>> Delivering Value

Struts 2.0Session - 9

June, 2009

Presentation ByTraining & Development

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Introduction

Good Evening everybody! Welcome to the Day 9 of the

Struts 2 training course

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Introduction Day 1 – History of Web applications

Day 2 – Concepts to be known before learning Struts 2

Day 3 – Struts 2 environment setup, Hello World Project, PhD Project

Day 4 – Dissection of Hello World, Value Stack, OGNL

Day 5 – Action Interface, Action Support class, Validation, Resource Bundle and Internationalization

Day 6 – Validation Framework provided by Struts 2

Day 7 – Writing your own interceptors

Day 8 – Action Context, accessing Session and SessionAware interface

It will be another exciting day of learning. Let us now do a quick recap on what we have

learned so far:

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Ms. Natasha

Let us now let Thinker, Dumbo and Jumbo refresh your memory by

providing reviews of important points over the past week

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Dumbo Vs. Jumbo

What should be the name of the result (or thereturn string) in case of validation errors?

Input

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Mr. Thinker

What is the name of theinterceptor that transfersdata from the screen fieldsto the action fields?

params

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Dumbo Vs. Jumbo

To which interface does the interceptor’s doInterceptmethod type cast the action class to check for anyerrors?

ValidationAware

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Mr. Thinker

To which interface does theinterceptor’s doInterceptmethod type cast the actionclass to invoke the validatemethod?

Validateable

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Mr. Thinker

What is the name of the filethat provides details aboutthe resource bundle file etc

struts.properties

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Dumbo Vs. Jumbo

Where are field errors displayed in the page?

Above the corresponding field

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Mr. Thinker

What are theparameters foraddFieldError?

Field Name, Error message

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Mr. Thinker

What is the method to retrievean error message from theresource bundle?

this.getText

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Dumbo Vs. Jumbo

if there is a validation error in the XML then will thevalidate method of action class be executed?

Yes, it will be executed because it is always better to display the errors at one go to the user

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Dumbo Vs. Jumbo

How do you enclose OGNL expression insidevalidation xml?

${ }

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Mr. Thinker

What should be the name of thevalidation xml if I have justone business method inside theaction class?

<action class>-validation.xml

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Mr. Thinker

What is the validator type to beused for complex validations?

Expression

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Dumbo Vs. Jumbo

When is the error message displayed for a validatortype of expression?

If the expression mentioned inside param returns false

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Dumbo Vs. Jumbo

What are the two re-alignments that we need to makeas far as our understanding on the value stack isconcerned?

Value Stack is not the top level object. This itself is placed on Action Context object. 2. Value stack itself does not contain named objects like application, session etc. it is contained by Action Context

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Mr. Thinker

If I have the attribute with the samename in multiple locations inside thevalue stack, which one will be returned?

The first occurrence’s value will be returned from the top.

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Mr. Thinker

Why don’t we give the ognlexpression asactionObject.person.name instead ofperson.name?

The OGNL expression starts looking from inside each top level object. Any OGNL expression is hence considered to start from inside the top level object

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Mr. Thinker

What is the scope of ValueStack?

Value stack is created for each request

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Dumbo Vs. Jumbo

Value Stack is placed under which top level object?

Action Context Object

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Dumbo Vs. Jumbo

How many kinds of objects are placed inside actioncontext object

6 kinds of objects

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Dumbo Vs. Jumbo

What are the 6 kinds of objects

Value Stack, parameters, application, session, attr, request

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Mr. Thinker

What are parameters?

Parameters are nothing but request.getParameters. In other words all form fields are stored in parameters

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Mr. Thinker

What is attr? What is thedifference between attr andparameters?

attr contains all the attributes set in any scope starting from page, request, session, servlet and application. Parameters are just form field. The advantage with attr is that you can get the attribute in any scope without actually specifying the scope. Whichever scope is encountered first, it will be automatically returned. The order of search would be in the same order mentioned above

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Mr. Thinker

Does Value Stack containInfrastructure data?

No, value stack does not contain infrastructure data?

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Mr. Thinker

What kind of data does valuestack contain then?

Value stack contains only application data (or domain data)

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Dumbo Vs. Jumbo

What is the scope of actioncontext?

Action Context is also created per request like value stack (i.e. per action).

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Dumbo Vs. Jumbo

If I am requesting a static HTML page willthe action context be created? Providereasoning for your answer

If it is static html page, then ideally the request will never pass beyond Filter. Hence action context will not be created

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Dumbo Vs. Jumbo

What is the advantage of actioncontext?

Clean MVC implementation. We do not have traces of servlet API inside action class

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Mr. Thinker

Which class creates ActionContext?

Action Context and in fact value stack are created by Default Action Invocation class which implements Action Invocation interface. The method is init method

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Mr. Thinker

What are the parameters forexecute method in struts 2?

None

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Mr. Thinker

What is the method to get thesession from action context?

ActionContext.getContext( ).getSession( )

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Mr. Thinker

What does it return?

It returns a Map

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Dumbo Vs. Jumbo

Why does it return a Map andnot HttpSession?

Because of the same reason explained previously. i.e. clean mvc implementation. We do not want traces of servlet api inside action class

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Dumbo Vs. Jumbo

How do we avoid even performing the methodinvocation of ActionContext.getContext().getSession( )?

You use SessionAware interface

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Mr. Thinker

What are the methods inSessionAware interface?

There is just one method called setSession(Map)

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Mr. Thinker

What are you supposed toinside setSession method?

Have a class level variable and set it from setSession method

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Ms. Natasha

Thanks a lot. Let us now continue with today’s lessons. Today we will be discussing

about two very important interfaces and their usages.

The Preparable interface and ModelDriven interface

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Preparable Interface

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Mr. Thinker

When are action objectscreated? (i.e. at what scope?) Action objects are

created for each request (i.e. for each action)

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Mr. Thinker

Since action objects are created for eachrequest, the variables inside actionobjects are also created every time

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A Very Typical Scenario

An object contains 20 fields

The first 10 fields are data entered in the first page

The next 5 fields are data entered in the second page

The next 5 fields are data entered in the third page

At the end of the third page, the data from the object is stored into the database

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How is this implemented traditionally?

Step 1: On click of next in the first page, store all the fields in the object and store the object in the session

Step 2: On click of next in the second page, retrieve the object from the session. Then start storing the next 5 fields in the object. Put the object back into the session

Step 3: On click of next in the third page, retrieve the object from the session. Then start storing the next 5 fields in the object. Then store the data from the object into the database

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The Preparable Interface

As the name suggests, the Preparable Interface is used to implement any preparatory work before invoking the action’s actual business method (i.e. execute or corresponding method)

Preparable interface provides one method called “void prepare( )”

prepare is a call back method

The method is called by an interceptor called “PrepareInterceptor”

So, if your stack contains PrepareInterceptor, then the prepare method would be called

defaultStack already contains PrepareInterceptor

Let us now look into the source code of this interceptor and also check how defaultStack has configured this interceptor

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The Preparable Interface (Contd.)

Using the Preparable Interface to implement the above scenario:– Make your action implement the Preparable interface– In the prepare method prepare the corresponding object by retrieving it

from session and storing it in the object– Because of outsourcing these preparatory logic to the prepare method, the

action method now will focus only on pure business logic

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ModelDriven Interface

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Mr. Thinker

What are the contents of theValue Stack? Action Object, Model

Object and temporary object in the order of bottom to top

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Dumbo Vs. Jumbo

In all the examples we have been seeing, the person object contains,name, age etc and also other details like address etc. In the JSP Ihave been referring these fields as person.firstName, person.age,person.address.street etc. Is there a way we can avoid the person andjust firstName, age, address.street etc?

Yes, there is a way. There is an interface called ModelDriven interface. Let the trainer explain more on this

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ModelDriven Interface

This interface contains one method called getModel( )

If your action implements ModelDriven interface, then you should implement getModel( ) method and in this method just return the Person object (or whichever you feel is the model object)

The getModel method is called by an interceptor called ModelDrivenInterceptor

If your stack contains this interceptor then from the JSP you can just refer as firstName instead of person.firstName

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Dumbo Vs. Jumbo

May I know what happens in the background

Yes, the ModelDriven Interceptor invokes the getModel method and whatever be the return it just pushes it into the Value Stack. This is what we have been referring as Model Object above Action Object in the value stack. Please see the diagram once again in the next slide

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Block Diagram

TEMPORARY OBJECT

MODEL OBJECT

ACTION OBJECT

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Dumbo Vs. Jumbo

:I understand that it is responsible for pushing the datainto value stack and now I have model object. But howdoes it ensure that I can just refer them as firstNameinstead of Person.firstName

I thought you should be knowing the answer for this. We have been telling time and again that in the OGNL expression you can ignore all top level objects. That is why we have been referring as person.firstName instead of actionObject.person.firstName. In the case of our discussion, the Person object would be returned from getModel method. So, the Person object itself would be pushed into the stack. The Person object would be a top level object in the stack. So, I can ignore person and start from next level. Is it clear?

Thanks it was a nice explanation.

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Model Driven Interface (Contd.)

Let us now look into the source code of this interface and ModelDriveInterceptor

Let us also check how the defaultStack also implements this interface

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Exercise My application contains the following screens:

Screen 1:– Heading – Borrower Details– First Name– Last Name– Age– No of dependents– Next button– Validations Age should be greater than 18 First Name, Age and no of dependents are mandatory

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Exercise Screen 2:

– Heading – Contact Details– Address 1– Address 2– City– State– Country– Next Button– Validations Address 1, city, state and country are mandatory

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Exercise Screen 3:

– Heading – Employment Details– Employer Name– Employer Address1– Employer Address2– Employer City– Employer State– Employer Country– No of years– Submit Button– Validations Employer Name, address 1, city, state, country and no of years

are mandatory

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Exercise Screen 4:

– Heading – Consolidated View– Provide a message, your application has been successfully submitted.

The following are the details submitted by you:– <Here provide all the details of the first three screens under the

corresponding headings in one page>

Object Design– Person is the top level object and inside that Address and employer

are child objects. Respective variables should be stored accordingly in the correct object

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Exercise Assignment:

– Implement the above screens using Preparable and ModelDriven interface

– Hint: getModel should return Person Object– Also modoify the already developed session interceptor such that if I

type the second, third or fourth page directly, it should lead me to the first page

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Session 8

I hope you had a very good exercise as it covers many of the concepts that we have

learned till now.You have a very good night and we will see

you back soon

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Session 9

Thank You !!