Expression Language - Beyond binding (mini-tutorial)

Question

Expression Language - Beyond binding (mini-tutorial)
Creator has a binding logic that is very easy to use. You right click on a component and choose which properties to bind onto your backing beans. Then you manipulate the backing bean property to change the bound property of your component. Or users input information into your app and the component writes through that input to your backing bean. It's cool.
But sometimes you find yourself binding things to other things and you wonder if this is the simplest way to do things. A classic example is the old "how do I highlight a row of this table based on some datafield?" question. This has been answered by Tor Norbyte and the method works but to me it looks a little disjointed and clumsy.
The classic solution involves binding your tableColumn's style property to a String type bean. Then inside the bean you write the logic that selectively returns a "background-color:'yellow'" string or something similar (Tor used css). The problem is that within the bean you still need to figure out what is the current row of your table. To do this you need to invoke and EL expression from the Java code which, if you were trying to avoid EL, is circular logic. Worse still, imagine you want to color columns separately.
Here's an alternative solution using only Expression Language. It's self contained, easy to debug and maintain and once you've taken a look at it is pretty simple.
Here's the JSP tag that does this:
<ui:tableColumn binding="#{myHomePage.tableColumn7}"
headerText="Expired" id="tableColumn7" 
style="#{currentRow.value['expired']>0?'background-color: red':''}">To make it work I had to manually type the expression in the JSP file. The property editor will work with some EL statements but not all. Sometimes it mangles your EL expression and this can take a bit of debugging. <tip>Use the JSP editor...</tip>
The bit I had to manually type is this:
style="#{currentRow.value['expired']>0?'background-color: red':''}"All it does is check if some field in my TableRowGroup called 'expired' is greater than zero and if so highlight the cell. The construction is #{[condition]?[true value]:[false value]}
You can also mix text and expressions in an EL expression:
<ui:staticText ....  text="#{SessionBean1.userName}'s Home Page"/>If "Milo" is logged in he'll see "Milo's Home Page" where I placed this static text component.
Here's an example I've got that's part of a Button tag in my JSP:
visible="#{currentRow.value['d2.ReplacedBy']!=null}"In this case I don't want users pressing the button if the field is null, so I hide it for rows where the field is null.
Bean or EL?
Well strictly speaking it's not either or! In order for a Bean to be bound to a component you need an EL expression! So the question is do you implement you logic in a bean or an EL? Well that's pretty easy -- if the logic is simple and can be expressed in one line then use an EL only. Advantages: less dependencies and Bean properties floating around. If the logic is more complex use a backing bean and simple binding.
More to explore
Tor's blog entry:
http://blogs.sun.com/roller/page/tor/20050113#creator_how_to_highlight_specific
Winston's Blog entry
http://blogs.sun.com/roller/page/winston?entry=setting_table_column_style
[How do I add row numbers to a table?]
http://forum.sun.com/jive/thread.jspa?threadID=112916&tstart=0
[How do I use EL to bind to an Object's public property or field?]
http://forum.sun.com/jive/thread.jspa?threadID=112434&tstart=0
Addendum 1-Dec-06 Highlighting every 2nd row in a table
The above thread on how to add row numbers to a table also exposes a pretty simple method to highlight every nth row in a table (a fairly common request)
Something like this should work:
<ui:tableColumn ...... style="#{currentRow.tableRow.rowId%2==1?'background-color: yellow':''}">Message was edited by:
yossarian

Answer 1

Hi Yossarian,
Thanks for posting the simple but elegant solution. I liked the solution.
To make it work I had to manually type the expression in the JSP file.
The property editor will work with some EL statements but not all.
Sometimes it mangles your EL expression and this can take a bit of
debugging.Please file a bug against this at
http://www.netbeans.org./community/issues.html
Use visualweb as the sub category.
Thanks
- Winston
http://blogs.sun.com/winston