Struts - Day6

>> Delivering Value

Struts 2.0 Session - 6 June, 2009 Presentation By Training & Development

Introduction Good Evening everybody! Welcome back to the Day 6 of the Struts 2 training course

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Introduction • Day 1 – History of Web applications It will be another exciting day of learning. Let us now do a quick recap on what we have learned so far:

• 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

• Let us now let Thinker, Dumbo and

Jumbo refresh your memory by providing reviews of important points over the past week 3

Mr. Thinker Can filters act on both request and response?

Yes Filters can act on prerequest as well as postresponse.

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Dumbo Vs. Jumbo Lets see, whether you can say this Jumbo, What are annotations?

Annotations are syntactic form of metadata. Annotations are added to the Java Source Code. Annotations are not method calls

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Mr. Thinker What is the problem that Command pattern attempts to solve?

Decoupling the sender of a command from the receiver of the command. The sender does not know anything specific about the intricacies of the command handled by the receiver

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Dumbo Vs. Jumbo Web Work is based on what Framework?

X Work

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Mr. Thinker X Work implements which design pattern?

Command Pattern

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Mr. Thinker What are the advantages of Struts 2? (whatever we have realized till now?)

No request. get parameters, no type conversions, no forwards/redirects, etc

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Dumbo Vs. Jumbo What is the name of the interface that our action classes can implement?

Action

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Mr. Thinker ActionSupport class What is the name of the class that already implements Action and few other interfaces and that which is of huge utility to developers?

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Mr. Thinker What are the interfaces that ActionSupport implements?

Action, Validateable, ValidationAware, TextProvider, LocaleProvider

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Dumbo Vs. Jumbo Action interface contains how many methods and what are their names?

Just one method called execute

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Dumbo Vs. Jumbo Name some constants defined by Action Interface

ERROR, INPUT, SUCCESS etc

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Mr. Thinker What are these used for?

These are used as return strings of execute / validate / other business logic methods

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Mr. Thinker How do you add an action level error message?

By invoking addActionError method

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Mr. Thinker How do you add a field level error message?

By invoking addFieldError method

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Dumbo Vs. Jumbo What are the parameters for addFieldError method?

The field name (as specified in JSP) and the error message

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Dumbo Vs. Jumbo What should be the name of the result (or the return string) in case of validation errors?

Input

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Dumbo Vs. Jumbo What is the tag that should be added for the errors to be displayed in the JSP

<s:actionerror />

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Mr. Thinker params What is the name of the interceptor that transfers data from the screen fields to the action fields?

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Mr. Thinker Workflow What is the name of the interceptor that invokes the validate method of the action class and acts according to the result of the validate method? (either pass the control back to the page or invoke the action method)

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Mr. Thinker What is the name of the method in any interceptor that always gets invoked during the request processing chain?

doIntercept

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Dumbo Vs. Jumbo To which interface does the interceptor’s doIntercept method type cast the action class to check for any errors?

ValidationAware

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Mr. Thinker To which interface does the interceptor’s doIntercept method type cast the action class to invoke the validate method?

Validateable

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Mr. Thinker What is the name of the file that provides details about the resource bundle file etc

struts.propertie s

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Dumbo Vs. Jumbo Where are action errors displayed in the page?

In the top of the page

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Dumbo Vs. Jumbo Where are field errors displayed in the page?

Above the corresponding field

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Ms. Natasha Unbelievable!! You guys are of immense help to us in this training. Now go back to your abode. We will let the trainer start today’s session

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Exercise  Run the latest project (Address Capture) and provide a string value

for Salary

 Observe the behavior

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Dumbo Vs. Jumbo Well, not without a reason. I did not do any checking but automatically I get an error in the page complaining about the string that I entered!! Is this also the power of struts?

Yes of course. This is part of the automatic type conversion that happens in Struts. Salary is of type float or double. There is an interceptor called “conversionError” which checks the conversions to java types and spits out any conversion type errors. In our case, we have entered a string value for salary. Since there is a mismatch between the types, the error is automatically thrown. Please also note that the conversionError is located after the params interceptor which is responsible for transferring the data from screen to action class. OK, now let us move on to today’s topic, the Validation Framework 31

Struts 2 - Validation Framework

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Introduction • We have already discussed about one form of validation which is using a

validate method inside the action class. Let us now focus our attention on a complete framework provided by Struts for Validation

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Various ways of Validation in Struts 2  Automatic type conversion validation – (automatically provided by

Framework)

 validate method in action class  validation Framework

We have already covered the first two aspects. Now let us look into the Validation Framework provided by Struts 2

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Constituents of the Validation Framework – Domain Data – Data to be validated – Validators – Classes provided by framework which actually performs the

validation – Validation meta data – the component that wires domain data with validators

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Constituents of the Validation Framework (Contd.)  As depicted in the above picture, there are various validators provided by the

framework each developed for a specific purpose – required – requiredstring – int – date – double – etc

 Domain Data are the actual data that need to be validated. Most of the

cases, this is the data available in the action classes

 Validation Meta data can be one of the two: – An XML file – Annotated Java Source 36

Constituents of the Validation Framework (Contd.)  Validation Meta data is the layer that wires a particular data with a particular

validator

 For example if Salary is mandatory then the meta data can be used to wire

this field with the validator called required

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Exercise  Let us learn the validation framework, the hard

way. Yes, through an exercise

 Our objective is to do the following validations in

the Address Capture application: – Name is mandatory – Age is mandatory

 Create a new XML file under the action

directory called “AddressAction-validation.xml”

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Exercise

 Type the following inside this XML: 
//OpenSymphony Group//XWork Validator 1.0.2//EN""http://www.opensymphony.com/xwork/ xwork-validator-1.0.2.dtd"> <message> Name is mandatory 39

Exercise  Clean, build and run the Address Capture Application  Click submit button without entering any value in the

Name field

 You will get a validation error in the screen with

respect to the name field

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Dumbo Vs. Jumbo Again for me, it looks like a magic. What happened internally, I am at a loss to understand

Yes, I agree. Let us give some time to the trainer to explain this

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Internals of the validation framework  In a nutshell what we did – Created an XML file named -validation.xml – In the XML, attached a particular field to a particular type of validator – Already provided a result with name=input – Already provided the tag <s:actionerror/> in the JSP  Let us now learn in detail about validation framework – – – –

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Internals of the validation framework (Contd.)  The above is a set of interceptors defined in struts-default.xml  All of us know that params interceptor is for transferring data from the

form fields to value stack

 conversionError is for displaying error messages with respect to conversion

of data types from screen to java types

 The last interceptor, the workflow interceptor is the one who was

responsible for invoking the validate method in the action class, do you remember?

 Now the only interceptor left out is the validation interceptor in bold above  As it is evident, validation interceptor is invoked after conversion error and

before workflow

 This is the interceptor which is responsible for invoking the -

validation.xml to check for any validation as mentioned in the XML

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Dumbo Vs. Jumbo The explanation was good. But he did not tell me the purpose of this XML.

OK. But why should I use this approach when I already have the validate method in the action class? Why cannot I always use the validate method in the action class?

Not smart. There are so many advantages of using the XML framework. Our trainer will explain you in more detail in the next slide

Well, I thought it is self-explanatory. This XML wires one or more input fields with one or more validators. For example, in the above example, the field myName was wired with the validator called “requiredstring”. Similarly there can be many fields and many validators in this XML

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Advantages of XML approach to validation  Validation is externalized and not part of the action class  Less coding and just a matter of seconds before you wire any validator for

any field

 Supports field level as well as form level validations  Generally all field level validations can be handled in the XML validators and

the validate method of action class is generally meant for complex validations involving multiple fields and business logic

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Dumbo Vs. Jumbo If there are validation errors how does the framework route me back to the same page

Whether it is this approach or the validate method approach, the framework checks for any errors using hasErrors method of validationAware interface. (that means any errors are added to the instance of ValidationAware interface). If there are errors, then a result with name “input” is passed back and hence the input page is displayed 46

Dumbo Vs. Jumbo OK. Is it possible that both the XML validation and the validate method get executed simultaneously so that the user gets to know all the error messages at once rather than one by one? Yes. Perfect

Good question. Yes, in fact, that is the default behavior of the framework. Even if there are errors in validation done through XML, the framework proceeds with the invocation of the validate method in action class. Only then it routes the page back to the user’s page. Am I clear?

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Exercise  Create one more input field in the screen called

email

 Check whether the email entered is valid. If not,

throw an appropriate error message

 Email is a mandatory field  Name should be a minimum of 3 characters in

length

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Exercise – I am a tough guy. You know. So, you are

going to be in soup. I will give a tough hint – liame dellac epyt rotadilav esu  Hint 1: – Read the above sentence from right to left to

get a hint

 Hint 2: – For Name, use a validator type of stringlength – You should use a param with

name=minLength

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Dumbo Vs. Jumbo I have a question. What if I have two validations for a particular field. Is there a way I can have multiple validations for a particular field?

Yes, it is possible. Just have multiple field-validator elements under the field element. As simple as that.

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Dumbo Vs. Jumbo By the way, in the previous example, the param was very useful for minLength of name. But the only problem is that I hardcoded the message that the length should be minimum 3 characters. Is there a way I can take that value from the variable itself instead of hardcoding?

A very valid question. And the answer is yes. You can use the variable in the message like this. “The name should be minimum of ${minLength} characters”. As you would have guessed, the minLength is an OGNL expression enclosed within ${ }

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Exercise  In the previous exercise do not hard code the

number 3. Instead get the value from the variable itself

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Dumbo Vs. Jumbo Cool Man. One more question. Is there a way I can mention the value of a particular field in the message? For example I want to call out the name of the person before I display the error.

Yes, it is possible. Just like ${minLength}, you can use any form field’s value in the XML. You can enclose them in ${ } and it should work fine.

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Exercise  In the previous exercise display the message like

this: “Hi ! The Name should be minimum of <minLength> characters.

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Mr. Thinker How do I represent OGNL in a JSP

Embed the expression within %{ }

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Mr. Thinker How do I represent OGNL in a validation XML?

Embed the expression within ${ }

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Dumbo Vs. Jumbo Initially we talked about i18n and how struts 2 supports it. But in this XML again we are hard coding the message!! What’s up?

A very good catch. But Struts 2 has a way out. Instead of <message>, you can use <message key=‘msg.key’/> which will then look in the properties file with a key as ‘msg.key’.

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Exercise  In the previous exercise retrieve the message

from the message bundle instead of hard coding

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Dumbo Vs. Jumbo Till this point, we have only looked into validations which are specific to a field. What about validations which span multiple fields? Or validations which contain an expression? Does Struts 2 support this?

Yes, Struts 2 supports expression inside validations. Let us see an example

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Expressions in Validations  

<param name="expression">person.age > 10

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<message key="address.age.10.error"/>

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 The above is an example of expressions in validations  The above validation checks if age is greater than 10. If less than or equal

to 10, an error is displayed

 Please note that the expression inside the param is the expression for PASS.

If the expression returns false, then an error is displayed

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Exercise  Do the above change in the latest exercise that

you completed

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Mr. Thinker For all my previous validations, the errors were displayed above the field. But for this error is displayed at the top!!

Because this is an action scope message whereas all the previous messages were field scoped messages

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Dumbo Vs. Jumbo One last question for the day. Why did I create the validation XML inside action directory? Is there any specific reason?

The validation XML is loaded by the class loader. Hence we created that file under the same location where you have action classes

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Dumbo Vs. Jumbo Sorry one more last question  If I have more than one action represented in an action class, (i.e. more than one business logic method), will I be able to have one xml for each of these business methods?

I know you are talking about a case similar to the validate method in action class (like how we have one “validatedo” method for one action method). Yes it is possible. Let the trainer explain.

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Multiple validations for same action class  To create multiple validations file (one for each business method), the

following should be the name of the validation XML:

 --validation.xml  In this case, depending on the action name the corresponding validation.xml

will automatically be invoked

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Session - 6 Guys! We are making steady progress. Today we looked into the validation framework in great detail. We will see you all tomorrow with another great topic

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Session 6 Thank You !

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