MyBatis总结(二十六)--任务35：MyBatis_映射文件_select_resultMap_关联查询_collection分步查询&延迟加载

忘是亡心i 2022-05-12 08:16 190阅读 0赞

本文内容来自上硅谷

与[MyBatis总结(二十五)--任务34：MyBatis\_映射文件\_select\_resultMap\_关联查询\_collection定义关联集合封装规则][MyBatis_--_34_MyBatis_select_resultMap_collection]相比较实现的功能都是一样的，不过本文注重的是分步，与延迟加载

**目录**

1分步

1.1Junit

1.2dao

1.3xml

1.4第二个sql的dao

1.5第二个sql的xml

1.5第二个sql对应的javabean(及每部门下面的每个员工信息)

2延迟加载

3扩展

3.0使用实例如下

3.1多列的值传递过去

3.2在全局配置了延迟加载的情况下可以在collection的fetchType属性可以设置该sql是否执行延迟加载

--------------------

# 1分步 #

## 1.1Junit ##

@Test
    	public void test06() throws IOException{
    		SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
    		SqlSession openSession = sqlSessionFactory.openSession();
    		
    		try{
    			DepartmentMapper mapper = openSession.getMapper(DepartmentMapper.class);
    			Department deptByIdStep = mapper.getDeptByIdStep(1);
    			System.out.println(deptByIdStep.getDepartmentName());
    			System.out.println(deptByIdStep.getEmps());
    		}finally{
    			openSession.close();
    		}
    	}

## 1.2dao ##

package com.atguigu.mybatis.dao;
    
    import com.atguigu.mybatis.bean.Department;
    
    public interface DepartmentMapper {
    	
    	public Department getDeptById(Integer id);
    	
    	public Department getDeptByIdPlus(Integer id);
    
    	public Department getDeptByIdStep(Integer id);
    }

## 1.3xml ##

collection中的select属性就规定了第二段查询的语句

<resultMap type="com.atguigu.mybatis.bean.Department" id="MyDeptStep">
    		<id column="id" property="id"/>
    		<id column="dept_name" property="departmentName"/>
    		<collection property="emps"
    			select="com.atguigu.mybatis.dao.EmployeeMapperPlus.getEmpsByDeptId"
    			column="{deptId=id}" fetchType="lazy"></collection>
    	</resultMap>
    	
    	<select id="getDeptByIdStep" resultMap="MyDeptStep">
    		select id,dept_name from tbl_dept where id=#{id}
    	</select>

## 1.4第二个sql的dao ##

package com.atguigu.mybatis.dao;
    
    import java.util.List;
    
    import com.atguigu.mybatis.bean.Employee;
    
    public interface EmployeeMapperPlus {
    	
    	public Employee getEmpById(Integer id);
    	
    	public Employee getEmpAndDept(Integer id);
    	
    	public Employee getEmpByIdStep(Integer id);
    	
    	public List<Employee> getEmpsByDeptId(Integer deptId);
    
    }

## 1.5第二个sql的xml ##

<select id="getEmpsByDeptId" resultType="com.atguigu.mybatis.bean.Employee">
    		select * from tbl_employee where d_id=#{deptId}
    	</select>

## 1.5第二个sql对应的javabean(及每部门下面的每个员工信息) ##

package com.atguigu.mybatis.bean;
    
    import org.apache.ibatis.type.Alias;
    
    @Alias("emp")
    public class Employee {
    	
    	private Integer id;
    	private String lastName;
    	private String email;
    	private String gender;
    	private Department dept;
    	
    	public Employee() {
    		super();
    	}
    	
    	public Employee(Integer id, String lastName, String email, String gender) {
    		super();
    		this.id = id;
    		this.lastName = lastName;
    		this.email = email;
    		this.gender = gender;
    	}
    	
    	
    	
    
    	public Department getDept() {
    		return dept;
    	}
    
    	public void setDept(Department dept) {
    		this.dept = dept;
    	}
    
    	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;
    	}
    	@Override
    	public String toString() {
    		return "Employee [id=" + id + ", lastName=" + lastName + ", email="
    				+ email + ", gender=" + gender + "]";
    	}
    	
    	
    
    }

# 2延迟加载 #

代码语句一致，只是在mybatis配置文件中需要配置

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

在执行JUnit时

![70][]

当执行完215后且执行216之前时，只会执行第一个sql语句。到执行216行时，才会调用第二个sql

# 3扩展 #

## 3.0使用实例如下 ##

## 3.1多列的值传递过去 ##

格式如下： column="\{key1=column1,key2=column2\}"

## 3.2在全局配置了延迟加载的情况下可以在collection的fetchType属性可以设置该sql是否执行延迟加载 ##

fetchType="lazy"：表示使用延迟加载；  
         - lazy：延迟  
         - eager：立即

[MyBatis_--_34_MyBatis_select_resultMap_collection]: https://blog.csdn.net/lsx2017/article/details/82721385
[70]: /images/20220512/60dfeab08d1e4e57b40aa2c9c8f387cc.png