MyBatis映射利用mysql left join 解决N+1查询问题

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1.权限是几乎每个系统都需要的

2.一般在用户请求某个url的时候，都需要验证用户是否拥有该url的访问权限

3.最简单的权限系统需要用户表，角色表，用户角色表，权限表，角色权限表

# Host: 127.0.0.1  (Version: 5.6.22)
# Date: 2015-04-09 10:52:58
# Generator: MySQL-Front 5.3  (Build 4.13)

/*!40101 SET NAMES utf8 */;

#
# Source for table "privilege"
#

CREATE TABLE `privilege` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  `url` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

#
# Data for table "privilege"
#

INSERT INTO `privilege` VALUES (1,'产品增加','/product/add'),(2,'产品删除','/product/delete'),(3,'产品修改','/product/update'),(4,'产品查询','/product/query');

#
# Source for table "role"
#

CREATE TABLE `role` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

#
# Data for table "role"
#

INSERT INTO `role` VALUES (1,'admin'),(2,'manager'),(3,'guest');

#
# Source for table "role_priv"
#

CREATE TABLE `role_priv` (
  `role_id` int(11) NOT NULL,
  `priv_id` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

#
# Data for table "role_priv"
#

INSERT INTO `role_priv` VALUES (1,1),(2,1),(1,3);

#
# Source for table "user"
#

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

#
# Data for table "user"
#

INSERT INTO `user` VALUES (1,'jack'),(2,'jaychang'),(3,'jasaon'),(4,'lucy'),(5,'marry');

#
# Source for table "user_role"
#

CREATE TABLE `user_role` (
  `user_id` int(11) NOT NULL DEFAULT '0',
  `role_id` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

#
# Data for table "user_role"
#

INSERT INTO `user_role` VALUES (1,1),(1,3),(3,2),(2,2);

4.表对应的实体类

放在了sample包下

public class Privilege {
	private Integer id;
	private String name;
	private String url;
...省略get,set方法
}

public class User {
	private Integer id;
	private String name;
	private List<Role> roles;
...省略get,set方法
}

public class Role {
	private Integer id;
	private String name;
	private List<Privilege> privileges;
...省略get,set方法
}

5.mybatis配置文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
	<typeAliases>
		<typeAlias type="sample.User" alias="User" />
		<typeAlias type="sample.Role" alias="Role" />
		
	</typeAliases>
	<environments default="development">
		<environment id="development">
			<transactionManager type="JDBC" />
			<dataSource type="POOLED">
				<property name="driver" value="com.mysql.jdbc.Driver" />
				<property name="url" value="jdbc:mysql://127.0.0.1:3306/temp" />
				<property name="username" value="root" />
				<property name="password" value="123456" />
			</dataSource>
		</environment>
	</environments>
	<mappers>
		<mapper resource="mybatis/mapper/UserMapper.xml" />
	</mappers>
</configuration>

6.UserMapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" 
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="sample.User">
	<resultMap type="User" id="userResultMap">
		<id property="id" column="user_id" />
		<result property="name" column="user_name" />
		<collection property="roles" ofType="sample.Role">
			<result property="id" column="role_id"/>
			<result property="name" column="role_name"/>
			<collection property="privileges" ofType="sample.Privilege">
				<result property="id" column="priv_id"/>
				<result property="name" column="priv_name"/>
				<result property="url" column="priv_url"/>
			</collection>
		</collection>
	</resultMap>
	
	<resultMap type="Role" id="roleResultMap">
		<id property="id" column="role_id" />
		<result property="name" column="role_name" />
		<collection property="privileges" ofType="sample.Privilege">
			<result property="id" column="priv_id"/>
			<result property="name" column="priv_name"/>
			<result property="url" column="priv_url"/>
		</collection>
	</resultMap>
	
	<select id="selectUserById" parameterType="long" resultMap="userResultMap">
		select u.id as user_id,u.name as user_name,r.id as role_id,
			   r.name as role_name,p.id as priv_id,p.name as priv_name,p.url as priv_url
		from user u
		left join user_role ur on u.id = ur.user_id
		left join role r on r.id = ur.role_id
		left join role_priv rp on r.id = rp.role_id 
    	left join privilege p on p.id = rp.priv_id
    	where u.id = #{id}
	</select>
	
</mapper>

		测试
		List<User> users = sqlSession.selectList("selectUserById", 1L);
		for (User u : users) {
			System.out.println("用户名：" + u.getName());
			List<Role> roles = u.getRoles();
			if (null != roles) {
				for (Role role : roles) {
					System.out.println("    拥有角色：" + role.getName());
					List<Privilege> privs = role.getPrivileges();
					if (null != privs && privs.size() > 0) {
						System.out.println("        该角色【" + role.getName()
								+ "】，拥有的权限:");
						for (Privilege priv : privs) {
							System.out.println("            " + priv.getName()
									+ "," + priv.getUrl());
						}
					}
				}
			}
		}