（八）mybatis学习之高级映射（一对一、一对多、多对多）
一对一关联查询
resultType
resultType的对于关联查询是实时查询的。
使用resultType来开发的话，就要将sql查询出来的结果映射到某个pojo中。
resultType的开发比较简单，就是查询结果是什么，就去定义一个pojo类，添加对应的属性名进行映射就OK。
resultMap
resultMap对于管理查询，可以设置一些延迟加载的配置。
resultMap里配置一对一关联，主要使用到association这个标签，指定类型用javaType。
订单pojo类：
public class OrderInfos {
private String id;
private String orderNo; //订单号
private String mark; //留言（描述）
private String address; //地址（具体地址）
private String phone; //收货人电话
private Integer quantity; //数量
private Double totalSum; //订单总金额
private Date createTime; //订单生成时间
private User user;
//....get set
用户信息pojo类：
public class User {
private String id;
private String userName; //用户名
private String nickName; //昵称
private String password; //登录密码
private Integer sex; //性别
private Date birthday; //出生日期
private String province; //省
private String city; //市
private String county; //县
private Date createTime; //创建时间
//... get set
配置文件内容：
<resultMap type="orderInfos" id="findOrderMap"> <!-- 映射订单信息 id:指定查询列中的唯一标识，如果有组合主键，则配置对个id --> <id column="id" property="id"/> <result column="orderNo" property="orderNo"/> <result column="mark" property="mark"/> <result column="address" property="address"/> <result column="createTime" property="createTime"/> <!-- 订单所对应的用户信息 --> <!-- association：用于映射关联查询单个对象的信息 property：要讲关联查询的用户信息映射到订单pojo列中的哪个属性里 javaType:表示关联查询的对象类型（可以使用别名） --> <association property="user" javaType="pojo.User"> <id column="user_id" property="id"/> <result column="userName" property="userName"/> <result column="sex" property="sex"/> <result column="birthday" property="birthday"/> <result column="password" property="password"/> </association> </resultMap> <!-- 获取订单里的用户信息 --> <select id="findOrder" resultMap="findOrderMap"> select A.*,B.userName,B.sex,B.birthday,B.`password` from tab_orderinfos as A inner join tab_user as B on A.user_id=B.id </select>
一对多关联查询
配置一对多关联，需要在resultMap里使用collection标签来实现，指定类型用ofType
订单pojo类：
public class OrderInfos {
private String id;
private String orderNo; //订单号
private String mark; //留言（描述）
private String address; //地址（具体地址）
private String phone; //收货人电话
private Integer quantity; //数量
private Double totalSum; //订单总金额
private Date createTime; //订单生成时间
private List<OrderInfosDetail> detailList;
//... get set
订单明细pojo类：
public class OrderInfosDetail {
private String id;
private String color; //颜色
private String size; //尺寸
private Double unitPrice; //价格
private Integer quantity; //数量
private Double totalSum; //总金额
//... get set
配置文件内容：
<resultMap type="orderInfos" id="oneToManyFindOrderMap"> <!-- 映射订单信息 id:指定查询列中的唯一标识，如果有组合主键，则配置对个id --> <id column="id" property="id"/> <result column="orderNo" property="orderNo"/> <result column="mark" property="mark"/> <result column="address" property="address"/> <result column="createTime" property="createTime"/> <!-- 订单所对应的用户信息 --> <!-- association：用于映射关联查询单个对象的信息 property：要讲关联查询的用户信息映射到订单pojo列中的哪个属性里 javaType:表示关联查询的对象类型（可以使用别名） --> <association property="user" javaType="pojo.User"> <id column="user_id" property="id"/> <result column="userName" property="userName"/> <result column="sex" property="sex"/> <result column="birthday" property="birthday"/> <result column="password" property="password"/> </association> <!-- 获取订单明细信息,一个订单关联查询出多条明细，要使用collection来进行映射 --> <!-- collection：对关联查询到的多条记录进行映射 property：要讲关联查询的明细信息映射到订单pojo列中的哪个属性里 ofType:指要映射到集合属性中pojo的类型 --> <collection property="detailList" ofType="orderInfosDetail"> <id column="detailId" property="id"/> <result column="quantity" property="quantity"/> <result column="color" property="color"/> <result column="unitPrice" property="unitPrice"/> </collection> </resultMap> <select id="oneToManyFindOrder" resultMap="oneToManyFindOrderMap"> select A.*,B.userName,B.sex,B.birthday,B.`password`,C.id detailId,C.quantity,C.color,C.unitPrice from tab_orderinfos as A inner join tab_user as B on A.user_id=B.id left join tab_orderinfosdetail as C on A.id=C.orderInfos_id </select>
这里我们发现订单信息与用户信息的配置，跟上面的一对一里的配置一样。这种情况可以使用继承，就不需要再进行配置了。
使用extends属性设置了继承以后，配置文件内容变为：
<resultMap type="orderInfos" id="oneToManyFindOrderMap" extends="findOrderMap"> <!-- 获取订单明细信息,一个订单关联查询出多条明细，要使用collection来进行映射 --> <!-- collection：对关联查询到的多条记录进行映射 property：要讲关联查询的明细信息映射到订单pojo列中的哪个属性里 ofType:指要映射到集合属性中pojo的类型 --> <collection property="detailList" ofType="orderInfosDetail"> <id column="detailId" property="id"/> <result column="quantity" property="quantity"/> <result column="color" property="color"/> <result column="unitPrice" property="unitPrice"/> </collection> </resultMap> <select id="oneToManyFindOrder" resultMap="oneToManyFindOrderMap"> select A.*,B.userName,B.sex,B.birthday,B.`password`,C.id detailId,C.quantity,C.color,C.unitPrice from tab_orderinfos as A inner join tab_user as B on A.user_id=B.id left join tab_orderinfosdetail as C on A.id=C.orderInfos_id </select>
多对多关联查询
多多对多的关联查询其实都是基于多对一，一对多的基础上进行配置的。
会了一对一、一对多，多对多也就会了。这里不再多说。
延迟加载
什么是延迟加载？就是需要用到的时候再进行查询，这就是延迟加载。
mybatis里association和collection都支持延迟加载。
这里就拿association来测试一下，collection用法一样。
需要说明的是，延迟加载跟上面所说的一对一、一对多的查询开发方式是不一样的。上面的查询都是实时的查询出所有的数据，不是延迟加载。
针对上面所说的一对一，我们来实现一下它的延迟加载。
此时需要配置两个sql配置：
（1）查询订单信息
（2）根据订单信息中的user_id，去查询对应的用户信息
最后的配置文件内容如下：
<resultMap type="orderInfos" id="lazyMap">
<!-- 映射订单信息 -->
<id column="id" property="id"/>
<result column="orderNo" property="orderNo"/>
<result column="mark" property="mark"/>
<result column="address" property="address"/>
<result column="createTime" property="createTime"/>
<!-- 延迟加载订单所对应的用户信息
select:指定延迟加载需要执行的sql配置id
column:订单信息中关联查询用户信息的列（说白了，就是说订单表中是根据那个列去查询用户信息的）
-->
<association property="user" javaType="pojo.User" select="lazyFindUserById" column="user_id"></association>
</resultMap>
<!-- 查询订单信息,用户信息需要延迟加载,因为延迟加载是在association或collection中才能实现，所以这里需要使用resultMap来做映射 -->
<select id="findOrderAll" resultMap="lazyMap">
select * from tab_orderinfos
</select>
<!-- 根据id查询用户信息 -->
<select id="lazyFindUserById" parameterType="java.lang.String" resultType="user">
select * from tab_user where id=#{id}
</select>
注意：mybatis默认是不开启延迟加载的。所以我们需要在sqlMapConfig.xml的settings里配置开启延迟加载。具体的settings的信息请看文档《3.1.2 settings》
在sqlMapConfig.xml配置settings开启延迟加载：
<settings> <!-- 开启延迟加载 lazyLoadingEnabled:全局配置懒加载，如果设置为false，则所有关联查询都会被初始化加载。默认值是fasle。所以这里要设为true aggressiveLazyLoading:当设置为true时，会对全部查询发出sql了。所以我们应该要设置为false。为false时，需要用什么就查询什么，按需加载。默认值是true --> <setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/> </settings>
测试类：
@Test
public void test() throws Exception {
//获取代理对象
OrderMapper orderMapper = (OrderMapper) context.getBean("orderMapper");
List<OrderInfos> list = orderMapper.findOrderAll();
for(OrderInfos order :list){
User user = order.getUser();
System.out.println(user.getUserName());
}
System.out.println(list);
}
}结果：
2016-04-04 16:16:04,370 [main] DEBUG [mapper.OrderMapper.findOrderAll] - ==> Preparing: select * from tab_orderinfos 2016-04-04 16:16:04,418 [main] DEBUG [mapper.OrderMapper.findOrderAll] - ==> Parameters: 2016-04-04 16:16:04,980 [main] DEBUG [mapper.OrderMapper.findOrderAll] - <== Total: 22 2016-04-04 16:16:04,987 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@4fb686] 2016-04-04 16:16:04,987 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource 2016-04-04 16:16:18,278 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 16:16:18,279 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@104f568 [wrapping: com.mysql.jdbc.JDBC4Connection@1bd7522]] will not be managed by Spring 2016-04-04 16:16:18,281 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 16:16:18,284 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Parameters: 2c9381ed460abc1101460abddf830000(String) 2016-04-04 16:16:18,302 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - <== Total: 1 2016-04-04 16:16:18,303 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource abc 2016-04-04 16:16:30,678 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 16:16:30,679 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@43690f [wrapping: com.mysql.jdbc.JDBC4Connection@1bd7522]] will not be managed by Spring 2016-04-04 16:16:30,681 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 16:16:30,684 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Parameters: 2c9381ed460abc1101460abddf830000(String) 2016-04-04 16:16:30,702 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - <== Total: 1 2016-04-04 16:16:30,703 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource abc 2016-04-04 16:16:41,684 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 16:16:41,685 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@160e1bb [wrapping: com.mysql.jdbc.JDBC4Connection@1bd7522]] will not be managed by Spring 2016-04-04 16:16:41,685 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 16:16:41,685 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Parameters: 2c9381ed460abc1101460abddf830000(String) 2016-04-04 16:16:41,689 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - <== Total: 1 2016-04-04 16:16:41,690 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource abc 2016-04-04 16:16:41,690 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 16:16:41,690 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@163eb77 [wrapping: com.mysql.jdbc.JDBC4Connection@1bd7522]] will not be managed by Spring 2016-04-04 16:16:41,690 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 16:16:41,691 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Parameters: 2c9381ed460abc1101460abddf830000(String) 2016-04-04 16:16:41,695 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - <== Total: 1 2016-04-04 16:16:41,696 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource abc 2016-04-04 16:16:41,696 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 16:16:41,696 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@edd5f4 [wrapping: com.mysql.jdbc.JDBC4Connection@1bd7522]] will not be managed by Spring 2016-04-04 16:16:41,697 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 16:16:41,698 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Parameters: 2c9381ed460abc1101460abddf830000(String) 2016-04-04 16:16:41,702 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - <== Total: 1 2016-04-04 16:16:41,702 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource abc 2016-04-04 16:16:41,702 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 16:16:41,702 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@b759cd [wrapping: com.mysql.jdbc.JDBC4Connection@1bd7522]] will not be managed by Spring 2016-04-04 16:16:41,702 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 16:16:41,703 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Parameters: 2c9381ed460abc1101460abddf830000(String) 2016-04-04 16:16:41,705 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - <== Total: 1 2016-04-04 16:16:41,705 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource abc 2016-04-04 16:16:41,705 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 16:16:41,705 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@1497c25 [wrapping: com.mysql.jdbc.JDBC4Connection@175982a]] will not be managed by Spring 2016-04-04 16:16:41,705 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 16:16:41,706 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Parameters: 2c9381ed460abc1101460abddf830000(String) 2016-04-04 16:16:41,727 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - <== Total: 1 2016-04-04 16:16:41,727 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource abc 2016-04-04 16:16:41,728 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 16:16:41,728 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@1825bfb [wrapping: com.mysql.jdbc.JDBC4Connection@175982a]] will not be managed by Spring 2016-04-04 16:16:41,728 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 16:16:41,728 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - ==> Parameters: 2c9381ed460abc1101460abddf830000(String) 2016-04-04 16:16:41,731 [main] DEBUG [mapper.OrderMapper.lazyFindUserById] - <== Total: 1 2016-04-04 16:16:41,731 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource abc
通过上面的日志打印，可以很清楚的看到，在for循环遍历订单信息时，当我们使用getUser（）时，才发出sql，查询数据获取结果。这样就实现了延迟加载。