Groovy Documentation

## groovy.lang
[Java] Annotation Type Delegate

    java.lang.Object
      groovy.lang.Delegate

    @Retention(RetentionPolicy.SOURCE)
    @Target({ElementType.FIELD})
    @GroovyASTTransformationClass("org.codehaus.groovy.transform.DelegateASTTransformation")
    public @interface Delegate

Field annotation to automatically delegate part of the functionality of an owner class to the annotated field.

All public instance methods present in the type of the annotated field and not present in the owner class will be added to owner class at compile time. The implementation of such automatically added methods is code which calls through to the delegate as per the normal delegate pattern.

As an example, consider this code:

     class Event {
         @Delegate Date when
         String title, url
     }

     def gr8conf = new Event(title: "GR8 Conference",
                               url: "http://www.gr8conf.org",
                              when: Date.parse("yyyy/MM/dd", "2009/05/18"))

     def javaOne = new Event(title: "JavaOne",
                               url: "http://java.sun.com/javaone/",
                              when: Date.parse("yyyy/MM/dd", "2009/06/02"))

     assert gr8conf.before(javaOne.when)

In this example, the `Event` class will have a method called `before(Date otherDate)` as well as other public methods of the `Date` class. The implementation of the `before()` method will look like this:

         public boolean before(Date otherDate) {
             return when.before(otherDate);
         }

By default, the owner class will also be modified to implement any interfaces implemented by the field. So, in the example above, because `Date` implements `Cloneable` the following will be true:

     assert gr8conf instanceof Cloneable

This behavior can be disabled by setting the annotation's `interfaces` element to false, i.e. `@Delegate(interfaces = false)`, e.g. in the above example, the delegate definition would become:

         @Delegate(interfaces = false) Date when

and the following would be true:

     assert !(gr8conf instanceof Cloneable)

If multiple delegate fields are used and the same method signature occurs in more than one of the respective field types, then the delegate will be made to the first defined field having that signature. If this does occur, it might be regarded as a smell (or at least poor style) and it might be clearer to do the delegation by long hand.

By default, methods of the delegate type marked as `@Deprecated` are not automatically added to the owner class. You can force these methods to be added by setting the annotation's `deprecated` element to true, i.e. `@Delegate(deprecated = true)`.

For example, in the example above if we change the delegate definition to:

         @Delegate(deprecated = true) Date when

then the following additional lines will execute successfully (during 2009):

     assert gr8conf.year + 1900 == 2009
     assert gr8conf.toGMTString().contains(" 2009 ")

Otherwise these lines produce a groovy.lang.MissingPropertyException or groovy.lang.MissingMethodException respectively as those two methods are `@Deprecated` in `Date`.

Technical notes:

- Static methods, synthetic methods or methods from the `GroovyObject` interface are not candidates for delegation
- Non-abstract non-static methods defined in the owner class or its superclasses take precedence over methods with identical signatures from a `@Delegate` field
- All methods defined in the owner class (including static, abstract or private etc.) take precedence over methods with identical signatures from a `@Delegate` field
- Recursive delegation to your own class is not allowed
- Mixing of `@Delegate` with default method arguments is known not to work in some cases. We recommend not using these features together.

Authors:
  Alex Tkachman
  Paul King

Optional Element Summary

`boolean`

`deprecated`

Deprecated:

@default false

Returns:
  true if owner class should delegate to methods annotated with

`boolean`

`interfaces`

@default true

Returns:
  true if owner class should implement interfaces implemented by field

Method Summary

Methods inherited from class java.lang.Object
java.lang.Object#wait(long), java.lang.Object#wait(long, int), java.lang.Object#wait(), java.lang.Object#equals(java.lang.Object), java.lang.Object#toString(), java.lang.Object#hashCode(), java.lang.Object#getClass(), java.lang.Object#notify(), java.lang.Object#notifyAll()

Element Detail

### deprecated

    public boolean deprecated

  Deprecated:
  @default false
  Returns:
    true if owner class should delegate to methods annotated with

### interfaces

    public boolean interfaces

  @default true
  Returns:
    true if owner class should implement interfaces implemented by field

Groovy Documentation