三、MyBatis系列：Mapper 映射 之 关联对象属性及延迟加载

在开发时，经常有这种需求，我拿到了一个order表单实体时，我需要知道该表单是属于哪个用户的。

这时访问 order.getUser() 获取用户信息，这时应该得到一个用户的信息实例。

解决这个问题的方法有三种：

1、通过1条SQL语句进行关联查询，同时读取表单和用户信息；

2、同时发送2条SQL，用于读取 order 与 user信息；

3、先用1条SQL读取order表单信息，访问getUser()时再用另1个SQL读取user信息；

 

实体定义如下：

public class User {
    private Integer id;
    private String username;
    private Date birthday;
    private String sex;
    private String address;
}
public class Order {
    private int id;
    private User user;
    private int number;
    private Date createtime;
    private String note;
}

第一种方法：使用关联查询的 mapper 的配置

<resultMap type="cn.xleos.mybatis.po.Order" id="orderUserMap">
    <id property="id" column="orders_id" />
    <result property="createtime" column="orders_createtime" />
    <result property="number" column="orders_number" />
    <result property="note" column="orders_note" />
    <association property="user" javaType="cn.xleos.mybatis.po.User">
        <id property="id" column="user_id" />
        <result property="username" column="user_username" />
        <result property="birthday" column="user_birthday" />
        <result property="sex" column="user_sex" />
        <result property="address" column="user_address" />
    </association>
</resultMap>
<select id="getOrdersAndUser">
    SELECT
    orders.id AS orders_id,
    orders.number AS orders_number,
    orders.createtime AS orders_createtime,
    orders.note AS orders_note,
    `user`.id AS user_id,
    `user`.username AS user_username,
    `user`.birthday AS user_birthday,
    `user`.sex AS user_sex,
    `user`.address AS user_address
    FROM orders
    INNER JOIN `user` ON `user`.id = orders.user_id
</select>

第二种方法，使用二条SQL语句来完成读取表单与用户信息

在读取order关联时 association 节使用了 select="getUserById" 属性，它表示使用 getUserById 这个SQL来读取用户信息。

<resultMap type="cn.xleos.mybatis.po.Order" id="orderMap">
    <id property="id" column="id" />
    <result property="createtime" column="createtime" />
    <result property="number" column="number" />
    <result property="note" column="note" />
    <association property="user" column="user_id"
        javaType="cn.xleos.mybatis.po.User" select="getUserById" />
</resultMap>
<select id="getOrders" resultMap="orderMap">
    SELECT id, user_id, number, createtime, note, FROM orders
</select>
<select id="getUserById" parameterMap="int" resultType="cn.xleos.mybatis.po.User">
    SELECT id, username, birthday, sex, address FROM user where id = #{value}
</select>

编写一个单元测试，通过断步来看看MyBatis是如何提交SQL至数据库的。

@Test
public void getOrdersTest() {
    SqlSession sqlSession = sqlSessionFactory.openSession();
    try {
        System.out.println("1. 读取 order 表单数据对象. ");
        Order order = sqlSession.selectOne("test.getOrders", 3);
        System.out.println("2. 访问表单的 user 属性. ");
        User user = order.getUser();
        System.out.println(user);
    } finally {
        sqlSession.close();
    }
}

以下是日志输出：

DEBUG [main] - Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.
DEBUG [main] - PooledDataSource forcefully closed/removed all connections.
1. 读取 order 表单数据对象. 
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Created connection 2063763486.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7b02881e]
DEBUG [main] - ==>  Preparing: SELECT id, user_id, number, createtime, note FROM orders WHERE id = ? 
DEBUG [main] - ==> Parameters: 3(Integer)
DEBUG [main] - ====>  Preparing: SELECT id, username, birthday, sex, address FROM user where id = ? 
DEBUG [main] - ====> Parameters: 1(Integer)
DEBUG [main] - <====      Total: 1
DEBUG [main] - <==      Total: 1
2. 访问表单的 user 属性. 
User [id=1, username=王五, birthday=Thu Jul 10 00:00:00 CST 2014, sex=2, address=北京市
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7b02881e]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7b02881e]
DEBUG [main] - Returned connection 2063763486 to pool.

通过日志可以发生在读取 order 数据时，就同时发送了2条SQL语句至数据库，读取 order 与 user 数据。

第三种方法，使用延时加载来访问 getUser() 时使用SQL语句读取user信息

1、需要在全局配置文件中添加lazyLoadingEnabled=true；
2、在关联映射（association）中设置 fetchType="lazy" 属性；
3、其它均与第二种方法一致；

<resultMap type="cn.xleos.mybatis.po.Order" id="orderMap">
    <!-- 省略其它属性映射 -->
    <association property="user" column="user_id"
        javaType="cn.xleos.mybatis.po.User" select="getUserById" fetchType="lazy" />
</resultMap>

也编写一个单元测试，通过断步来看看MyBatis是如何提交SQL至数据库的。

@Test
public void getOrdersLazyTest() {
    SqlSession sqlSession = sqlSessionFactory.openSession();
    try {
        System.out.println("1. 读取 order 表单数据对象. ");
        Order order = sqlSession.selectOne("test.getOrders", 3);
        System.out.println("2. 访问表单的 user 属性. ");
        User user = order.getUser();
        System.out.println(user);
    } finally {
        sqlSession.close();
    }
}

以下是日志输出：

DEBUG [main] - Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.
DEBUG [main] - PooledDataSource forcefully closed/removed all connections.
1. 读取 order 表单数据对象. 
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Created connection 1071097621.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3fd7a715]
DEBUG [main] - ==>  Preparing: SELECT id, user_id, number, createtime, note FROM orders WHERE id = ? 
DEBUG [main] - ==> Parameters: 3(Integer)
DEBUG [main] - <==      Total: 1
2. 访问表单的 user 属性. 
DEBUG [main] - ==>  Preparing: SELECT id, username, birthday, sex, address FROM user where id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 1
User [id=1, username=王五, birthday=Thu Jul 10 00:00:00 CST 2014, sex=2, address=北京市
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3fd7a715]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3fd7a715]
DEBUG [main] - Returned connection 1071097621 to pool.

使用了延时加载后，在读取 order 表单时仅发送一个SQL语句，于数据库查询出 order 对象； 在访问 order.getUser() 时，再发送一条SQL至数据库，读取出 user 记录；