定义表名注解
package com.example.demo.learn.orm;import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;/*** 表的别名* 项目中使用注解，肯定会使用反射，反射应用场景， jdbc spring ioc 常用的框架， 注解实现*/
@Retention(RetentionPolicy.RUNTIME)
public @interface SetTable {String value();
}
定义描述表字段注解
package com.example.demo.learn.orm;import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;@Retention(RetentionPolicy.RUNTIME)
public @interface SetProperty {String name();int length();
}
使用注解
package com.example.demo.learn.orm;/*** 表示用户对应的实体类*/@SetTable("t_tag")
public class UserEntity {@SetProperty(name = "user_name", length = 20)private String userName;@SetProperty(name = "age", length = 10)private Integer age;public String getUserName() {return userName;}public void setUserName(String userName) {this.userName = userName;}public Integer getAge() {return age;}public void setAge(Integer age) {this.age = age;}@Overridepublic String toString() {return "UserEntity{" +"userName='" + userName + '\'' +", age=" + age +'}';}
}
实现ORM框架映射，通过使用注解和反射机制
package com.example.demo.learn.orm;import java.lang.annotation.Annotation;
import java.lang.reflect.Field;public class Test {private static String path = "com.example.demo.learn.orm.UserEntity";public static void main(String[] args) throws ClassNotFoundException {Class<?> objClass = Class.forName(path);/*** 表示该类上面使用的注解,只会获取到当前类上使用的注解*/Annotation[] annotations = objClass.getAnnotations();for (Annotation annotation : annotations) {System.out.println(annotation);}/***获取某个注解对象,以及注解上的值*/SetTable setTable = objClass.getAnnotation(SetTable.class);// 表的名称String tabName = setTable.value();// 所有实体类属性Field[] fields = objClass.getDeclaredFields();StringBuffer buffer = new StringBuffer();buffer.append("select ");for (int i = 0; i < fields.length; i++) {SetProperty property = fields[i].getAnnotation(SetProperty.class);String name = property.name();buffer.append(name);if (i == fields.length-1) {buffer.append(" from ");} else {buffer.append(" , ");}}buffer.append(tabName);System.out.println(buffer.toString());}
}