前面文章介紹了都是一個sql語句完成所有的查詢操作，包括在執行聯合查詢的時候也是一個sql語句完成查詢，然後將查詢結果通過resultMap進行封裝。mybatis也提供了分步查詢的功能，在完成一個操作之後，將查詢的結果使用到第二個三個查詢引數中完成所有的查詢。具體如何實現，還是結合示例做展示：

分步查詢

1）association
使用association實現分步查詢操作。

建立實體

Employee和Department

package org.mybatis.resultmap;

public class Employee {
    private Integer id;
    private
 
 String lastName;
    private String email;
    private String gender;
    private Department department;

    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getLastName() {
        return lastName;
    }
    public
 
 void setLastName(String lastName) {
        this.lastName = lastName;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this
 
.gender = gender;
    }
    public Department getDepartment() {
        return department;
    }
    public void setDepartment(Department department) {
        this.department = department;
    }
    @Override
    public String toString() {
        return "Employee [id=" + id + ", lastName=" + lastName + ", email="
                + email + ", gender=" + gender + ", department=" + department
                + "]";
    }

}

package org.mybatis.resultmap;


public class Department {

    private Integer id;

    private String deptName;

    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getDeptName() {
        return deptName;
    }
    public void setDeptName(String deptName) {
        this.deptName = deptName;
    }

    public String toString() {
        return "Department [id=" + id + ", deptName=" + deptName + "]";
    }

}

建立介面類
DepartmentMapper和EmployeeMapper

package org.mybatis.resultmap;

public interface DepartmentMapper {

    public Department getDeptById(Integer id);

}

package org.mybatis.resultmap;

public interface EmployeeMapper {

    /**
     * 分步查詢
     * @param id
     * @return
     */
    public Employee getEmpAndDeptByStep(Integer id);

}

建立對映配置檔案
resultmapmapper.xml對應Employee和resultmapdepartmentmapper.xml對應Department

<?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="org.mybatis.resultmap.EmployeeMapper">
    <!-- 分步查詢 -->
    <resultMap type="org.mybatis.resultmap.Employee" id="mstep">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <result column="email" property="email"/>
        <result column="gender" property="gender"/>
        <!--org.mybatis.resultmap.DepartmentMapper.getDeptById為定義的DepartmentMapper的查詢介面-->
        <!--dept_id是Employee中對應Department的id值，作為傳遞引數-->
        <association property="department" select="org.mybatis.resultmap.DepartmentMapper.getDeptById" column="dept_id"></association>
    </resultMap>
    <select id="getEmpAndDeptByStep" resultMap="mstep">
        select * from mybatis_employee where id = #{id}
    </select>
</mapper>

<?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="org.mybatis.resultmap.DepartmentMapper">

    <select id="getDeptById" resultType="org.mybatis.resultmap.Department">
        select id,deptName from mybatis_department where id = #{id}
    </select>

</mapper>

建立主配置檔案
mybatis-config-resultmap.xml

<?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"></properties>

    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>

    <typeAliases>
        <!-- 為包下的所有類起別名，預設為類名大小寫（不區分大小寫） -->
        <package name="org.mybatis.resultmap"/>
    </typeAliases>


    <!-- 預設development是開發環境，如果改成test則表示使用測試環境 -->
    <environments default="dev_mysql">
        <environment id="dev_mysql">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="${mysql.driver}" />
                <property name="url" value="${mysql.url}" />
                <property name="username" value="${mysql.username}" />
                <property name="password" value="${mysql.password}" />
            </dataSource>
        </environment>

        <environment id="test_mysql">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="${mysql.drivertest}" />
                <property name="url" value="${mysql.urltest}" />
                <property name="username" value="${mysql.usernametest}" />
                <property name="password" value="${mysql.passwordtest}" />
            </dataSource>
        </environment>

    </environments>

    <mappers>
        <mapper resource="mapper/resultmapmapper.xml"/>
        <mapper resource="mapper/resultmapdepartmentmapper.xml"/>
    </mappers>
</configuration>

建立測試類

package org.mybatis.resultmap;

import java.io.IOException;
import java.io.InputStream;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

/**
 * 使用介面式程式設計
 */
public class MybatisTest {

    @Test
    public void testMybatis() {
        String resource = "mybatis-config-resultmap.xml";//全域性配置檔案

        InputStream inputStream = null;
        SqlSessionFactory sqlSessionFactory = null;
        SqlSession sqlSession = null;
        try {
            inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            sqlSession = sqlSessionFactory.openSession();

            EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);

            //分步查詢
            Employee emp4 = mapper.getEmpAndDeptByStep(2);
            System.out.println(emp4.getLastName());
            System.out.println("`######");
            System.out.println(emp4.getDepartment());


            sqlSession.close();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
        }
    }
}

執行結果：

DEBUG - ooo Using Connection [[email protected]]
DEBUG - ==> Preparing: select * from mybatis_employee where id = ?
DEBUG - ==> Parameters: 2(Integer)
DEBUG - ooo Using Connection [[email protected]]
DEBUG - ==> Preparing: select id,deptName from mybatis_department where id = ?
DEBUG - ==> Parameters: 2(Integer)
lisi
·######
Department [id=2, deptName=測試部]
DEBUG - Resetting autocommit to true on JDBC Connection [[email protected]]
DEBUG - Closing JDBC Connection [[email protected]]
DEBUG - Returned connection 1384722895 to pool.

與之前展示的不一樣的是本次操作傳送了兩次sql請求，先獲取到第一條資料的內容，然後從第一次請求獲取的結果中使用某個關聯值作為第二次查詢的引數。完成分步的查詢操作。

1）collection

使用collection實現分步查詢操作，跟使用association基本相似。

Employee和Department實體類不做任何修改。

DepartmentMapper新增查詢方法

public Department getDeptWithEmpByStep(Integer id);

EmployeeMapper新增查詢方法

/**
 * 通過部門id查詢Employee
 * @param id 部門id
 */
public List<Employee> getEmpByDeptId(Integer id);

Employee對應的resultmapmapper.xml配置檔案新增如下查詢方法：

<select id="getEmpByDeptId" resultType="org.mybatis.resultmap.Employee">
    select id,last_name,email,gender,dept_id from mybatis_employee where dept_id = #{deptId}
</select>

Department對應的resultmapdepartmentmapper.xml配置檔案新增如下查詢方法：

<!-- 分步查詢 collection-->
<resultMap type="org.mybatis.resultmap.Department" id="mstep2">
    <id column="did" property="id"/>
    <result column="deptName" property="deptName"/>
    <!-- 一個department對應多個employee -->
    <collection property="emps" select="org.mybatis.resultmap.EmployeeMapper.getEmpByDeptId" column="did">
        <id column="eid" property="id"/>
        <result column="last_name" property="lastName"/>
        <result column="email" property="email"/>
        <result column="gender" property="gender"/>
    </collection>
</resultMap>
<select id="getDeptWithEmpByStep" resultMap="mstep2">
    SELECT
        md.id did,
        md.deptName deptName
    FROM
        mybatis_department md
    WHERE
        md.id = #{id}
</select>

測試collection分步查詢

@Test
public void testMybatis() {
    String resource = "mybatis-config-resultmap.xml";//全域性配置檔案

    InputStream inputStream = null;
    SqlSessionFactory sqlSessionFactory = null;
    SqlSession sqlSession = null;
    try {
        inputStream = Resources.getResourceAsStream(resource);
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        sqlSession = sqlSessionFactory.openSession();

        DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);

        Department dept2 = mapper.getDeptWithEmpByStep(2);
        System.out.println(dept2);
        System.out.println(dept2.getEmps());

        sqlSession.close();
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
    }
}

測試結果

DEBUG - ooo Using Connection [[email protected]]
DEBUG - ==> Preparing: SELECT md.id did, md.deptName deptName FROM mybatis_department md WHERE md.id = ?
DEBUG - ==> Parameters: 2(Integer)
DEBUG - ooo Using Connection [[email protected]]
DEBUG - ==> Preparing: select id,last_name,email,gender,dept_id from mybatis_employee where dept_id = ?
DEBUG - ==> Parameters: 2(Integer)
Department [id=2, deptName=測試部]
[Employee [id=2, lastName=lisi, [email protected], gender=2], Employee [id=5, lastName=huanhuan, [email protected], gender=2], Employee [id=6, lastName=huanan, [email protected], gender=2]]
DEBUG - Resetting autocommit to true on JDBC Connection [[email protected]]
DEBUG - Closing JDBC Connection [[email protected]]
DEBUG - Returned connection 1282811396 to pool.

同樣傳送了兩次sql請求，資料封裝完成。
如果在第二步查詢的過程中需要傳遞多個引數，可以使用Map的形式進行傳遞，如：column=”did”可以寫成column=”{deptId=did}”。

2.延遲載入

在分步查詢的基礎之上，介紹一下延遲載入的內容。
關聯查詢的使用中，可能每次查詢只使用其中一個類的屬性，關聯類的屬性不需要使用，能夠不載入呢？mybatis提供延遲載入方案，設定延遲載入引數配置，可在使用的時候才載入某些資料。
如上面展示一次傳送兩個sql請求，將Employee和Department中的資料都查詢出來了。如果我想在使用Department的時候才去載入Department該如何實現呢？
在主配置檔案中<settings> 標籤新增如下配置：

<setting name="lazyLoadingEnabled" value="true" />
<setting name="aggressiveLazyLoading" value="false"/>

再次測試

DEBUG - ooo Using Connection [[email protected]]
DEBUG - ==> Preparing: select * from mybatis_employee where id = ?
DEBUG - ==> Parameters: 2(Integer)
lisi
`######
DEBUG - ooo Using Connection [[email protected]]
DEBUG - ==> Preparing: select id,deptName from mybatis_department where id = ?
DEBUG - ==> Parameters: 2(Integer)
Department [id=2, deptName=測試部]
DEBUG - Resetting autocommit to true on JDBC Connection [com.[email protected]]
DEBUG - Closing JDBC Connection [[email protected]]
DEBUG - Returned connection 1291113768 to pool.

結果顯示在使用Department之前，只發送了一個sql語句，當呼叫getDepartment()方法時才從後臺獲取資料。