在Spring Boot中大量使用了@Inherited注解。我们来了解一下这个注解的用法，注解的源码：
package java.lang.annotation;/** * Indicates that an annotation type is automatically inherited. If * an Inherited meta-annotation is present on an annotation type * declaration, and the user queries the annotation type on a class * declaration, and the class declaration has no annotation for this type, * then the class's superclass will automatically be queried for the * annotation type. This process will be repeated until an annotation for this * type is found, or the top of the class hierarchy (Object) * is reached. If no superclass has an annotation for this type, then * the query will indicate that the class in question has no such annotation. * * <p>Note that this meta-annotation type has no effect if the annotated * type is used to annotate anything other than a class. Note also * that this meta-annotation only causes annotations to be inherited * from superclasses; annotations on implemented interfaces have no * effect. * * @author Joshua Bloch * @since 1.5 * @jls 9.6.3.3 @Inherited */@Documented@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.ANNOTATION_TYPE)public @interface Inherited {}
注解的作用：
当某个注解类在它的类上定义了@Inherited注解，例如SpringBoot中的 @SpringBootApplication注解，@SpringBootApplication注解类就定义了@Inherited注解，看下源码中的红色部分：
@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@SpringBootConfiguration@EnableAutoConfiguration@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })public @interface SpringBootApplication { // .....省略}
那么现在有一个我们自己开发的类使用了这个注解，例如：
@SpringBootApplication
@Servicepublic class Person {}
然后有个类Employee继承了Person
public class Employee extends Person{}
那么现在在判断Employee类上有没有@SpringBootApplication时，通过代码验证：
@Test public void test1(){ Class clazz = Employee.class ; if(clazz.isAnnotationPresent(SpringBootApplication.class)){ System.out.println("true"); } }
上面这个测试用例执行将输出true，也就是子类中能查找到@SpringBootApplication ，但同样，你用上述代码查找Employee类上是否有Spring的@Service注解时，会输出false，至此你应该明白@Inherited注解的用意了吧。
经过这样的分析，我们再来读一下JDK的文档，就会比较容易理解了，否则会觉的有些绕，下面列出 @interface注解的中文文档：
指示注释类型被自动继承。如果在注释类型声明中存在 Inherited 元注释，并且用户在某一类声明中查询该注释类型，同时该类声明中没有此类型的注释，则将在该类的超类中自动查询该注释类型。此过程会重复进行，直到找到此类型的注释或到达了该类层次结构的顶层 (Object) 为止。如果没有超类具有该类型的注释，则查询将指示当前类没有这样的注释。
注意，如果使用注释类型注释类以外的任何事物，此元注释类型都是无效的。还要注意，此元注释仅促成从超类继承注释；对已实现接口的注释无效。
联系客服
