Struts imp

Struts is based on the MVC design pattern. Struts components can be categories into Model, View and Controller.
Model: Components like business logic / business processes and data are the part of Model.
View: JSP, HTML etc. are part of View
Controller: Action Servlet of Struts is part of Controller components which works as front controller to handle all the requests.

De= scribe is ActionServlet
ActionServlet provides the "controller" in the Model-View-Controller (MVC) design pattern for web applications that is commonly known as "Model 2". This nomenclature originated with a description in the JavaServerPages Specification, version 0.92, and has persisted ever since (in the absence of a better name).Controller is responsible for handling all the requests.
   
Wh= at is an Action Class used for?
An Action is an adapter between the contents of an incoming HTTP request and the corresponding business logic that should be executed to process this request. The controller (ActionServlet) will select an appropriate Action for each request, create an instance (if necessary), and call the perform method. 
Actions must be programmed in a thread-safe manner, because the controller will share the same instance for multiple simultaneous requests. In this means you should design with the following items in mind: 
=95 Instance and static variables MUST NOT be used to store information related to the state of a particular request. They MAY be used to share global resources across requests for the same action. 
=95 Access to other resources (JavaBeans, session variables, etc.) MUST be synchronized if those resources require protection. (Generally, however, resource classes should be designed to provide their own protection where necessary. 
When an Action instance is first created, the controller servlet will call setServlet() with a non-null argument to identify the controller servlet instance to which this Action is attached. When the controller servlet is to be shut down (or restarted), the setServlet() method will be called with a null argument, which can be used to clean up any allocated resources in use by this Action. 

Wh= at is ActionForm?
An ActionForm is a JavaBean optionally associated with one or more ActionMappings. Such a bean will have had its properties initialized from the corresponding request parameters before the corresonding action's perform() method is called.<= br> When the properties of this bean have been populated, but before the perform() method of the action is called, this bean's validate() method will be called, which gives the bean a chance to verify that the properties submitted by the user are correct and valid. If this method finds problems, it returns an error messages object that encapsulates those problems, and the controller servlet will return control to the corresponding input form. Otherwise, the validate() method returns null(), indicating that everything is acceptable and the corresponding Action's perform() method should be called.
This class must be subclassed in order to be instantiated. Subclasses should provide property getter and setter methods for all of the bean properties they wish to expose, plus override any of the public or protected methods for which they wish to provide modified functionality. 

Wh= at is Struts Validator Framework?
Struts Framework provides the functionality to validate the form data. It can be use to validate the data on the users browser as well as on the server side. Struts Framework emits the java scripts and it can be used validate the form data on the client browser. Server side validation of form can be accomplished by sub classing your From Bean with DynaValidatorForm class.  

Wh= at are the core classes of the Struts Framework?
Core classes of Struts Framework are ActionForm, Action, ActionMapping, ActionForward, ActionServlet etc.= 

Wh= at are Tag Libraries provided with Struts?
Struts provides a number of tag libraries that helps to create view components easily. These tag libraries are:
a) Bean Tags: Bean Tags are used to access the beans and their properties.
b) HTML Tags: HTML Tags provides tags for creating the view components like forms, buttons, etc..
c) Logic Tags: Logic Tags provides presentation logics that eliminate the need for scriptlets.
d) Nested Tags: Nested Tags helps to work with the nested context.

Wh= at are difference between ActionErrors and ActionMessage?
ActionMessage: A class that encapsulates messages. Messages can be either global or they are specific to a particular bean property.
Each individual message is described by an ActionMessage object, which contains a message key (to be looked up in an appropriate message resources database), and up to four placeholder arguments used for parametric substitution in the resulting message.
ActionErrors: A class that encapsulates the error messages being reported by the validate() method of an ActionForm. Validation errors are either global to the entire ActionForm bean they are associated with, or they are specific to a particular bean property (and, therefore, a particular input field on the corresponding form).

Ho= w you will handle exceptions in Struts?
In Struts you can handle the exceptions in two ways:
a) Declarative Exception Handling: You can either define global exception handling tags in your struts-config.xml or define the exception handling tags within .. tag. 
Example:

  key=3D"database.error.duplicate" 
  path=3D"/UserExists.jsp" 
  type=3D"mybank.account.DuplicateUserException"/>
b) Programmatic Exception Handling: Here you can use try{}catch{} block to handle the exception. 

Gi= ve the Details of XML files used in Validator Framework?
The Validator Framework uses two XML configuration files validator-rules.xml and validation.xml
The validation-rules.xml is provided with the Validator Framework and it declares and assigns the logical names to the validation routines. It also contains the client-side javascript code for each validation routine. The validation routines are java methods plugged into the system to perform specific validations.

Following table contains the details of the elements in this file:
Element=  
Attributes and Description

form-validation 
This is the root node. It contains nested elements for all of the other configuration settings.

global 
The validator details specified within this, are global and are accessed by all forms.<= span style=3D"color: navy;">

validator 
The validator element defines what validators objects can be used with the fields referenced by the formset elements.<= /span>

The attributes are: 

name: Contains a logical name for the validation routine

classname: Name of the Form Bean class that extends the subclass of ActionForm class

method<= /b>: Name of the method of the Form Bean class

methodParams: parameters passed to the method

msg= :Validator uses Struts' Resource Bundle mechanism for externalizing error messages. Instead of having hard-coded error messages in the framework, Validator allows you to specify a key to a message in the ApplicationResources.properties file that should be returned if a validation fails. Each validation routine in the validator-rules.xml file specifies an error message key as value for this attribute.<= span style=3D"color: navy;">

depends= : If validation is required, the value here is specified as 'required' for this attribute.

jsFunctionName= : Name of the javascript function is specified here. 

javascript 
Contains the code of the javascript function used for client-side validation. Starting in Struts 1.2.0 the default javascript definitions have been consolidated to commons-validator. The default can be overridden by supplying a element with a CDATA section, just as in struts 1.1.<= /span>


The Validator plug-in (validator-rules.xml) is supplied with a predefined set of commonly used validation rules such as Required, Minimum Length, Maximum length, Date Validation, Email Address validation and more. This basic set of rules can also be extended with custom validators if required.

Structure of validation.xml This validation.xml configuration file defines which validation routines that is used to validate Form Beans. You can define validation logic for any number of Form Beans in this configuration file. Inside that definition, you specify the validations you want to apply to the Form Bean's fields. The definitions in this file use the logical names of Form Beans from the struts-config.xml file along with the logical names of validation routines from the validator-rules.xml file to tie the two together.


Example of form in the validation.xml file:

 
 
 
 
 
  mask
  ^[0-9a-zA-Z]*$
 
 
   
Element=  
Attributes and Description

form-validation 
This is the root node. It contains nested elements for all of the other configuration settings

global 
The constant details are specified in element within this element.

constant 
Constant properties are specified within this element for pattern matching.

constant-name 
Name of the constant property is specified here

constant-value 
Value of the constant property is specified here.

formset 
This element contains multiple

elements 

form 
This element contains the form details.
The attributes are:
name:Contains the form name. Validator uses this logical name to map the validations to a Form Bean defined in the struts-config.xml file= 

field 
This element is inside the form element, and it defines the validations to apply to specified Form Bean fields.

The attributes are:

property: Contains the name of a field in the specified Form Bean

depends= : Specifies the logical names of validation routines from the validator-rules.xml file that should be applied to the field.

arg 
A key for the error message to be thrown incase the validation fails, is specified here

var 
Contains the variable names and their values as nested elements within this element.

var-name 
The name of the criteria against which a field is validated is specified here as a variable

var-value 
The value of the field is specified here
= 

= Ho= w you will display validation fail errors on jsp page?
Following tag displays all the errors:

  
Ho= w you will enable front-end validation based on the xml in validation.xml?
The tag to allow front-end validation based on the xml in validation.xml. For example the code: generates the client side java script for the form "logonForm" as defined in the validation.xml file. The when added in the jsp file generates the client site validation script.

Ca= n I setup Apache Struts to use multiple configuration files?
Yes Struts can use multiple configuration files. Here is the configuration example:
 
  banking
  org.apache.struts.action.ActionServlet
 
  = config
  = /WEB-INF/struts-config.xml,
  = = = /WEB-INF/struts-authentication.xml,
  = = = /WEB-INF/struts-help.xml
 
  1


Ho= w you will make available any Message Resources Definitions file to the Struts Framework Environment?<= /span>
Message Resources Definitions file are simple .properties files and these files contains the messages that can be used in the struts project. Message Resources Definitions files can be added to the struts-config.xml file through tag.
Example:


Wh= at is Struts Flow?
Struts Flow is a port of Cocoon's Control Flow to Struts to allow complex workflow, like multi-form wizards, to be easily implemented using continuations-capable JavaScript. It provides the ability to describe the order of Web pages that have to be sent to the client, at any given point in time in an application. The code is based on a proof-of-concept Dave Johnson put together to show how the Control Flow could be extracted from Cocoon. (Ref: http://struts.sourceforge.net/struts-flow/index.html )

Wh= at are the difference between and ?= 
: This tag is used to output locale-specific text (from the properties files) from a MessageResources bundle.
: This tag is used to output property values from a bean. is a commonly used tag which enables the programmers to easily present the data.

Wh= at is LookupDispatchAction?
An abstract Action that dispatches to the subclass mapped execute method. This is useful in cases where an HTML form has multiple submit buttons with the same name. The button name is specified by the parameter property of the corresponding ActionMapping. 

Wh= at do you understand by DispatchAction?
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.


SAVE
SAVE AS NEW 
DELETE
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 =3D 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:

  path=3D"/reg/dispatch" 
  type=3D"app.reg.RegDispatch" 
  name=3D"regForm" 
  scope=3D"request" 
  validate=3D"true" 
  parameter=3D"dispatch"/>
If you wanted to use the property "o" instead, as in o=3Dcreate, you would change the mapping to

  path=3D"/reg/dispatch" 
  type=3D"app.reg.RegDispatch" 
  name=3D"regForm" 
  scope=3D"request" 
  validate=3D"true" 
  parameter=3D"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. 

Is= struts threadsafe?Give an example?
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

Wh= at are the uses of tiles-def.xml file, resourcebundle.properties file, validation.xml file?= 
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.

Wh= at is the difference between perform() and execute() methods?
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.  
Ho= w Struts relates to J2EE?
Struts framework is built on J2EE technologies (JSP, Servlet, Taglibs), but it is itself not part of the J2EE standard.

Wh= at is Struts actions and action mappings?
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.