How To Pass A Null Value To A Custom Tag Library

Today I found something very interesting.  I wanted to pass a java.math.BigDecimal to a custom JSP Tag Library and format it as a percentage, however if the BigDecimal was null I didn't want to show it.

Easy, I thought. So I created a class similar to the following...

However, when I called this taglib with a null value I still got a value of 0.

The output:
0

After a bit of trial and error I discovered that the JSP tab library framework does a little bit of magic on known classes, such as Strings and Numeric objects. Classes of Numeric types that are null get converted to 0 and Strings get converted into the empty string ""

I discovered that if you change your internal value to an java.lang.Object instead of a java.math.BigDecimal the magic doesn't know what to do and just passes null to your class.

So the new class looks like the following...

So now when I use the tag and pass a null BigDecimal to it I get the required result of no output.

WIN

I have an example web app demonstrating this, if anyone wants a copy just let me know.

Labels: java, jstl, null, tag library, taglib