Editing Apache Groovy (section)

===Prototype extension===
Groovy offers support for [[Prototype-based programming|prototype extension]] through <code>ExpandoMetaClass</code>, Extension Modules (only in Groovy 2), Objective-C-like [[Objective-C#Categories|Categories]] and <code>DelegatingMetaClass</code>.<ref>{{cite web |url=http://groovy.codehaus.org/JN3525-MetaClasses |title=JN3525-MetaClasses |access-date=2012-10-07 |archive-url=https://web.archive.org/web/20121001122409/http://groovy.codehaus.org/JN3525-MetaClasses |archive-date=2012-10-01 |url-status=dead }}</ref>

<code>ExpandoMetaClass</code> offers a [[domain-specific language]] (DSL) to express the changes in the class easily, similar to [[Ruby (programming language)|Ruby's]] open class concept:

<syntaxhighlight lang="groovy">
Number.metaClass {
  sqrt = { Math.sqrt(delegate) }
}

assert 9.sqrt() == 3
assert 4.sqrt() == 2
</syntaxhighlight>

Groovy's changes in code through prototyping are not visible in Java, since each attribute/method invocation in Groovy goes through the metaclass registry. The changed code can only be accessed from Java by going to the metaclass registry.

Groovy also allows overriding methods as <code>getProperty()</code>, <code>propertyMissing()</code> among others, enabling the developer to intercept calls to an object and specify an action for them, in a simplified [[Aspect-oriented programming|aspect-oriented]] way. The following code enables the class <code>java.lang.String</code> to respond to the <code>hex</code> property:

<syntaxhighlight lang="groovy">
enum Color {
  BLACK('#000000'), WHITE('#FFFFFF'), RED('#FF0000'), BLUE('#0000FF')
  String hex
  Color(String hex) { 
    this.hex = hex 
  }
}

String.metaClass.getProperty = { String property ->
  def stringColor = delegate
  if (property == 'hex') {
    Color.values().find { it.name().equalsIgnoreCase stringColor }?.hex
  }
}

assert "WHITE".hex == "#FFFFFF"
assert "BLUE".hex == "#0000FF"
assert "BLACK".hex == "#000000"
assert "GREEN".hex == null
</syntaxhighlight>

The Grails framework uses metaprogramming extensively to enable [[Grails (framework)#Persistence|GORM]] dynamic finders, like <code>User.findByName('Josh')</code> and others.<ref>{{cite web |url=http://www.slideshare.net/zenMonkey/metaprogramming-techniques-in-groovy-and-grails |title=Metaprogramming Techniques in Groovy and Grails |date=11 Jun 2009}}</ref>