testing for empty groovy closure?

Question

I want to let users supply a groovy class with a property that is a file-selector closure which I pass on to AntBuilder's 'copy' task:

  class Foo {
    def ANT = { fileset(dir:'/tmp/tmp1') }
  }

in my code, I pick up the ANT property as 'fAnt' and pass to Ant:

  ant.copy(todir:'/tmp/tmp2', fAnt)

This works - but, if the user passes in an empty closure (def ANT={}) or with a selector that doesn't select anything (maybe the fileset dir doesn't exist) then it blows up. I tried surrounding the ant copy with a try-catch to catch the InvokerInvocationException, but somehow the exception comes through anyway ... while I'm tracking that down, is there a way to read back a groovy Closure's contents as a string, or to test if it's empty?

Answer 1

One mystery solved - the exception I need to catch is org.apache.tools.ant.BuildException - so I can just catch that to trap errors, but the original question remains - is there a way to examine a Closure's contents?

Answer 2

In short: No. You can't decompile a closure in a meanngful way at runtime. If it's user supplied, the Closure could even be a Java class.

Long answer: If you want to do a lot of work, you might be able to, but it's probably not worth it. The Groovy parser is part of the API, so if you have access to the source, you can theoretically examine the AST and determine if the closure is empty. Look into the SourceUnit class.

It's almost certainly not worth the effort though. You're better off catching the exception and adding a helpful message like "You may have passed an empty closure or invalid fileset".