an action that comes with Struts 1.1
or later, that lets you combine Struts actions into one class, each with their own method.
The org.apache.struts.action.DispatchAction class allows multiple operation
to mapped to the different functions in the same Action class.
For example:
A package might include separate RegCreate, RegSave, and RegDelete Actions,
which just perform different operations on the same RegBean object. Since
all of these operations are usually handled by the same JSP page, it would
be handy to also have them handled by the same Struts Action.
A very simple way to do this is to have the submit button modify a field in the form which indicates which operation to perform.
<html:hidden property="dispatch" value="error"/>
<SCRIPT>function set(target) {document.forms[0].dispatch.value=target;}</SCRIPT>
<html:submit onclick="set('save');">SAVE</html:submit>
<html:submit onclick="set('create');">SAVE AS NEW</html:submitl>
<html:submit onclick="set('delete);">DELETE</html:submit>
Then, in the Action you can setup different methods to handle the different operations, and branch to one or the other depending on which value is passed in the dispatch field.
String dispatch = myForm.getDispatch();
if ("create".equals(dispatch)) { ...
if ("save".equals(dispatch)) { ...
The Struts Dispatch Action [org.apache.struts.actions] is designed to do exactly the same thing, but without messy branching logic. The base perform method will check a dispatch field for you, and invoke the indicated method. The only catch is that the dispatch methods must use the same signature as perform. This is a very modest requirement, since in practice you usually end up doing that anyway.
To convert an Action that was switching on a dispatch field to a DispatchAction, you simply need to create methods like this
public ActionForward create(
ActionMapping mapping,
ActionForm form,
HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException { ...
public ActionForward save(
ActionMapping mapping,
ActionForm form,
HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException { ...
Cool. But do you have to use a property named dispatch? No, you don't. The only other step is to specify the name of of the dispatch property as the "parameter" property of the action-mapping. So a mapping for our example might look like this:
<action
path="/reg/dispatch"
type="app.reg.RegDispatch"
name="regForm"
scope="request"
validate="true"
parameter="dispatch"/>
If you wanted to use the property "o" instead, as in o=create, you would change the mapping to
<action
path="/reg/dispatch"
type="app.reg.RegDispatch"
name="regForm"
scope="request"
validate="true"
parameter="o"/>
Again, very cool. But why use a JavaScript button in the first place? Why not use several buttons named "dispatch" and use a different value for each?
You can, but the value of the button is also its
label. This means if the page designers want to label the button something
different, they have to coordinate the Action programmer. Localization
becomes virtually impossible. (Source: http://husted.com/struts/tips/002.html).
Question: How Struts relates to J2EE?
Answer: Struts framework is built on J2EE technologies (JSP,
Servlet, Taglibs), but it is itself not part of the J2EE standard.
Question: What is Struts actions and action mappings?
Answer: A Struts action is an instance of a subclass of an Action class, which implements a portion of a Web application and whose perform or execute method returns a forward.
An action can perform tasks such as validating a user name and password.
An action mapping is a configuration file entry that, in general, associates an action name with an action. An action mapping can contain a reference to a form bean that the action can use, and can additionally define a list of local forwards that is visible only to this action.
An action servlet is a servlet that is started by the servlet container of a Web server to process a request that invokes an action. The servlet receives a forward from the action and asks the servlet container to pass the request to the forward's URL. An action servlet must be an instance of an org.apache.struts.action.ActionServlet class or of a subclass of that class. An action servlet is the primary component of the controller.