org.springframework.core.annotation

## 注释类型 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

  - Explicit 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.
  - Explicit 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.
  - Implicit aliases within an annotation: if one or more attributes within an annotation are declared as attribute overrides for the same meta-annotation attribute (either directly or transitively), those attributes will be treated as a set of implicit aliases for each other, resulting in behavior analogous to that for explicit aliases within an 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

  - Explicit aliases within an annotation:
    1. Each attribute that makes up an aliased pair must be annotated with `@AliasFor`, and either `attribute()` or `value()` 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. `annotation()` should not be declared.
  - Explicit 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`, and `attribute()` must reference the attribute in the meta-annotation.
    2. Aliased attributes must declare the same return type.
    3. `annotation()` must reference the meta-annotation.
    4. The referenced meta-annotation must be meta-present on the annotation class that declares `@AliasFor`.
  - Implicit aliases within an annotation:
    1. Each attribute that belongs to a set of implicit aliases must be annotated with `@AliasFor`, and `attribute()` must reference the same attribute in the same meta-annotation (either directly or transitively via other explicit meta-annotation attribute overrides within the annotation hierarchy).
    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. `annotation()` must reference an appropriate meta-annotation.
    6. The referenced meta-annotation must be meta-present on the annotation class that declares `@AliasFor`.

  ### Example: Explicit Aliases within an Annotation

  In `@ContextConfiguration`, `value` and `locations` are explicit aliases for each other.

       public @interface ContextConfiguration {

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

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

          // ...
       }

  ### Example: Explicit Alias for Attribute in Meta-annotation

  In `@XmlTestConfig`, `xmlFiles` is an explicit alias for `locations` in `@ContextConfiguration`. In other words, `xmlFiles` overrides the `locations` attribute in `@ContextConfiguration`.

       @ContextConfiguration
       public @interface XmlTestConfig {

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

  ### Example: Implicit Aliases within an Annotation

  In `@MyTestConfig`, `value`, `groovyScripts`, and `xmlFiles` are all explicit meta-annotation attribute overrides for the `locations` attribute in `@ContextConfiguration`. These three attributes are therefore also implicit aliases for each other.

       @ContextConfiguration
       public @interface MyTestConfig {

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

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

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

  ### Example: Transitive Implicit Aliases within an Annotation

  In `@GroovyOrXmlTestConfig`, `groovy` is an explicit override for the `groovyScripts` attribute in `@MyTestConfig`; whereas, `xml` is an explicit override for the `locations` attribute in `@ContextConfiguration`. Furthermore, `groovy` and `xml` are transitive implicit aliases for each other, since they both effectively override the `locations` attribute in `@ContextConfiguration`.

       @MyTestConfig
       public @interface GroovyOrXmlTestConfig {

          @AliasFor(annotation = MyTestConfig.class, attribute = "groovyScripts")
          String[] groovy() default {};

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

  ### 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.

  从以下版本开始:
    4.2
  作者:
    Sam Brannen
  另请参阅:
    `AnnotatedElementUtils`, `AnnotationUtils`, `AnnotationUtils.synthesizeAnnotation(Annotation, java.lang.reflect.AnnotatedElement)`, `SynthesizedAnnotation`

- - ### 可选元素概要

    可选元素
    限定符和类型 | 可选元素和说明
    --- | ---
    `Class<? extends Annotation>` | `annotation` The type of annotation in which the aliased `attribute()` is declared.
    `String` | `attribute` The name of the attribute that this attribute is an alias for.
    `String` | `value` Alias for `attribute()`.

- - ### 元素详细资料

    - #### value

          @AliasFor(value="attribute")
          public abstract String value

      Alias for `attribute()`.

      Intended to be used instead of `attribute()` when `annotation()` is not declared — for example: `@AliasFor("value")` instead of `@AliasFor(attribute = "value")`.

      默认值:
        ""

  - - #### attribute

          @AliasFor(value="value")
          public abstract String attribute

      The name of the attribute that this attribute is an alias for.
      另请参阅:
        `value()`
      默认值:
        ""

  - - #### annotation

          public abstract Class<? extends 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.

      默认值:
        java.lang.annotation.Annotation.class