使用mybatis，进行多表联合查询

在配置文件中添加读取properties文件

添加日志

添加多个映射（mapper）

<?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>

    <properties resource="db.properties" />
<settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${user}"/>
                <property name="password" value="${pass}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/qfedu/mapper/OrderMapper.xml"/>
        <mapper resource="com/qfedu/mapper/UserMapper.xml"/>
    </mappers>
</configuration>

mapper配置文件设置返回类型为resultMap,会根据下面的标签填值

在resuletMap标签中

1. id代表主键，分别设置属性（property）、数据库列（column）对应关系
2. result代表普通字段映射，如果字段名和属性名一致，可以省略
3. association关联，只要对一关系都可以使用

property代表order类中的u（用于储存另一个表查询后的数据），column代表order数据库表的uid字段（两表之间的联系）

select:另一个xml的namespace和id

OrderMapper.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="com.qfedu.pojo.OrderMapper">
    
    <select id="getOrderByOid" resultMap="orderMap">
      select * from orders where oid = #{oid}
    </select>
    
    <resultMap id="orderMap" type="com.qfedu.pojo.Order">
    <id property="oid" column="oid" ></id>
    <result column="price" property="price" />
        <result column="addr" property="addr" />
        <result column="payType" property="payType" />
        <association property="u" column="uid" select="com.qfedu.pojo.UserMapper.getUserByUid"></association>
        </resultMap>
</mapper>

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="com.qfedu.pojo.UserMapper">
    
    <select id="getUserByUid" resultType="com.qfedu.pojo.User">
      select * from users where uid = #{uid}
    </select>
</mapper>

order.java

public class Order {

    private String oid;
    private double price;
    private String addr;
    private String payType;
    private User u;
    private List<Detail> details;

    @Override
    public String toString() {
        return "Order{" +
                "oid='" + oid + '\'' +
                ", price=" + price +
                ", addr='" + addr + '\'' +
                ", payType='" + payType + '\'' +
                ", u=" + u +
                ", details=" + details +
                '}';
    }

    public String getOid() {
        return oid;
    }

    public void setOid(String oid) {
        this.oid = oid;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    public String getAddr() {
        return addr;
    }

    public void setAddr(String addr) {
        this.addr = addr;
    }

    public String getPayType() {
        return payType;
    }

    public void setPayType(String payType) {
        this.payType = payType;
    }

    public User getU() {
        return u;
    }

    public void setU(User u) {
        this.u = u;
    }

    public List<Detail> getDetails() {
        return details;
    }

    public void setDetails(List<Detail> details) {
        this.details = details;
    }
}