Reprinted from ：https://www.cnblogs.com/lqtbk/p/10141455.html

Yuan notes ( Notes to notes )

1. @Retention

It is defined in front of an annotation class , Used to describe the life cycle of the annotation .
　　 It has the following parameters ：
　　　　RetentionPolicy.SOURCE： Specifies that the annotation is only kept in the source file .
　　　　RetentionPolicy.CLASS： Specifies that the annotation is reserved only in class In file .( default )
　　　　RetentionPolicy.RUNTIME： Specifies that annotations can be retained during program execution .

2. @Target

It is defined in front of an annotation class , Used to indicate which elements the annotation can be declared in front of .( By default, it can be placed before any element )
　　 It has the following parameters ：
　　　　ElementType.TYPE： Note that the annotation can only be declared in one class 、 Interface 、 Before enumeration .
　　　　ElementType.FIELD： The annotation can only be declared before the field of a class .
　　　　ElementType.METHOD： The annotation can only be declared before the method of a class .
　　　　ElementType.PARAMETER： Indicates that the annotation can only be declared before a method parameter .
　　　　ElementType.CONSTRUCTOR： The annotation can only be declared before the constructor of a class .
　　　　ElementType.LOCAL_VARIABLE： The annotation can only be declared before a local variable .
　　　　ElementType.ANNOTATION_TYPE： Indicates that the annotation can only be declared before an annotation type .
　　　　ElementType.PACKAGE： The annotation can only be declared before a package name .

3. @Inherited Indicates that the annotation will be inherited by subclasses .

It should be noted that , Annotation with this meta annotation , Only when used on class elements can it work . This is JDK General original words :

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

But annotations on other elements , As long as you don't override the elements in the parent class , It will be inherited . That's why there are getDeclaredAnnotations() and getAnnotations() Why .

4. @Documented

Indicates that the JavaDoc When the document , The annotation will also appear in javaDoc In the document .