Document @AliasFor

Issue: SPR-11512

spring-core/src/main/java/org/springframework/core/annotation/AliasFor.java

@@ -24,18 +24,120 @@ import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
 /**
- * TODO Document @AliasFor.
+ * {@code @AliasFor} is an annotation that is used to declare aliases for
+ * annotation attributes.
+ *
+ * <h3>Usage Scenarios</h3>
+ * <ul>
+ * <li><strong>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>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>
+ * </ul>
+ *
+ * <h3>Usage Requirements</h3>
+ * <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 the utility methods in
+ * {@link AnnotationUtils}. Behind the scenes, Spring will <em>synthesize</em>
+ * an annotation by wrapping it in a dynamic proxy that transparently enforces
+ * <em>attribute alias</em> semantics for annotation attributes that are
+ * annotated with {@code @AliasFor}. Similarly, {@link AnnotatedElementUtils}
+ * supports explicit meta-annotation attribute overrides when {@code @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.
+ *
+ * <h3>Implementation Requirements</h3>
+ * <ul>
+ * <li><strong>Aliases within an annotation</strong>:
+ * <ol>
+ * <li>Each attribute that makes up an aliased pair must be annotated with
+ * {@code @AliasFor}, and the {@link #attribute} must reference the
+ * <em>other</em> attribute in the pair.</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>The {@link #annotation} attribute may remain set to the default,
+ * although setting it to the declaring class for both attributes in the
+ * pair is also valid.</li>
+ * </ol>
+ * </li>
+ * <li><strong>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}; the {@link #attribute} must
+ * reference the aliased attribute in the meta-annotation; and the
+ * {@link #annotation} must reference the meta-annotation.</li>
+ * <li>Aliased attributes must declare the same return type.</li>
+ * </ol>
+ * </li>
+ * </ul>
+ *
+ * <h3>Example: Aliases within an Annotation</h3>
+ * <pre class="code"> public &#064;interface ContextConfiguration {
+ *
+ *    &#064;AliasFor(attribute = "locations")
+ *    String[] value() default {};
+ *
+ *    &#064;AliasFor(attribute = "value")
+ *    String[] locations() default {};
+ *
+ *    // ...
+ * }</pre>
+ *
+ * <h3>Example: Alias for Attribute in Meta-annotation</h3>
+ * <pre class="code"> &#064;ContextConfiguration
+ * public &#064;interface MyTestConfig {
+ *
+ *    &#064;AliasFor(annotation = ContextConfiguration.class, attribute = "locations")
+ *    String[] xmlFiles();
+ * }</pre>
+ *
+ * <h3>Spring Annotations Supporting Attribute Aliases</h3>
+ * <p>As of Spring Framework 4.2, the following annotations within core
+ * Spring have been updated to use {@code @AliasFor} to configure their
+ * internal attribute aliases.
+ * <ul>
+ * <li>{@link org.springframework.jmx.export.annotation.ManagedResource}</li>
+ * <li>{@link org.springframework.test.context.ActiveProfiles}</li>
+ * <li>{@link org.springframework.test.context.ContextConfiguration}</li>
+ * <li>{@link org.springframework.test.context.TestExecutionListeners}</li>
+ * <li>{@link org.springframework.test.context.TestPropertySource}</li>
+ * <li>{@link org.springframework.test.context.jdbc.Sql}</li>
+ * <li>{@link org.springframework.web.bind.annotation.ControllerAdvice}</li>
+ * <li>{@link org.springframework.web.bind.annotation.RequestMapping}</li>
+ * </ul>
  *
  * @author Sam Brannen
  * @since 4.2
+ * @see AnnotatedElementUtils
+ * @see AnnotationUtils
+ * @see AnnotationUtils#synthesizeAnnotation(Annotation, java.lang.reflect.AnnotatedElement)
+ * @see SynthesizedAnnotation
  */
 @Retention(RetentionPolicy.RUNTIME)
 @Target(ElementType.METHOD)
 @Documented
 public @interface AliasFor {
 
+	/**
+	 * The name of the attribute that <em>this</em> attribute is an alias for.
+	 */
 	String attribute();
 
+	/**
+	 * The type of annotation in which the aliased {@link #attribute} is declared.
+	 * <p>Defaults to {@link Annotation}, implying that the aliased attribute is
+	 * declared in the same annotation as <em>this</em> attribute.
+	 */
 	Class<? extends Annotation> annotation() default Annotation.class;
 
 }

spring-core/src/main/java/org/springframework/core/annotation/AnnotationConfigurationException.java

@@ -17,10 +17,13 @@
 package org.springframework.core.annotation;
 
 /**
- * Thrown by {@link AnnotationUtils} if an annotation is improperly configured.
+ * Thrown by {@link AnnotationUtils} and <em>synthesized annotations</em>
+ * if an annotation is improperly configured.
  *
  * @author Sam Brannen
  * @since 4.2
+ * @see AnnotationUtils
+ * @see SynthesizedAnnotation
  */
 @SuppressWarnings("serial")
 public class AnnotationConfigurationException extends RuntimeException {