how to access constants from JSTL (EL)

During our web development we may come across a situation where we want to use a constant (static final) variable for some reason in our JSP using JSTL's Expression Language (EL). As you know we access properties very easily using JSTL as below.

you might alreay know.. if we want to access a property called studentName and a method is defined as getStudentName() in the target object (lets cal it 'student') the EL would be "${student.studentName}".

But if we want to access a constant defined in the above class as
public static int MAX_RETRY_ATTEMPTS=10;

using the EL "${student.MAX_RETRY_ATTEMPTS}" , will fail
I am sure you will need to use these constants sometimes.

We can use a simple trick to solve this.

we will make use of a Map to store the constants , as below

   .....
   .....
   ......
   public final static int MAX_ALLOWED_USERS=10;
   public final static int MAX_LOGIN_ATTEMPTS=2;
   Map constants=new HashMap();
   
   public Map getConstants(){
       constants.put("MAX_ALLOWED_USERS", MAX_ALLOWED_USERS);
       constants.put("MAX_LOGIN_ATTEMPTS", MAX_LOGIN_ATTEMPTS);
       return constants;
   }

As you can see we have defined a method getConstants which returns a HashMap with our constants with appropriate keys.

Now how can we use this in our JSP ?

    The Maximum number of attempts is : "${MyForm.constants['MAX_LOGIN_ATTEMPTS']}"

clearly, we first get the Map using getConstants() method and then value of the constant using the key 'MAX_LOGIN_ATTEMPTS'.

Easy ?

Please let me know if there is a better way to do this

Some new ways to Develop web applications

Its been long since I started working on developing Java based web applications. Quite frankly, developing applications, although critical for earning rice :-) , I am no more enjoying developing them with the MVC models using heavy boilerplate code and scaringly big XML files for configuration.
The typical request response model sounded great for me in the initial days , probably I was used to develop using cgi and later php with code scattered all over the page.
The interest in some different stuff automatically dragged me to Ruby on Rails, and found it really cool, but again the basic approach was same but with emphasis on convention over configuration.

Looks like atlast I found some thing interesting in Seaside , a Continuation based framework in Smallatalk !! Yess.. I wrote it correctly , It is Smalltalk, that language we heard about in our College days.


So whats new about it ? Lets have a look at the following figure



In a typical Struts based or any Action based framework , Control flows from Browser to Action Class (or similar ) for processing some business logic by parsing the request parameters (many framework do this automatically ..but still we are dealing with fields and request parameters) and based upon the result control will be passed to render appropriate page, and this process repeats. This model allows business logic to be written in separate classes .

Now , lets us look at the Continuations based Seaside framework, In a striking contrast with the above approach , we can write our web applications , very similar to the way we write any console based application, like the below pseudo code

1) Get Values From Browser
.... Suspend Processing till user click on Submit
2) Process
3) Display Result
..Suspend processing till user responds with something
4 ....


I found it very different and very intuitive way of developing applications. There is a lot of Buzz around this framework , you too may find it interesting.

Currently I am exploring this framework and finding it a great way to develop the applications. I will write about this further in my future posts.

List of Values in Web Apps

I recall the days where we used to develop using Developer 2000 , and many applications (Desktop apps) used to feature a List of Values functionality (LOVs) where users click on a button to select from a set of options which can be better displayed in separate screen providing search/filter capabilities and an ID is returned to the Original Window's control. Although not common in websites , many web applications require this functionality ..Here is how they do it...

Let us consider a sample web application in which
1) User has to select a part ID from a list of available parts ,
2) This cannot be modled as a select(Combo) box as we need to show some extra info in a table
3) We just need part ID from popup where user can search based on part name or description

now..Look at the above figure, Page1 requires Part ID from Popup, and we are using Struts . The best way (In my opinion) is :
1) Create a JSP (Popup.jsp) with a table showing the information of Parts (array of Part Objects)
2) Create a mapping in struts-config.xml

As you know ...PartsForm is the FormBean,

1) LovParts is the Action name which will be invoked by Page1 (refer to figure above)
2) Popup.jsp is the popup that displays Parts information in a HTML table
3) PartsAction Contains(or calls) required business methods to get the list of Parts

We are almost done with this.. inorder to bring up the popup there are many methods but all involve using javascript's window.open, for our example let us assume that we just have one field in the Page 1 that can be sumbitted (using POST) to determine what to show in the Popup initially (Like pre-fetching values for a already existing Part ID)

Write a function in your JSP (or your js file) which does the following

1) but the target page shall not replace the existing one
2) Submits the contents of the form to a different window so that the resulting page will not replace the current one (we want the existing page and the Popup)

1. function postToPopup(winName){
2. var winProps = 'Width='+20+', Height='+30+', top='+10+', left='+50; //ADJUST THESE VALUES..
3. var hndWin = window.open("", winName, winProps); //OPEN A BLANK POPUP
4. document.forms[0].target=winName;
5. document.forms[0].submit(); //submit the form and open the content in popup identified by winName
   
6. }

now Just call this function on the button press :
postToPopup("partsIdPopup")


Congrats!! We have just completed the first part , ie., Opening the popup, Now after user selects the Required Part and clicks OK , the Popup should close and the PartID should be returned to the Page1. Doing this is also easy , we just need to use window.opener property to get the parent window which opened this Popup and use DOM to access the field we want to set the PartID

For example: the name of the field we want to set is partID then the code in the Popup.jsp will have this in the Button click event of OK

 
function okClicked(){
window.opener.document.forms[0].partsId=partID;
//NOw cose the Popup Window
window.close();
}

Thats it..

Hope it helped you