Java自带注解、元注解、自定义注解的区别

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override{   
}

@Target(ElementType...)
@Retention(RetentionPolicy...)
权限修饰符 @interface 注解名称(){
    //注解体，可以定义属性
    属性类型 属性名() default 默认值;
}

//Spring框架注解之Component
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Component{ 
  String value() default "";
}

@Tablemi("mi")
class Student2 {
    @Fieldmi(name = "id", type = "int", length = 10)
    private int id;
    @Fieldmi(name = "age", type = "int", length = 3)
    private int age;
    @Fieldmi(name = "name", type = "varchar", length = 5)
    private String name;
 
    public Student2() {
    }
 
    public Student2(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }
 
}//并提供get和set方法

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Tablemi{
    String value();
}
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface Fieldmi{
    String name();
    String type();
    int length();
}

Class c1 = Class.forName("com.mi.demo01.Student2");
//通过反射获取注解
Annotation[] annotations = c1.getAnnotations();
for (Annotation annotation : annotations) {
    System.out.println(annotation);
}
//获的注解的value值
Tablemi tablemi = (Tablemi)c1.getAnnotation(Tablemi.class);
String value = tablemi.value();
System.out.println(value);
//获得类指定的注解
Field field = c1.getDeclaredField("id");
Fieldmi fieldmi = field.getAnnotation(Fieldmi.class);
System.out.println(fieldmi.name());
System.out.println(fieldmi.type());
System.out.println(fieldmi.length());
//获得类指定的注解
Field field1 = c1.getDeclaredField("name");
Fieldmi fieldmi1 = field1.getAnnotation(Fieldmi.class);
System.out.println(fieldmi1.name());
System.out.println(fieldmi1.type());
System.out.println(fieldmi1.length());
//获得类指定的注解
Field field2 = c1.getDeclaredField("age");
Fieldmi fieldmi2 = field2.getAnnotation(Fieldmi.class);
System.out.println(fieldmi2.name());
System.out.println(fieldmi2.type());
System.out.println(fieldmi2.length());