org.springframework/spring-core/5.3.3 : org/springframework/core/annotation/AliasFor.java

AliasFor
https://github.com/spring-projects/spring-framework: Spring Core (Spring IO)
Apache License, Version 2.0
Juergen Hoeller
/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.annotation;

import java.lang.annotation.Annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@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 MergedAnnotations. 
Implementation Requirements

Explicit aliases within an annotation:

Each attribute that makes up an aliased pair should be annotated with @AliasFor, and either attribute or value must reference the other 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.
Aliased attributes must declare the same return type.
Aliased attributes must declare a default value.
Aliased attributes must declare the same default value.
annotation should not be declared.


Explicit alias for attribute in meta-annotation:

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.
Aliased attributes must declare the same return type.
annotation must reference the meta-annotation.
The referenced meta-annotation must be meta-present on the annotation class that declares @AliasFor.


Implicit aliases within an annotation:

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).
Aliased attributes must declare the same return type.
Aliased attributes must declare a default value.
Aliased attributes must declare the same default value.
annotation must reference an appropriate meta-annotation.
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. 
Author: Sam Brannen
See Also: MergedAnnotations
SynthesizedAnnotation
Since: 4.2/**
 * {@code @AliasFor} is an annotation that is used to declare aliases for
 * annotation attributes.
 *
 * <h3>Usage Scenarios</h3>
 * <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>
 * <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>
 * <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>
 * <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>
 * <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>
 * <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>
 * <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>
 * <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 {

	Alias for attribute. Intended to be used instead of attribute when annotation is not declared — for example: @AliasFor("value") instead of @AliasFor(attribute = "value"). /**
	 * Alias for {@link #attribute}.
	 * <p>Intended to be used instead of {@link #attribute} when {@link #annotation}
	 * is not declared &mdash; for example: {@code @AliasFor("value")} instead of
	 * {@code @AliasFor(attribute = "value")}.
	 */
	@AliasFor("attribute")
	String value() default "";

	The name of the attribute that this attribute is an alias for.
See Also: value/**
	 * The name of the attribute that <em>this</em> attribute is an alias for.
	 * @see #value
	 */
	@AliasFor("value")
	String attribute() default "";

	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.
/**
	 * 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;

}
Author:	Sam Brannen
See Also:	MergedAnnotations SynthesizedAnnotation
Since:	4.2
/

org.springframework/ spring-core/ 5.3.3/ org/springframework/core/annotation/AliasFor.java

Usage Scenarios

Usage Requirements

Implementation Requirements

Example: Explicit Aliases within an Annotation

Example: Explicit Alias for Attribute in Meta-annotation

Example: Implicit Aliases within an Annotation

Example: Transitive Implicit Aliases within an Annotation

Spring Annotations Supporting Attribute Aliases
