Annotations for java basics

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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