利用反射操作注解

如何利用反射操作注解

1. 根据全类名获取类加载器

Class c1 = Class.forName("com.peng.demo.Student");

2. 通过反射获得 注解

Annotation[] annotations = c1.getAnnotations();

3. 获得注解的value值

TablePeng tablePeng= (TablePeng) c1.getAnnotation(TablePeng.class);
        String value = tablePeng.value();
        System.out.println(value); // db_student

4. /获得指定注解的值

 Field name = c1.getDeclaredField("name");
        FieldPeng annotation = name.getAnnotation(FieldPeng.class);
        System.out.println(annotation.columnName()); //db_name
        System.out.println(annotation.type()); //varchar
        System.out.println(annotation.length()); //30

5. java代码

package com.peng.demo;
import java.lang.annotation.*;
import java.lang.reflect.Field;
/**
 * 利用反射操作注解
 */
public class ReflectionDemo02 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        //通过全类名获取Class类加载器
        Class c1 = Class.forName("com.peng.demo.Student");
        //通过反射获得 注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation); // @com.peng.demo.TablePeng(value="db_student")
        }
        // 获得注解的value值
        TablePeng tablePeng= (TablePeng) c1.getAnnotation(TablePeng.class);
        String value = tablePeng.value();
        System.out.println(value); // db_student

        // 获得指定注解的值
        Field name = c1.getDeclaredField("name");
        FieldPeng annotation = name.getAnnotation(FieldPeng.class);
        System.out.println(annotation.columnName()); //db_name
        System.out.println(annotation.type()); //varchar
        System.out.println(annotation.length()); //30
    }
}

@TablePeng("db_student") //模拟数据库的db_student表
class Student{
    @FieldPeng(columnName = "db_id",type = "int",length = 10) // 模拟数据库表的每列属性结构 columnName = "db_id",type = "int",length = 10
    private int id;
    @FieldPeng(columnName = "db_age",type = "int",length = 10)
    private int age;
    @FieldPeng(columnName = "db_name",type = "varchar",length = 30)
    private String name;

    public Student() {
    }

    public Student(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}

//自定义 类的注解 (String模拟数据库表名字)
@Target(ElementType.TYPE) //元注解@Target :描述注解能够作用的位置
@Retention(RetentionPolicy.RUNTIME) //元注解@Retention(保留,保持) :描述注解被保留的阶段
@interface TablePeng{
    String value();
}

//自定义 属性的注解 (属性模拟表的每列结构)
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FieldPeng{
    String columnName();
    String type();
    int length();
}