Package groovy.lang

# Annotation Interface Singleton

@Documented @Retention(SOURCE) @Target(TYPE) public @interface Singleton
Class annotation to make a singleton class. The singleton is obtained through normal property access using the singleton property (defaults to "instance"). Such classes can be initialized during normal static initialization of the class or lazily (on first access). To make the singleton lazy use `@Singleton(lazy=true)`. Lazy singletons are implemented with double-checked locking and a volatile backing field. By default, no explicit constructors are allowed. To create one or more explicit constructors use `@Singleton(strict=false)`. This could be used to:

- provide your own custom initialization logic in your own no-arg constructor - you will be responsible for the entire code (the `@Singleton` annotation becomes merely documentation)
- provide one or more constructors with arguments for a quasi-singleton - these constructors will be used to create instances that are independent of the singleton instance returned by the singleton property

- ## Optional Element Summary

  Optional Elements
  Modifier and Type
  Optional Element
  Description
  `boolean`
  `lazy`
  `String`
  `property`
  `boolean`
  `strict`

- ## Element Details

  - ### lazy

    boolean lazy
    Returns:
      if this singleton should be lazy
    Default:
      false

  - ### strict

    boolean strict
    Returns:
      if this singleton should have strict semantics
    Default:
      true

  - ### property

    String property
    Returns:
      the singleton property name
    Default:
      "instance"