11.2 注解的使用示例2 一对一关联关系

发表于 更新于 分类于 ， ， Waline： 本文字数： 7.8k 阅读时长 ≈ 7 分钟

11.2 注解的使用示例2 一对一关联关系

目录结构

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E:\workspace_web2\MyAOneToOneTest
├─src
│ ├─db.properties
│ ├─domain
│ │ ├─Card.java
│ │ └─Person.java
│ ├─fractory
│ │ └─SqlSessionFratoryTools.java
│ ├─log4j.xml
│ ├─mapper
│ │ ├─CardMapper.java
│ │ └─PersonMapper.java
│ ├─mybatis-config.xml
│ ├─mybatis_tb_card_tb_person.sql
│ └─test
│   └─OneToOneTest.java
└─WebContent
  ├─META-INF
  └─WEB-INF
    └─lib
      ├─ant-1.9.6.jar
      ├─ant-launcher-1.9.6.jar
      ├─asm-5.2.jar
      ├─cglib-3.2.5.jar
      ├─commons-logging-1.2.jar
      ├─javassist-3.22.0-CR2.jar
      ├─log4j-1.2.17.jar
      ├─log4j-api-2.3.jar
      ├─log4j-core-2.3.jar
      ├─mybatis-3.4.5.jar
      ├─mysql-connector-java-5.1.44-bin.jar
      ├─ognl-3.1.15.jar
      ├─slf4j-api-1.7.25.jar
      └─slf4j-log4j12-1.7.25.jar

创建数据库表

首先,给之前创建的mybatis数据库创建两个表tb_card和tb_person,并插入测试数据。SQL脚本如下使用方式:
保存成.sql文件,然后使用Navicat导入(设置编码格式utf-8,免得乱码).

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# 创建tb_card表
DROP TABLE IF EXISTS `tb_card`;
CREATE TABLE `tb_card` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `code` varchar(18) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
# 插入一条数据
INSERT INTO `tb_card` VALUES ('1', '43280119980091');
# 创建tb_person表
DROP TABLE IF EXISTS `tb_person`;
CREATE TABLE `tb_person` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(18) NOT NULL,
  `sex` varchar(18) NOT NULL,
  `age` int(11) DEFAULT NULL,
  `card_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `card_id` (`card_id`),
  CONSTRAINT `tb_person_ibfk_1` FOREIGN KEY (`card_id`)
    REFERENCES `tb_card` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
# 插入一条数据
INSERT INTO `tb_person` VALUES ('1', '小明', '男', '22', '1');

创建持久化类

Card.java

/MyAOneToOneTest/src/domain/Card.java
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package domain;
public class Card {
    private Integer id;
    private String code;
    public Card()
    {
        // TODO Auto-generated constructor stub
    }
    public Card(Integer id, String code)
    {
        this.id = id;
        this.code = code;
    }
    // 此处省略getter和setter方法,请自己补上
    @Override
    public String toString()
    {
        return "Card [id=" + id + ", code=" + code + "]";
    }
}

Person.java

/MyAOneToOneTest/src/domain/Person.java
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package domain;
public class Person {
    private Integer id;
    private String name;
    private String sex;
    private Integer age;
    private Card card;
    public Person()
    {
        super();
    }
    // 此处省略getter和setter方法,请自己补上
    @Override
    public String toString()
    {
        return "Person [id=" + id + ", name=" + name + ", sex=" + sex + ", age=" + age + ", card="
                + card + "]";
    }
}

mapper接口

CardMapper.java

/MyAOneToOneTest/src/mapper/CardMapper.java
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package mapper;

import org.apache.ibatis.annotations.Select;
import domain.Card;

public interface CardMapper {
    @Select("select * from tb_card where id=#{id}")
    Card selectCardById(Integer id);
}

PersonMapper.java

/MyAOneToOneTest/src/mapper/PersonMapper.java
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package mapper;

import org.apache.ibatis.annotations.One;
import org.apache.ibatis.annotations.Param;
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 domain.Person;

public interface PersonMapper {
    @Select("select * from tb_person where id=#{id}")
    @Results({
        @Result(column = "id",property = "id", id = true),
        @Result(column = "name",property = "name"),
        @Result(column = "sex",property = "sex"),
        @Result(column = "age",property = "age"),
        @Result(column = "card_id",property = "card", 
            one = @One(select = "mapper.CardMapper.selectCardById", fetchType = FetchType.LAZY)
        ) 
    })
    Person selectPersonById(@Param("id") Integer id);
}

selectPersonById方法使用了@Select注解,其根据id查询对应的Person数据。因为需要将Person对应的Card数据也查询出来,所以Person的Card属性使用了一个@Result结果映射。column="card_id",property="card"表示Person的Card属性对应tb_person表的card_id列,one属性表示是一个一对一关联关系,
@one注解的select属性表示需要关联执行的SQL语句,
fetchType表示查询的类型是立即加载(EAGER)还是懒加载(LAZY)。

测试类

/MyAOneToOneTest/src/test/OneToOneTest.java
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package test;

import org.apache.ibatis.session.SqlSession;
import domain.Person;
import fractory.SqlSessionFactoryTools;
import mapper.PersonMapper;

public class OneToOneTest {
    public static void main(String[] args) {
        SqlSession sqlSession = null;
        try {
            sqlSession = SqlSessionFactoryTools.getSqlSession();
            PersonMapper personMapper = sqlSession.getMapper(PersonMapper.class);
            // 执行查询
            Person person = personMapper.selectPersonById(1);
            // 设置懒加载，立即加载Card
            testLazyUseCardNow(person);
            // 设置懒加载，不立即加载Card
            // testLazyUseCardLazy(person);
            // 提交事务
            sqlSession.commit();
        } catch (Exception e) {
            // 出错回滚事务
            sqlSession.rollback();
            e.printStackTrace();
        } finally {
            // 关闭会话
            if (sqlSession != null)
                sqlSession.close();
        }
    }

    /**
     * 当设置为懒加载时，立即访问card成员变量的情况。
     * 
     * @param person
     */
    private static void testLazyUseCardNow(Person person) {
        // 调用toString方法会立即加载Card
        System.out.println(person.toString());
    }

    /**
     * 当设置为懒加载时，不立即访问card成员变量的。
     * 
     * @param person 查询数据库得到的对象
     */
    private static void testLazyUseCardLazy(Person person) {
        System.out.println(person.toStringSimple());
        System.out.println("=================================");
        System.out.println(person.getCard());
    }
}

运行效果

运行OneToOneTest类的main方法,该方法通过SqlSession的getMapper(Class<T> type)方法获得mapper接口的代理对象PersonMapper。调用PersonMapper的selectPersonById方法时会执行该方法上的注解。需要注意的是,Person的一对一关联使用的注解@one的select属性,要执行的SQL语句在CardMapper接口的selectCardById方法的注解中。控制台显示如下所示：

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DEBUG [main] ==>  Preparing: select * from tb_person where id=? 
DEBUG [main] ==> Parameters: 1(Integer)
DEBUG [main] <==      Total: 1
DEBUG [main] ==>  Preparing: select * from tb_card where id=? 
DEBUG [main] ==> Parameters: 1(Integer)
DEBUG [main] <==      Total: 1
Person [id=1, name=小明, sex=男, age=22, card=Card [id=1, code=450101199701243862]]

可以看到,查询Peson信息时Person对应的Card对象也被查询出来了.
在上面PersonMapper接口中的设置的是懒加载。
由于Person类的toString方法里面有访问到card属性，所以会执行关联查询。

懒加载验证

如果把上面的testLazyUseCardNow(person);语句注释掉，打开testLazyUseCardLazy(person);语句。如下所示：

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public static void main(String[] args) {
    SqlSession sqlSession = null;
    try {
        sqlSession = SqlSessionFactoryTools.getSqlSession();
        PersonMapper personMapper = sqlSession.getMapper(PersonMapper.class);
        // 执行查询
        Person person = personMapper.selectPersonById(1);
        // 设置懒加载，立即加载Card
        // testLazyUseCardNow(person);
        // 设置懒加载，不立即加载Card
        testLazyUseCardLazy(person);
        // 提交事务
        sqlSession.commit();
    } catch (Exception e) {
        // 出错回滚事务
        sqlSession.rollback();
        e.printStackTrace();
    } finally {
        // 关闭会话
        if (sqlSession != null)
            sqlSession.close();
    }
}

则先执行一条SQL语句,取出Person,当需要访问Person类的card成员时，才会执行下一条SQL语句。

运行结果如下：

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DEBUG [main] ==>  Preparing: select * from tb_person where id=? 
DEBUG [main] ==> Parameters: 1(Integer)
DEBUG [main] <==      Total: 1
Person [id=1, name=小明, sex=男, age=22]
=================================
DEBUG [main] ==>  Preparing: select * from tb_card where id=? 
DEBUG [main] ==> Parameters: 1(Integer)
DEBUG [main] <==      Total: 1
Card [id=1, code=450101199701243862]

这就是懒加载的效果。

立即加载验证

再PersonMapper接口中使用立即加载

public interface PersonMapper {
    @Select("select * from tb_person where id=#{id}")
    @Results({
        @Result(column = "id",property = "id", id = true),
        @Result(column = "name",property = "name"),
        @Result(column = "sex",property = "sex"),
        @Result(column = "age",property = "age"),
        @Result(column = "card_id",property = "card", 
            one = @One(select = "mapper.CardMapper.selectCardById", fetchType = FetchType.EAGER)
        ) 
    })
    Person selectPersonById(@Param("id") Integer id);
}

再次运行上面的测试类，运行结果如下：

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DEBUG [main] ==>  Preparing: select * from tb_person where id=? 
DEBUG [main] ==> Parameters: 1(Integer)
DEBUG [main] ====>  Preparing: select * from tb_card where id=? 
DEBUG [main] ====> Parameters: 1(Integer)
DEBUG [main] <====      Total: 1
DEBUG [main] <==      Total: 1
Person [id=1, name=小明, sex=男, age=22]
=================================
Card [id=1, code=450101199701243862]

可以看到在立即加载模式下，就算还没有访问Person类的card属性，也会执行关联查询，加载card内容。