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MyBatis延迟加载
===============

MyBatis在封装JDBC同时，对其进行了优化，除缓存以外，还提供了延迟加载机制：

- 如果在MyBatis中开启了延迟加载，在执行子查询时：
  - 首先执行第一次查询
  - 只有在用到子查询结果时，才会触发子查询
  - 如果不使用子查询的结果，子查询不会执行
- 借助延迟加载，可以减少对数据库不必要的访问

1 实体类

将Association项目cn.tedu.association.pojo包中的三个实体类
Department、Employee和DepartmentEmployees
复制到Dynamic项目cn.tedu.dynamic.pojo包中，并修改包名：

package cn.tedu.dynamic.pojo;

在/src/main/java/cn.tedu.dynamic.dao目录下添加数据访问接口：

2 DAO

public interface DepartmentDao {
    public DepartmentEmployees getDepartmentEmployees(String name);
}

public interface EmployeeDao {
    public List<Employee> getEmployees(int department);
}

3 配置MyBatis

在/src/main/resources/mybatis-config.xml文件中添加：

<typeAliases>
    ...
    <typeAlias type="cn.tedu.dynamic.pojo.Department" alias="Department" />
    <typeAlias type="cn.tedu.dynamic.pojo.Employee" alias="Employee" />
    <typeAlias type="cn.tedu.dynamic.pojo.DepartmentEmployees" alias="DepartmentEmployees" />
    ...
    <mappers>
        ...
        <mapper resource="mappers/DepartmentMapper.xml" />
        <mapper resource="mappers/EmployeeMapper.xml" />
        ...
    </mappers>
    ...
</typeAliases>

4 Mapper

在/src/main/resources/mappers目录下添加DepartmentMapper.xml文件：

<mapper namespace="cn.tedu.dynamic.dao.DepartmentDao">
    <resultMap id="departmentEmployees" type="DepartmentEmployees">
        <result column="id" property="department.id" />
        <result column="name" property="department.name" />
        <result column="description" property="department.description" />
        <association column="id" property="employees"
                     select="cn.tedu.dynamic.dao.EmployeeDao.getEmployees" />
    </resultMap>
    <select id="getDepartmentEmployees" resultMap="departmentEmployees">
        select id, name, description from t_department
        where name = #{name}
    </select>
</mapper>

在/src/main/resources/mappers目录下添加EmployeeMapper.xml文件：

<mapper namespace="cn.tedu.dynamic.dao.EmployeeDao">
    <select id="getEmployees" resultType="Employee">
        select number, fullname, age, department from t_employee
        where department = #{department}
    </select>
</mapper>

5 测试

public class DepartmentDaoTest {
    @Test
    public void testGetDepartmentEmployees() {
        DepartmentDao departmentDao = MyBatisUtil.getMapper(DepartmentDao.class);
        DepartmentEmployees departmentEmployees = departmentDao.getDepartmentEmployees("研发部");
        System.out.println(departmentEmployees.getDepartment());
    }
}

运行测试用例：

[main] DEBUG - Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.sg :Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.
[main] DEBUG - Opening JDBC Connectionsg :Opening JDBC Connection
[main]  INFO - {dataSource-1} initedsg :{dataSource-1} inited
[Finalizer] DEBUG - PooledDataSource forcefully closed/removed all connections.sg :PooledDataSource forcefully closed/removed all connections.
[main] DEBUG - ==>  Preparing: select id, name, description from t_department where name = ? sg :==>  Preparing: select id, name, description from t_department where name = ?
[main] DEBUG - ==> Parameters: 研发部(String)sg :==> Parameters: 研发部(String)
[main] DEBUG - ====>  Preparing: select number, fullname, age, department from t_employee where department = ? sg :====>  Preparing: select number, fullname, age, department from t_employee where department = ?
[main] DEBUG - ====> Parameters: 1(Integer)sg :====> Parameters: 1(Integer)
[main] DEBUG - <====      Total: 3sg :<====      Total: 3
[main] DEBUG - <==      Total: 1sg :<==      Total: 1
Department(id=1, name=研发部, description=研发软件产品)

执行了两次select，第一次查询t_department表，第二次查询t_employee表。

6 开启延迟加载

在DepartmentMapper.xml文件中将子查询的fetchType属性设置为lazy：

<mapper namespace="cn.tedu.dynamic.dao.DepartmentDao">
    ...
    <resultMap id="departmentEmployees" ...>
        ...
        <association ... fetchType="lazy" />
    </resultMap>
    ...
</mapper>

运行测试用例：

报错提示空指针异常，Error creating lazy proxy. Cause: java.lang.NullPointerException ...

7 升级MyBatis

在pom.xml文件中指定依赖更高版本的MyBatis：

<dependencies>
    ...
    <!-- MyBatis -->
    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>3.5.1</version>
    </dependency>
    ...
</dependencies>

运行测试用例：

[main] DEBUG - Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.sg :Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.
[main] DEBUG - Opening JDBC Connectionsg :Opening JDBC Connection
[main]  INFO - {dataSource-1} initedsg :{dataSource-1} inited
[Finalizer] DEBUG - PooledDataSource forcefully closed/removed all connections.sg :PooledDataSource forcefully closed/removed all connections.
[main] DEBUG - ==>  Preparing: select id, name, description from t_department where name = ? sg :==>  Preparing: select id, name, description from t_department where name = ?
[main] DEBUG - ==> Parameters: 研发部(String)sg :==> Parameters: 研发部(String)
[main] DEBUG - <==      Total: 1sg :<==      Total: 1
Department(id=1, name=研发部, description=研发软件产品)

只执行了对t_department表的第一次查询。

8 使用子查询结果

修改测试用例，增加对子查询结果的使用：

public class DepartmentDaoTest {
    @Test
    public void testGetDepartmentEmployees() {
        DepartmentDao departmentDao = MyBatisUtil.getMapper(DepartmentDao.class);
        DepartmentEmployees departmentEmployees = departmentDao.getDepartmentEmployees("研发部");
        System.out.println(departmentEmployees.getDepartment());
        System.out.println(departmentEmployees.getEmployees());
    }
}

运行测试用例：

[main] DEBUG - Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.sg :Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.
[main] DEBUG - Opening JDBC Connectionsg :Opening JDBC Connection
[main]  INFO - {dataSource-1} initedsg :{dataSource-1} inited
[Finalizer] DEBUG - PooledDataSource forcefully closed/removed all connections.sg :PooledDataSource forcefully closed/removed all connections.
[main] DEBUG - ==>  Preparing: select id, name, description from t_department where name = ? sg :==>  Preparing: select id, name, description from t_department where name = ?
[main] DEBUG - ==> Parameters: 研发部(String)sg :==> Parameters: 研发部(String)
[main] DEBUG - <==      Total: 1sg :<==      Total: 1
Department(id=1, name=研发部, description=研发软件产品)
[main] DEBUG - ==>  Preparing: select number, fullname, age, department from t_employee where department = ? sg :==>  Preparing: select number, fullname, age, department from t_employee where department = ?
[main] DEBUG - ==> Parameters: 1(Integer)sg :==> Parameters: 1(Integer)
[main] DEBUG - <==      Total: 3sg :<==      Total: 3
[Employee(number=1001, fullname=张飞, age=25, department=1), Employee(number=1002, fullname=关羽, age=30, department=1), Employee(number=1003, fullname=赵云, age=20, department=1)]

只有在实际用到子查询结果时才真正执行子查询操作。

9 提前关闭会话

修改测试用例，在使用子查询结果之前关闭会话：

public class DepartmentDaoTest {
    @Test
    public void testGetDepartmentEmployees() {
        DepartmentDao departmentDao = MyBatisUtil.getMapper(DepartmentDao.class);
        DepartmentEmployees departmentEmployees = departmentDao.getDepartmentEmployees("研发部");
        System.out.println(departmentEmployees.getDepartment());
        MyBatisUtil.closeSession();
        System.out.println(departmentEmployees.getEmployees());
    }
}

运行测试用例：

[main] DEBUG - Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.sg :Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.
[main] DEBUG - Opening JDBC Connectionsg :Opening JDBC Connection
[main]  INFO - {dataSource-1} initedsg :{dataSource-1} inited
[Finalizer] DEBUG - PooledDataSource forcefully closed/removed all connections.sg :PooledDataSource forcefully closed/removed all connections.
[main] DEBUG - ==>  Preparing: select id, name, description from t_department where name = ? sg :==>  Preparing: select id, name, description from t_department where name = ?
[main] DEBUG - ==> Parameters: 研发部(String)sg :==> Parameters: 研发部(String)
[main] DEBUG - <==      Total: 1sg :<==      Total: 1
Department(id=1, name=研发部, description=研发软件产品)
[main] DEBUG - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7d64e326]sg :Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7d64e326]
[main] DEBUG - Opening JDBC Connectionsg :Opening JDBC Connection
[main] DEBUG - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7d64e326]sg :Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7d64e326]
[main] DEBUG - ==>  Preparing: select number, fullname, age, department from t_employee where department = ? sg :==>  Preparing: select number, fullname, age, department from t_employee where department = ?
[main] DEBUG - ==> Parameters: 1(Integer)sg :==> Parameters: 1(Integer)
[main] DEBUG - <==      Total: 3sg :<==      Total: 3
[main] DEBUG - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7d64e326]sg :Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7d64e326]
[main] DEBUG - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7d64e326]sg :Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7d64e326]
[Employee(number=1001, fullname=张飞, age=25, department=1), Employee(number=1002, fullname=关羽, age=30, department=1), Employee(number=1003, fullname=赵云, age=20, department=1)]

子查询的延迟执行不受会话提前关闭的影响。

例程：Dynamic