spring之@AliasFor

目录

一、介绍

二、使用

2.1、Explicit aliases within an annotation

2.2、Explicit alias for attribute in meta-annotation

2.3、Implicit aliases within an annotation

参考

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一、介绍

@AliaFor是一个用于声明注解属性别名的注解。可以让同一个注解里的两个属性成为一个属性，两者互为别名，也可以覆盖元注解的属性。下面是他的定义：

@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;
}

有三个属性，value和attribute互为别名，下面是它的api文档说明：

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.
java.lang.String	attribute The name of the attribute that thisattribute is an alias for.
java.lang.String	value Alias for attribute().

value和attribute指定一个属性名，而被@AliasFor注解的属性名为该属性的别名。比如value是attribute的别名，attribute是value的别名。

二、使用

2.1、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.

意思就是同一个注解内可以相互声明别名，下面声明一个@ContextConfiguration

 public @interface ContextConfiguration {

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

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

    // ...
 }

然后value和location互为别名，注意返回类型和默认值要一致。

2.2、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.

通过aliasfor来覆盖元注解的属性，此时不需要默认值了，但需要使用annotation指定元注解，下面声明@XmlTestConfig

 @ContextConfiguration
 public @interface XmlTestConfig {

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

声明了@ContextConfiguration的locations属性，此时可以通过@XmlTestConfig的xmlFiles来为@ContextConfiguration的locations赋值。

下面看一个spring文档中的一个例子1.10.2. Meta-annotations

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Scope(WebApplicationContext.SCOPE_SESSION)
public @interface SessionScope {
    @AliasFor(annotation = Scope.class)
    ScopedProxyMode proxyMode() default ScopedProxyMode.TARGET_CLASS;

}

上面第一点说到，必须要使用attribute来指明覆盖的属性，但是这里没有使用，我猜它会自己主动在元注解中找同名的属性吧。

2.3、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.

就是说，同一个注解中的多个属性覆盖了元注解的一个相同的属性，此时，这些属性隐性的互为别名了，注意属性覆盖的传递性。

下面看一个声明：

 @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 {};
 }

此刻，三个属性同时是元注解locations的别名，那么这三个属性隐性互为别名。

下面看看隐性传递的例子：

 @MyTestConfig
 public @interface GroovyOrXmlTestConfig {

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

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

属性覆盖具有传递性，因此这两个属性最终覆盖了@ContextConfiguration的locations属性，因此这两个属性互为别名。

参考

https://docs.spring.io/spring-framework/docs/5.0.3.RELEASE/javadoc-api/org/springframework/core/annotation/AliasFor.html#annotation--

https://docs.spring.io/spring-framework/docs/current/spring-framework-reference/core.html#beans-meta-annotations

https://blog.youkuaiyun.com/jdbdh/article/details/82721178