Jelly uses the Jexl Expression Language, which is a superset of JSTL (Java Standard Tag Library). To quote from JSTL in Action,
"The empty operator determines whether a collection or string is empty or null. For instance, ${empty param.firstname} will be true only if a request parameter named firstname is not present. JSTL expressions can also compare items directly against the keyword null, as in ${param.firstname == null}, but this is an advanced use."
Although Jelly is not always in a web context (ie, doesn't have the param implicit object), the same rules apply. In addition, to quote from the jexl home page (http://jakarta.apache.org/commons/jexl.html):
Jexl has extended the JSTL in a few ways :
- Support for invocation of any accessible method (see example above).
- Added a general size() method, which works on String, returning length, Map, returning # of keys, and List and arrays, returning the number of elements.
- Optional syntax for the 'empty' function : empty(obj)
- Misc : '+' has been overloaded to be use as a String concatenation operator
The "invocation of any accessible method" means that you can call ${object.getProperty()}, and it will be the same as ${object.property}, or ${object.setProperty(someOtherObject)}. You can also call methods that do not correspond to properties, such as ${someString.endsWith(otherString)}.
Strongly recommend reading up on Jexl and JSTL.
Matt
Vikas Phonsa wrote:
Hi Guys,
I have this line in one of the jelly script in a maven plugin. I don't know jelly but have checked out its major tags. Could you pls help me understand this:
<j:if test="${empty outputDir}">
I undertand j:if test
But in ${empty outputDir}" does empty refers to some inbuilt function of checking the outputDir or what ?
Or is it supposed to be some function written somewhere in the jelly script that I'm using.
Please guide
Thanks
Vikas
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