1. Meta-annotation
1.1@Target
[Function]
is used to specify the location where the marked annotation can be used, for example:@Target(ElementType.METHOD): Indicates that it can be used on methods, but other structures cannot be used;@Target({ElementType .METHOD, ElementType.TYPE}): Indicates that it can be used on methods, interfaces, classes, and enumerations.
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1.2@Retention
[Function]
Use In the designated annotation retention stage, the annotation has three values:
@Retention(RetentionPolicy.SOURCE): indicates that it is retained until the source code stage and disappears after compilation
@Retention(RetentionPolicy.CLASS): Retains until the compilation phase and disappears after running
@Retention(RetentionPolicy.RUNTIME): Retains until the running phase , if you want to read the annotation information through reflection, you need to specify the annotation retention stage as RUNTIME
1.3@Inherited
[Function]
indicates this Whether the annotation can be inherited by subclasses.
1.4@Documented
[Function]
Indicates whether this annotation can be read into the document by Javadoc.
2. Annotation statement
[Format]
[Meta-annotation]
【修饰符】 @interface 注解名 { 注解体 }
[Example]
@Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) public @interface MyAnnotation { }
3. Configuration parameter declaration
[Format]
【数据类型】 参数名() default 默认值；
default Default value: When you need to set the default value, you can add it. When you need to set it, you don’t need to write it;
Data types can only be: basic data types, String, Class, enum, Annotation, and one-dimensional arrays of all the above types.
If there is only one parameter member or a frequently used parameter, you can define the parameter name: value. When using annotations, if the parameter name is value, you can omit it and write the input value directly.
[Example]
@Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) public @interface MyAnnotation { String name() default ""; }
4. Read annotation information
Only annotation annotations@Retention(RetentionPolicy.RUNTIME)It can be read through reflection.
Read annotation information through reflection, as follows:
import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; public class TestInterface { public static void main(String[] args) { MyAnnotation myAnnotation = MyClass.class.getAnnotation(MyAnnotation.class); String value = myAnnotation.value(); System.out.println(value); } } @MyAnnotation class MyClass {} @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @interface MyAnnotation { String value() default "我是一个注解"; }
Output results:
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