Package groovy.lang

Annotation Type Category

@Documented @Retention(SOURCE) @Target(TYPE) public @interface Category
Transforms an instance-style Groovy class or interface to become a static-style conventional Groovy category.

Groovy categories are the original mechanism used by Groovy when augmenting classes with new methods. Writing categories required using a class writing style where all methods were static and an additional self parameter was defined. The self parameter and static nature of the methods disappeared once applied by Groovy's metaclass framework but some regarded the writing style as a little noisy. This transformation allows you to write your categories without the "apparent noise" but adds it back in during compilation so that the classes appear as normal categories.

It might seem strange writing your class/object enhancements using a succinct notation, then having "noise" added, then having the noise removed during category application. If this worries you, then you may also like to consider using Groovy's ExpandoMetaClass mechanism which avoids the category definition altogether. If you already have an investment in categories or like some of the other features which categories currently give you, then read on.

The mechanics: during compilation, all methods are transformed to static ones with an additional self parameter of the type you supply as the annotation parameter (the default type for the self parameters is Object which might be more broad reaching than you like so it is usually wise to specify a type). Properties invoked using 'this' references are transformed so that they are instead invoked on the additional self parameter and not on the Category instance. (Remember that once the category is applied, the reverse will occur and we will be back to conceptually having methods on the this references again!)

Classes conforming to the conventional Groovy category conventions can be used within use statements or mixed in at runtime with the mixin method on classes.

An example showing a use statement (allowing fine-grained application of the category methods):

 class IntegerOps {
     def triple() {
         this * 3

 use (IntegerOps) {
     assert 25.triple() == 75
Or, "mixing in" your methods at runtime:
 class Shuffler {
     def shuffle() {
         def result = new ArrayList(this)

 class Sentence extends ArrayList {
     Sentence(Collection initial) { super(initial) }
 Sentence.mixin Shuffler

 def words = ["The", "quick", "brown", "fox"]
 println new Sentence(words).shuffle()
 // => [quick, fox, The, brown]       (order will vary)
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