Annotation - Annotation

 

1. Annotation definition

Annotations, also known as metadata, provide a formal way for us to add information to source code, allowing us to read useful information when needed

2. Meta annotations

   Meta-annotations are annotations that have been defined by JDK .    There are four standard meta-annotations defined by JDK :

 

	definition	value
@Target	Indicates the scope of the object modified by Annotation	ElementType： 　1. CONSTRUCTOR: used to describe the constructor 　2.FIELD: used to describe the domain 　3.LOCAL_VARIABLE: used to describe local variables 　4.METHOD: Used to describe the method 　5.PACKAGE: used to describe the package 　6.PARAMETER: used to describe parameters 　7.TYPE: Used to describe classes, interfaces ( including annotation types ) or enum declarations	@Target(ElementType.TYPE) public @interface Table {   public String tableName() default "className"; }
@Retention	Defines the length of time the Annotation is retained	RetentionPoicy： 　1. SOURCE: Valid in the source file (that is, the source file is retained) 　2. CLASS: Valid in class files (ie class reservation) 　3.RUNTIME: Valid at runtime (that is, reserved at runtime)
@Documented	A public API for describing other types of annotations that should be used as annotated program members , and thus can be documented by tools such as javadoc	is a markup annotation with no members
@Inherited	Explains that an annotated type is inherited. If an annotation type modified with @Inherited is used in a class , the annotation will be used in subclasses of that class	is a markup annotation with no members

 

 

 

3. Custom annotations

@interface is used to declare an annotation, each of which actually declares a configuration parameter ;

The name of the method ;

The return value type is the type of the parameter (the return value type can only be basic types, Class , String , enum ) ;

The default value of a parameter can be declared by default .

 

Example 1: (The annotation of a single parameter is generally named value , and the method of nama=XXX is not used when using it )

@Target(ElementType.FIELD)

@Retention(RetentionPolicy.RUNTIME)

@Documentedpublic @interface FruitName {

    String value() default "";

}

 

Example two:

@Target(ElementType.FIELD)

@Retention(RetentionPolicy.RUNTIME)

@Documented

public @interface FruitColor {

  publicenum Color{ BULE,RED,GREEN}; 

           Color fruitColor() default Color.GREEN;

}

 

 

4.通过反射获取注解信息

  （注意，只有RetentionPoicy=RUNTIME的时候才能通过反射获取到）

//判断该方法是否包含MyAnnotation注解

method.isAnnotationPresent(MyAnnotation.class)

//获取该方法的MyAnnotation注解实例

MyAnnotation myAnnotation = method.getAnnotation(MyAnnotation.class);

//获取myAnnotation属性值

String[] value1 = myAnnotation.value1();

//获取方法上的所有注解

Annotation[] annotations = method.getAnnotations();