Mybatis 46_基于嵌套select的特征

▲ 使用@One注解
<association…/> = @Result + @One
@One只需要指定select和fetchType,而column依然放在@Result中指定

项目0514基于嵌套select的1-1_注解

主类

package lee;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

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.itcheng.app.dao.AddressMapper;
import org.itcheng.app.dao.PersonMapper;
import org.itcheng.app.domain.Address;
import org.itcheng.app.domain.Person;

public class PersonManager
{
	// SqlSessionFactory应该是应用级别
	private static SqlSessionFactory sqlSessionFactory;
	public static void main(String[] args) throws IOException
	{
		String resource = "mybatis-config.xml";
		InputStream inputStream = Resources.getResourceAsStream(resource);
		// 1. 创建SqlSessionFactory
		sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
		// 2. 打开SqlSession
		SqlSession sqlSession = sqlSessionFactory.openSession();
		
		findPerson(sqlSession);
		/* 结果
		 * DEBUG [main] org.itcheng.app.dao.PersonMapper.getPerson ==>  Preparing: select * from person_inf where person_id = ? 
		 * DEBUG [main] org.itcheng.app.dao.PersonMapper.getPerson ==> Parameters: 3(Integer)
		 * DEBUG [main] org.itcheng.app.dao.PersonMapper.getPerson <==      Total: 1
		 * 白鼠精
		 * -----------------
		 * DEBUG [main] org.itcheng.app.dao.AddressMapper.getAddressByOwner ==>  Preparing: select * from address_inf where owner_id = ? 
		 * DEBUG [main] org.itcheng.app.dao.AddressMapper.getAddressByOwner ==> Parameters: 3(Integer)
		 * DEBUG [main] org.itcheng.app.dao.AddressMapper.getAddressByOwner <==      Total: 1
		 * 陷空山无底洞
		 * */
	
//		findAddresses(sqlSession);
		/* 结果
		 * DEBUG [main] org.itcheng.app.dao.AddressMapper.findAddressesById ==>  Preparing: select * from address_inf where addr_id > ? 
		 * DEBUG [main] org.itcheng.app.dao.AddressMapper.findAddressesById ==> Parameters: 1(Integer)
		 * DEBUG [main] org.itcheng.app.dao.AddressMapper.findAddressesById <==      Total: 4
		 * 花果山水帘洞
		 * DEBUG [main] org.itcheng.app.dao.PersonMapper.getPerson ==>  Preparing: select * from person_inf where person_id = ? 
		 * DEBUG [main] org.itcheng.app.dao.PersonMapper.getPerson ==> Parameters: 1(Integer)
		 * DEBUG [main] org.itcheng.app.dao.PersonMapper.getPerson <==      Total: 1
		 * 孙悟空
		 * 福陵山云栈洞
		 * 陷空山无底洞
		 * 积雷山摩云洞
		 * -----------------
		 * */
	}
	
	public static void findPerson(SqlSession sqlSession)
	{
		PersonMapper personMapper = sqlSession.getMapper(PersonMapper.class);
		
		Person p = personMapper.getPerson(3);
		System.out.println(p.getName());
		System.out.println("-----------------");
		
		// Person获取关联实体是延迟加载，要等到真正访问关联实体时才去执行额外的SELECT语句
		System.out.println(p.getAddr().getDetail());
		
		// 4. 提交事务
		sqlSession.commit();
		// 5. 关闭资源
		sqlSession.close();	
	}
	
	public static void findAddresses(SqlSession sqlSession)
	{
		AddressMapper addressMapper = sqlSession.getMapper(AddressMapper.class);
		
		List<Address> addrList = addressMapper.findAddressesById(1);
		//  由于Address获取关联实体是立即加载，因此它会立即执行额外的SELECT语句
		// 对于立即加载策略,即使不访问关联实体,它也会立即执行额外的SELECT语句
		addrList.forEach(e -> {
			System.out.println(e.getDetail());
			// 此处只获取水帘洞的主人
			if(e.getDetail().contains("水帘洞"))
			{
				System.out.println(e.getPerson().getName());
			}
		});
		System.out.println("-----------------");
		
		// 4. 提交事务
		sqlSession.commit();
		// 5. 关闭资源
		sqlSession.close();	
	}	
		
}

接口类

package org.itcheng.app.dao;

import java.util.List;

import org.apache.ibatis.annotations.One;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.ResultMap;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.mapping.FetchType;
import org.itcheng.app.domain.Address;

// Mapper组件相当于DAO组件
public interface AddressMapper
{
	@Select("select * from address_inf where addr_id > #{id}")
	@Results(id = "addrMap", value = {
		@Result(column="addr_id", property="id", id=true),
		@Result(column="addr_detail", property="detail"),
		@Result(property="person", column="owner_id", 
			one=@One(select="org.itcheng.app.dao.PersonMapper.getPerson",
			 fetchType=FetchType.LAZY))
	})
	List<Address> findAddressesById(Integer id);
	
	@Select("select * from address_inf where owner_id = #{id}")
	@ResultMap("addrMap")  // 该注解表明引用一个已有ResultMap的id
	List<Address> getAddressByOwner(Integer ownerId);	
}

package org.itcheng.app.dao;

import org.apache.ibatis.annotations.One;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.mapping.FetchType;
import org.itcheng.app.domain.Person;

// Mapper组件相当于DAO组件
public interface PersonMapper
{
	@Select("select * from person_inf where person_id = #{id}")
	@Results({ @Result(column = "person_id", property = "id", id = true),
		@Result(column = "person_name", property = "name"),
		@Result(column = "person_age", property = "age"),
		@Result(property="addr",  column="person_id", 
			one = @One(select = "org.itcheng.app.dao.AddressMapper.getAddressByOwner",
			fetchType = FetchType.LAZY))
	})
	Person getPerson(Integer id);
}

嵌套select的特征

嵌套select（包括1-1、1-N、N-N）都存在一个显著的"1+N"select语句的性能问题：
比如你要查询Address及其关联实体，执行过程如下：
（1） MyBatis执行第一条select语句获取多条Address记录。
（2） MyBatis要为【每条】Address记录分别执行一条额外的select语句来获取关联的对象。
这意味着第一条SQL语句返回了N条记录，第二次就需要额外执行N条select语句来获取关联对象。

嵌套select（1+N）的优势：

1. MyBatis开始只需要加载被查询的记录，完全可以不加载关联记录（延迟加载）,
   根本就不获取关联记录，因此可以节省内存空间。
2. 程序即使要访问关联对象，但可能只访问部分关联对象，此时需要也不需要额外增加N条select语句
   【结论】：如果使用嵌套select的关联查询，绝大部分情况都应该使用延迟加载！