Struts - Day5

>> Delivering Value

Struts 2.0 Session - 5 May, 2009 Presentation By Training & Development

Introduction Good Evening Guys! Welcome back to the Struts 2 training program. We are on Day 5, the end of first week of Struts training. We have seen 5 10% of Struts 2

As I have always been maintaining if you lay a strong foundation by learning the concepts thoroughly the rest will just be a cake walk So, the point is, the first 20% of Struts 2 is really important and the rest is just based on this 20%

2

Introduction  On Day 1 we learned the history of Web Let us do a quick overview on what we have learned so far

Applications starting from HTTP till Struts 2

 On Day 2 we learned about a few important

concepts which form the basis of Struts 2 namely Filters, Command pattern, Annotations and Dependency Injection

 On Day 3 we learned the background of how

Struts 2 was formed. We also setup the environment and did two great projects namely Hello World and Ph.D in Mathematics

 On Day 4 we dissected the Hello World

Project and Ph.D Project to know the internals. We also got introduced to important concepts called Interceptors, Value Stack and OGNL

 Let us now do a random recap on the first 4

days

3

Mr. Thinker What are the main problems with Servlets?

Separation of Concern is nonexistent. HTML code is embedded in Java making it difficult to maintain

4

Dumbo Vs. Jumbo Jumbo! Does JSPs completely replace Servlets?

No Dumbo. JSPs still get converted to Servlets. So, for each JSP we see, there will be a corresponding servlet in the background

5

Mr. Thinker What are the problems with JSPs?

Separation of concern is still a problem. Java code is embedded in HTML which is equally undesirable

6

Mr. Thinker How do Filters help?

Repetitive tasks can be moved to a Filter. (Eg. Session checking, authentication etc)

7

Mr. Thinker How is dependency injection achieved generally?

Dependency Injection is achieved generally by performing Setter Injection.

8

Dumbo Vs. Jumbo Command Pattern is basis of what framework?

Command Pattern is the basic building of Struts Framework

9

Mr. Thinker What is the ancestor framework of Struts2?

Webwork

10

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

X Work

11

Mr. Thinker X Work implements which design pattern?

Command Pattern

12

Mr. Thinker What is the first component in Struts 2 that receives the request from the client.

FilterDispatcher. It is a Filter class

13

Dumbo Vs. Jumbo What is the name of the file that contains all the configuration details related to the framework?

struts.xml

14

Mr. Thinker Can action class be a normal Java class?

Yes, the action class can be a normal java class without extending or implementing anything else

15

Mr. Thinker What is the return type of the execute method in Action class?

The return type is String

16

Dumbo Vs. Jumbo With what is the return of action class checked?

The return of action class is checked with the action element’s result element in the struts.xml file

17

Mr. Thinker Interceptors are similar to what?

Interceptors are similar to Filters

18

Mr. Thinker Where are interceptors configured?

Interceptors are configured in struts.xml

19

Mr. Thinker Where is the default setting of interceptor set?

The default interceptor to be used is set in a file called “strutsdefault.xml”

20

Dumbo Vs. Jumbo What is the difference between an Interceptor and an Interceptor Stack?

An interceptor stack is a stack of interceptors. What a great answer!!

21

Dumbo Vs. Jumbo What is the order in which the interceptors get executed in an interceptor stack?

The interceptors in the interceptor stack are executed in the order in which they are defined in the Interceptor stack

22

Mr. Thinker What is OGNL?

Object Graph Notation Language

23

Mr. Thinker What is value stack?

Value stack is a stack of objects

24

Dumbo Vs. Jumbo I have an object called PurchaseOrder inside which I have Asset inside which I have Invoice. Inside Invoice I have invoiceNo. How can I access invoiceNo using OGNL?

purchaseOrder.asset.invoice.invoiceNo

25

Mr. Thinker What is the order of objects in value Stack?

Temporary Objects, Model Object, Action Object, Named Object (Application, session, request etc)

26

Mr. Thinker What is the name of the interceptor that sets the variables of action class from the screen fields?

ParamsInterceptor

27

Mr. Thinker What is the kind of dependency Injection when the params interceptor sets the value of action variables from screen fields?

Setter Injection

28

Ms. Natasha Oh My God! That was a very thorough recap!! Good Job guys. Now go back to your work. You can always come back when required Now let us give some time to the trainer to start what he has to say today ! Action, Action , More Action!

29

Primary responsibilities of Action

30

Primary responsibilities of Action  Actions encapsulate the unit of work  Actions provide locus for data transfer  Actions return control string for result routing

31

Encapsulating the unit of work  Struts 2 is an action based framework  The primary purpose of the action then is to fulfill the business logic. (i.e.

serve as the starting point of model layer)

 The primary purpose of the execute method is exactly this. Executing

business logic

32

Dumbo Vs. Jumbo Just one quick question. I know that execute method should purely only contain the business logic. But before executing the business logic there might be a need for other things. One of them is populating the business objects which I know is handled by the params interceptor. But how about any other validations that need to happen before executing the business logic methods?

That’s not a bad question considering your stature. Yes, validations are definitely a likely requirement before executing the business logic. Struts 2 is not so dumb to not address this issue. This is addressed. This will be addressed later on today. But for now, listen to what he has to say. The execute method should only contain business logic

33

Locus for data transfer  Being the model component, action is responsible for taking around the

model data all around

 As we have already seen, action class has model variables which are class

level variables

 Hence it is so easy to access these model data from within the action class  We have also already seen that throughout the framework it is easy to access

these data because of the magic called “Value Stack”

34

Returning control string for result routing  After executing the business logic it is the action’s responsibility to inform

which page should be displayed

 For this, the action method uses the return string  The return string is checked against the struts.xml’s action element’s result

element and the corresponding page is displayed to the user

35

Dumbo Vs. Jumbo That was good explanation about action. Tell me something. You had mentioned that Struts 2 is an action based framework. Hence Action class is the hero even though there are other character artists like value stack, OGNL, interceptors etc. Being the Hero, I was expecting more help from the framework in this area. But I do not see any help being rendered by the framework in the action class. Action class is a java class entirely written by the developer. So, what is the fun!!!

I know you are always pessimistic and sarcastic. But there is an answer from Struts 2. Do not worry. Struts 2 provides a lot of help in the action layer. Stay tuned to know more

36

The Action Interface  The Action Interface is the first answer to Dumbo’s question  Action Interface is provided by the framework  The interface contains one method called execute  The package is “com.opensymphony.xwork2.Action”  Your actions can implement this Action Interface in order to tap the

benefits provided by this interface

37

Constituents of Action Interface  The Action Interface provides just one method as follows – String execute( ) throws Exception  The Action Interface provides various constants: – public static final String ERROR "error" – public static final String INPUT "input" – public static final String LOGIN "login" – public static final String NONE "none" – public static final String SUCCESS "success"  That is all what the Action Interface provides

38

Dumbo Vs. Jumbo Well!! Are you fooling me? What is so great about this interface? It just provides one method which anyway I am using and few string constants. First tell me what are these string constants and then tell me something better than this action interface.

Answer to your question: These string constants can be used by the developer as return strings in the execute method. So, instead of saying return “success”, you can say return SUCCESS. Just a convenient method. Similarly the framework uses these as well. Answers to your second question are the next few slides

39

The ActionSupport class  Dumbo is very true in stating that Action interface is not so great.

Fortunately we have an answer

 ActionSupport is the best example of how flexible and easy the framework

is

 ActionSupport provides implementations of various important interfaces

including Action

 If your actions extend this class, you automatically gain access to these

important default behaviors of various interfaces

 Interceptors combine with ActionSupport class to provide us with a great

channel to solve complex problems

40

The ActionSupport class (Contd.) Interceptor

+

Action Support

Great Combination to solve complex problems

• The following provides the relationship between the components that we saw

now.

«interface» Action

ActionSupport

YourAction

41

Exercise • Change the Address Capture project such that it uses ActionSupport

class

42

The ActionSupport class  The following are the interfaces that the ActionSupport class implements: – Action – Validateable – ValidationAware – TextProvider – LocaleProvider

43

Exercise  We already have a Person class  In the Person class add one more attribute called age  In the screen where you captured the address, capture the age of the

person also

 Now run this program and display the age in the result screen apart

from the addresses that you already displaying

44

Dumbo Vs. Jumbo We have seen the details of Action Support class. But how do we use it in practice? What are the advantages of using this class?

There are lots of advantages. Yes that would be the next topic for the day

45

Validation  The action support class implements an interface called Validateable which

has a method called validate( )

 If your action extends ActionSupport you can override this validate method

to include any validations

46

Exercise  In the Address Capture Application, include the following

in the action class:

public void validate() { if (this.getPerson().getAge() < 18) { this.addActionError("The Age should be greater than 18. Sorry!!"); } }  Now run the Address Capture application. Enter the age

as a value less than 18

 Observe the behavior

47

Exercise  Now add the following element to the struts action

element for the address capture action

/AddressCapture.jsp  Now run the address capture application. Enter the age

as a value less than 18

 Observe the behavior  Now add the following to AddressCapture.jsp inside the

form tag

<s:actionerror />  Now run the address capture application. Enter the age

as a value less than 18

48

Dumbo Vs. Jumbo Lot of magic has happened over the past few minutes. Naturally I would like to know what happens internally

That’s why the trainer is here

49

The Internals of the Validation flow  Let us take a close look at struts-default.xml

... < interceptor-stack name="defaultStack"> ... ... <param name="excludeMethods">input,back,cancel,browse ...

50

The Internals of the Validation flow (Contd.)  When the user submits the request from the browser, struts-default.xml is referred  The default stack is mentioned as defaultStack  So each and every interceptor in this stack is invoked one by one  When the params interceptor is reached, the screen values are copied to Action

class variables

 After params interceptor, somewhere down the line, there is an interceptor called

DefaultWorkflowInterceptor with a reference called “workflow”

 So, the doIntercept method of DefaultWorkflowInterceptor is called  This method in turn invokes the validate method of OUR ACTION  Please note that the validate method does not return any value  The validate method just puts an error message using addActionError method 51

The Internals of the Validation flow (Contd.)  The doIntercept method after invoking validate method checks if there any errors

raised, by using hasErrors method

 If there are errors then the workflow is cut short. The action method is not

executed. Instead the workflow is returned back to the page by passing the return string as “input”

 If there are no errors, then the next interceptor is invoked

52

Dumbo Vs. Jumbo Things are beginning to fall in place but I have so many questions. What is the importance of the two interfaces Validateable and ValidationAware? Are these used anywhere apart from the ActionSupport class

Yes of course. The trainer will show you the source code of DefaultWorkflowInterceptor where you can see that the interfaces Validateable and ValidationAware are used to typecast the Action class to these interface types and then invoke the validate and hasErrors methods respectively. Now you know the power of interfaces and type casting!!

53

Dumbo Vs. Jumbo Yes. Cool man. My next question. You remember when we did our exercise, there were a few errors which gradually got corrected by themselves. As soon as I just added the validate method and ran the application I got some exception that there is no result definition blah blah blah. What was that?

A Small correction. The errors did not vanish by themselves. To answer your question, whenever a validation error is raised (like in our case), the interceptor returns a string called “input”. You can then catch this string in the struts.xml and provide a corresponding page. When we first ran the application, there was no result with name as input. As a result it threw an exception. After we added a new result row with name as input, that issue was resolved 54

Dumbo Vs. Jumbo But then we faced the next problem. The system just displayed the input page but did not show any error message or nothing. It just showed whatever I entered. Is not a bug in struts?

No, definitely not. Since we have mentioned this jsp in our result element the control is passed back to this page. However we did not specify to the struts framework that the error should be displayed. We then added the code <s:actionerror /> which tells the framework that the error should be displayed on the page. So, the error was promptly displayed after we included this code in the action

55

Dumbo Vs. Jumbo Wow man. That was great explanation. One last question. I was surprised to see that whatever I entered originally was still there even after the error was raised. How are the values maintained over requests.

That is the power of value stack. We saw that the params interceptor is responsible for passing the values from screen to action. This gets executed before validation. Hence the values will be available in the action class. When the error is thrown, the JSP page retrieves the value from the value stack which in turn gets it back from the action class. So, the values are still there. So, it is automatic. You do not have to do anything extra to achieve this functionality. 56

Dumbo Vs. Jumbo Technology has improved so much aaa!! Now what next!! BTW, the error message is hard coded in the action class. It’s a bad practice you know. Is there a better way?

Yes, that’s our next topic

57

Resource Bundles and Internationalization  As already seen, the ActionSupport class implements TextProvider and

LocaleProvider interfaces

 These are the interfaces used for internationalization and resource bundling  Let us now see how we can take advantage of this provision from Struts

58

Exercise  Create a properties file called struts.properties under default package  Add the following: – struts.custom.i18n.resources=guest  Create a properties file called guest.properties under default package  Add the following: – address.age.error = Age should be greater than 18!!  In the Action class instead of hardcoding the error use the following: – this.addActionError(this.getText("address.age.error");  Run and observe the behavior  Any error messages can be added in future by just adding a set of key

value pairs in the guest.properties and correspondingly referring them from the action class 59

Dumbo Vs. Jumbo All is well. But is it mandatory that the resource bundle should be named as guest. Can I name it as whatever I want?

Good Man. One more question. I see that the error message is displayed at the top of the page. Is there a way I can have error messages displayed above each field instead of at the top

Yes it is possible. Let us do this exercise to understand:

Yes of course. That’s why we have struts.properties. you can name it as whatever in struts.properties instead of guest

60

Exercise  Note down the name of the age field in the Address Capture JSP  Now in the action class instead of the addActionError do the

following: – this.addFieldError(“”,this.getText("add ress.age.error"));

 Run and observe the behavior  You will find that the error message is displayed above the Age field

instead of at the top

61

Exercise  Add a new field called salary  Add a validation for this field as follows: – If Age > 25 then Salary is mandatory  This should be displayed at the field level  If there are no validation errors, the next page should display all

details including Salary

62

Session 5 We are coming to the end of an interesting session on Struts. 2.. Stay tuned to learn more interesting concepts in Struts 2 next week. Have a great weekend.

63

Session 5 Thank You !

64