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javax.inject
Annotation Type Scope

@Target(value=ANNOTATION_TYPE)
@Retention(value=RUNTIME)
@Documented
public @interface Scope

Identifies scope annotations. A scope annotation applies to a class containing an injectable constructor and governs how the injector reuses instances of the type. By default, if no scope annotation is present, the injector creates an instance (by injecting the type's constructor), uses the instance for one injection, and then forgets it. If a scope annotation is present, the injector may retain the instance for possible reuse in a later injection. If multiple threads can access a scoped instance, its implementation should be thread safe. The implementation of the scope itself is left up to the injector.

In the following example, the scope annotation @Singleton ensures that we only have one Log instance:

   @Singleton
   class Log {
     void log(String message) { ... }
   }

The injector generates an error if it encounters more than one scope annotation on the same class or a scope annotation it doesn't support.

A scope annotation:

  • is annotated with @Scope, @Retention(RUNTIME), and typically @Documented.
  • should not have attributes.
  • is typically not @Inherited, so scoping is orthogonal to implementation inheritance.
  • may have restricted usage if annotated with @Target. While this specification covers applying scopes to classes only, some injector configurations might use scope annotations in other places (on factory method results for example).

For example:

   @java.lang.annotation.Documented
   @java.lang.annotation.Retention(RUNTIME)
   @javax.inject.Scope
   public @interface RequestScoped {}

Annotating scope annotations with @Scope helps the injector detect the case where a programmer used the scope annotation on a class but forgot to configure the scope in the injector. A conservative injector would generate an error rather than not apply a scope.

See Also:
@Singleton

Copyright (C) 2009 The JSR-330 Expert Group. Licensed under the Apache License, Version 2.0.