spring-core中@AliasFor的源码解析说明

spring版本为5.0.11

 @AliasFor 表示别名，它可以注解到自定义注解的两个属性上，表示这两个互为别名，也就是说这两个属性其实同一个含义。

同时若自定义注解继承了另一个注解，要想让调用方能够设置继承过来的属性值，就必须在自定义注解中重新定义一个属性，同时声明该属性是父注解某个属性的别名。

注意：两个属性必须属性值相同的

/**
 * {@code @AliasFor} is an annotation that is used to declare aliases for
 * annotation attributes.
 *
 * <h3>Usage Scenarios</h3>使用场景
 * <ul>
 * <li><strong>Explicit aliases within an annotation</strong>: within a single
 * annotation, {@code @AliasFor} can be declared on a pair of attributes to
 * signal that they are interchangeable aliases for each other.</li>
 * <li><strong>Explicit alias for attribute in meta-annotation</strong>: if the
 * {@link #annotation} attribute of {@code @AliasFor} is set to a different
 * annotation than the one that declares it, the {@link #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 {@code @AliasFor} it is even possible to declare
 * an alias for the {@code value} attribute of a meta-annotation.</li>
 * <li><strong>Implicit aliases within an annotation</strong>: 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 <em>implicit</em> aliases
 * for each other, resulting in behavior analogous to that for explicit
 * aliases within an annotation.</li>
 * </ul>
 *
 * <h3>Usage Requirements</h3>使用需求
 * <p>Like with any annotation in Java, the mere presence of {@code @AliasFor}
 * on its own will not enforce alias semantics. For alias semantics to be
 * enforced, annotations must be <em>loaded</em> via {@link MergedAnnotations}.
 *
 * <h3>Implementation Requirements</h3>实现需求
 * <ul>
 * <li><strong>Explicit aliases within an annotation</strong>:
 * <ol>
 * <li>Each attribute that makes up an aliased pair should be annotated with
 * {@code @AliasFor}, and either {@link #attribute} or {@link #value} must
 * reference the <em>other</em> attribute in the pair. Since Spring Framework
 * 5.2.1 it is technically possible to annotate only one of the attributes in an
 * aliased pair; however, it is recommended to annotate both attributes in an
 * aliased pair for better documentation as well as compatibility with previous
 * versions of the Spring Framework.</li>
 * <li>Aliased attributes must declare the same return type.</li>
 * <li>Aliased attributes must declare a default value.</li>
 * <li>Aliased attributes must declare the same default value.</li>
 * <li>{@link #annotation} should not be declared.</li>
 * </ol>
 * </li>
 * <li><strong>Explicit alias for attribute in meta-annotation</strong>:
 * <ol>
 * <li>The attribute that is an alias for an attribute in a meta-annotation
 * must be annotated with {@code @AliasFor}, and {@link #attribute} must
 * reference the attribute in the meta-annotation.</li>
 * <li>Aliased attributes must declare the same return type.</li>
 * <li>{@link #annotation} must reference the meta-annotation.</li>
 * <li>The referenced meta-annotation must be <em>meta-present</em> on the
 * annotation class that declares {@code @AliasFor}.</li>
 * </ol>
 * </li>
 * <li><strong>Implicit aliases within an annotation</strong>:
 * <ol>
 * <li>Each attribute that belongs to a set of implicit aliases must be
 * annotated with {@code @AliasFor}, and {@link #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).</li>
 * <li>Aliased attributes must declare the same return type.</li>
 * <li>Aliased attributes must declare a default value.</li>
 * <li>Aliased attributes must declare the same default value.</li>
 * <li>{@link #annotation} must reference an appropriate meta-annotation.</li>
 * <li>The referenced meta-annotation must be <em>meta-present</em> on the
 * annotation class that declares {@code @AliasFor}.</li>
 * </ol>
 * </li>
 * </ul>
 *
 * <h3>Example: Explicit Aliases within an Annotation</h3>示例：显式别名
 * <p>In {@code @ContextConfiguration}, {@code value} and {@code locations}
 * are explicit aliases for each other.
 *
 * <pre class="code"> public &#064;interface ContextConfiguration {
 *
 *    &#064;AliasFor("locations")
 *    String[] value() default {};
 *
 *    &#064;AliasFor("value")
 *    String[] locations() default {};
 *
 *    // ...
 * }</pre>
 *
 * <h3>Example: Explicit Alias for Attribute in Meta-annotation</h3>示例：显式别名在元注解
 * <p>In {@code @XmlTestConfig}, {@code xmlFiles} is an explicit alias for
 * {@code locations} in {@code @ContextConfiguration}. In other words,
 * {@code xmlFiles} overrides the {@code locations} attribute in
 * {@code @ContextConfiguration}.
 *
 * <pre class="code"> &#064;ContextConfiguration
 * public &#064;interface XmlTestConfig {
 *
 *    &#064;AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
 *    String[] xmlFiles();
 * }</pre>
 *
 * <h3>Example: Implicit Aliases within an Annotation</h3>示例：隐式别名
 * <p>In {@code @MyTestConfig}, {@code value}, {@code groovyScripts}, and
 * {@code xmlFiles} are all explicit meta-annotation attribute overrides for
 * the {@code locations} attribute in {@code @ContextConfiguration}. These
 * three attributes are therefore also implicit aliases for each other.
 *
 * <pre class="code"> &#064;ContextConfiguration
 * public &#064;interface MyTestConfig {
 *
 *    &#064;AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
 *    String[] value() default {};
 *
 *    &#064;AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
 *    String[] groovyScripts() default {};
 *
 *    &#064;AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
 *    String[] xmlFiles() default {};
 * }</pre>
 *
 * <h3>Example: Transitive Implicit Aliases within an Annotation</h3>示例：过渡隐式别名
 * <p>In {@code @GroovyOrXmlTestConfig}, {@code groovy} is an explicit
 * override for the {@code groovyScripts} attribute in {@code @MyTestConfig};
 * whereas, {@code xml} is an explicit override for the {@code locations}
 * attribute in {@code @ContextConfiguration}. Furthermore, {@code groovy}
 * and {@code xml} are transitive implicit aliases for each other, since they
 * both effectively override the {@code locations} attribute in
 * {@code @ContextConfiguration}.
 *
 * <pre class="code"> &#064;MyTestConfig
 * public &#064;interface GroovyOrXmlTestConfig {
 *
 *    &#064;AliasFor(annotation = MyTestConfig.class, attribute = "groovyScripts")
 *    String[] groovy() default {};
 *
 *    &#064;AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
 *    String[] xml() default {};
 * }</pre>
 *
 * <h3>Spring Annotations Supporting Attribute Aliases</h3>spring注解支持属性别名
 * <p>As of Spring Framework 4.2, several annotations within core Spring
 * have been updated to use {@code @AliasFor} to configure their internal
 * attribute aliases. Consult the Javadoc for individual annotations as well
 * as the reference manual for details.
 *
 * @author Sam Brannen
 * @since 4.2
 * @see MergedAnnotations
 * @see SynthesizedAnnotation
 */
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
@Documented
public @interface AliasFor {
    @AliasFor("attribute")
    String value() default "";

    @AliasFor("value")
    String attribute() default "";

    Class<? extends Annotation> annotation() default Annotation.class;
}