org.springframework.core.annotation

Annotation Type AliasFor

@Retention(value=RUNTIME)
 @Target(value=METHOD)
 @Documented
public @interface AliasFor

@AliasFor is an annotation that is used to declare aliases for annotation attributes.

Usage Scenarios

• Aliases within an annotation: within a single annotation, @AliasFor can be declared on a pair of attributes to signal that they are interchangeable aliases for each other.
• Alias for attribute in meta-annotation: if the annotation() attribute of @AliasFor is set to a different annotation than the one that declares it, the attribute() is interpreted as an alias for an attribute in a meta-annotation (i.e., an explicit meta-annotation attribute override). This enables fine-grained control over exactly which attributes are overridden within an annotation hierarchy. In fact, with @AliasFor it is even possible to declare an alias for the value attribute of a meta-annotation.

Usage Requirements

Like with any annotation in Java, the mere presence of @AliasFor on its own will not enforce alias semantics. For alias semantics to be enforced, annotations must be loaded via the utility methods in AnnotationUtils. Behind the scenes, Spring will synthesize an annotation by wrapping it in a dynamic proxy that transparently enforces attribute alias semantics for annotation attributes that are annotated with @AliasFor. Similarly, AnnotatedElementUtils supports explicit meta-annotation attribute overrides when @AliasFor is used within an annotation hierarchy. Typically you will not need to manually synthesize annotations on your own since Spring will do that for you transparently when looking up annotations on Spring-managed components.

Implementation Requirements

• Aliases within an annotation:
  1. Each attribute that makes up an aliased pair must be annotated with @AliasFor, and the attribute() must reference the other attribute in the pair.
  2. Aliased attributes must declare the same return type.
  3. Aliased attributes must declare a default value.
  4. Aliased attributes must declare the same default value.
  5. The annotation() attribute may remain set to the default, although setting it to the declaring class for both attributes in the pair is also valid.
• Alias for attribute in meta-annotation:
  1. The attribute that is an alias for an attribute in a meta-annotation must be annotated with @AliasFor; the attribute() must reference the aliased attribute in the meta-annotation; and the annotation() must reference the meta-annotation.
  2. Aliased attributes must declare the same return type.

Example: Aliases within an Annotation

 public @interface ContextConfiguration {

    @AliasFor(attribute = "locations")
    String[] value() default {};

    @AliasFor(attribute = "value")
    String[] locations() default {};

    // ...
 }

Example: Alias for Attribute in Meta-annotation

 @ContextConfiguration
 public @interface MyTestConfig {

    @AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
    String[] xmlFiles();
 }

Spring Annotations Supporting Attribute Aliases

As of Spring Framework 4.2, several annotations within core Spring have been updated to use @AliasFor to configure their internal attribute aliases. Consult the Javadoc for individual annotations as well as the reference manual for details.

Since:

4.2

Author:

Sam Brannen

See Also:

AnnotatedElementUtils, AnnotationUtils, AnnotationUtils.synthesizeAnnotation(Annotation, java.lang.reflect.AnnotatedElement), SynthesizedAnnotation

Required Element Summary

Required Elements

Modifier and Type	Required Element and Description
java.lang.String	attribute The name of the attribute that this attribute is an alias for.

Optional Element Summary

Optional Elements

Modifier and Type	Optional Element and Description
java.lang.Class<? extends java.lang.annotation.Annotation>	annotation The type of annotation in which the aliased attribute() is declared.

Element Detail

attribute

public abstract java.lang.String attribute

The name of the attribute that this attribute is an alias for.

annotation

public abstract java.lang.Class<? extends java.lang.annotation.Annotation> annotation

The type of annotation in which the aliased attribute() is declared.

Defaults to Annotation, implying that the aliased attribute is declared in the same annotation as this attribute.

Default:

java.lang.annotation.Annotation.class