Dynamic Groovy: Groovy's Equivalent to Ruby Open Classes

A lot of hype has been made of Ruby's open classes, and for good reason as they're pretty darn cool. Since Groovy classes compile down to byte code (ie a Groovy class is a Java class) it has always been a little more problematic to add dynamic features to classes you don't have control over.

If you're the class implementor for example you could override invokeMethod and getProperty, but if you're not your only option was Groovy categories, which aren't nearly as elegant, or implementing your own custom MetaClass which exposed you to Groovy internals and wasn't very fun.

However, this is all changing because I've just committed major improvements and written the documentation for Groovy's dynamic meta class mechanism, the ExpandoMetaClass. This is the system I discussed with Neal Ford at JavaOne who felt, prior to our discussion, that Ruby had the upper hand because of open classes. After our discussion and a short demo of what ExpandoMetaClass is capable of, Neal changed his mind and upgraded Groovy's rating on his language scale.

So in the next beta release, you'll see a new improved version of ExpandoMetaClass with features such as:

• The ability to add methods onto interfaces
• The ability to override invokeMethod, getProperty and setProperty to provide "missing method" behaviour
• The ability to "borrow" methods from other classes
• The ability to dynamically construct method property names at runtime
  
• Improvements to thread safety whilst performing MetaClass modifications

Here is a little example:

String.metaClass.swapCase = {->
   def sb = new StringBuffer()
   delegate.each {
        sb << (Character.isUpperCase(it as char) ?                    
               Character.toLowerCase(it as char) :                     
               Character.toUpperCase(it as char))       
   }       
   sb.toString() 
}  
assert "UpAndDown" == "uPaNDdOWN".swapCase() 

ExpandoMetaClass has been at the heart of Grails for a while, so is completely production ready. Now its time to bring it to the masses with the next beta release of Groovy targeted for the end of June.