[07] 延迟加载

如今咱们有 Author 做者类，其属性中有 List<Book> bookList 用来表示该做者所出版的书籍。咱们配置好映射关系以后，但愿在获取到一个 Author 类时，其 bookList 的值只有当咱们在用到的时候才会去数据库获取，而不是每次都给咱们所有的完整数据，这也就是MyBatis延迟加载要解决的问题：即映射集合内存在级联时，咱们实际须要的数据少于数据库查出来的数据。这就会形成数据库多查出来的数据派不上用场，同时也加大了数据库负担。

咱们在提到 MyBatis 的内联时，必然会涉及到的就是两个元素 association 和 collection，在以前的阐述中咱们也说过，对于内联查询的方式有一种利用到了 select 属性，也便是咱们延迟加载的主人公。咱们说使用 select 其实是将SQL语句分红了两部分单独执行，而不是采用联表查询的SQL，也正是这样，能让咱们实现延迟加载。

在MyBatis中实现延迟加载有两种方式，若是你但愿全局开启延迟加载，那么你须要在mybatis-config.xml全局配置文件的<settings>中加入下面这两个配置：

<!--开启全局延迟加载（懒加载）-->
<setting name="lazyLoadingEnabled" value="true" />
<!--任何方法调用均加载该对象全部属性-->
<setting name="aggressiveLazyLoading" value="false"/>

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<!--开启全局延迟加载（懒加载）-->

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

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<!--任何方法调用均加载该对象全部属性-->

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

若是你并不想全局开启，只是针对某个类的某些属性进行延迟加载开启，那么则须要你在 resultMap 中的 association 或 collection 设置属性值 fetchType="lazy"，以下示例：

<association fetchType="lazy" property="author" column="author_id" javaType="dulk.learn.mybatis.pojo.Author" select="findAuthorById"/>

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<association fetchType="lazy" property="author" column="author_id" javaType="dulk.learn.mybatis.pojo.Author" select="findAuthorById"/>

刚才咱们提到了做者和书之间的一对多关系，下面咱们就来以此为例看下相关的代码和配置。

Author类：

public class Author {
    private long id;
    private String name;
    private int age;
    private List<Book> bookList;

    //... getter and setter
}

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public class Author {

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    private long id;

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    private String name;

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    private int age;

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    private List<Book> bookList;

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    //... getter and setter

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}

Dao接口：

public interface AuthorDao {
    Author findAuthorById(long id);
    Book findBookByAuthorId(long authorId);
}

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public interface AuthorDao {

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    Author findAuthorById(long id);

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    Book findBookByAuthorId(long authorId);

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}

mapper.xml：

<mapper namespace="dulk.learn.mybatis.dao.AuthorDao">

    <resultMap id="authorResultMap" type="dulk.learn.mybatis.pojo.Author">
        <id property="id" column="id"/>
        <result property="name" column="name" />
        <result property="age" column="age" />
        <!--采用select形式，设定fetchType延迟加载，column为关联查询的列-->
        <collection fetchType="lazy" property="bookList" column="id" select="findBookByAuthorId" />
    </resultMap>

    <select id="findAuthorById" parameterType="long" resultMap="authorResultMap">
        SELECT *
        FROM author
        WHERE id = #{id}
    </select>

    <select id="findBookByAuthorId" parameterType="long" resultType="dulk.learn.mybatis.pojo.Book">
        SELECT *
        FROM book
        WHERE author_id = #{authorId}
    </select>

</mapper>

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<mapper namespace="dulk.learn.mybatis.dao.AuthorDao">

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    <resultMap id="authorResultMap" type="dulk.learn.mybatis.pojo.Author">

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        <id property="id" column="id"/>

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        <result property="name" column="name" />

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        <result property="age" column="age" />

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        <!--采用select形式，设定fetchType延迟加载，column为关联查询的列-->

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        <collection fetchType="lazy" property="bookList" column="id" select="findBookByAuthorId" />

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    </resultMap>

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    <select id="findAuthorById" parameterType="long" resultMap="authorResultMap">

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        SELECT *

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        FROM author

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        WHERE id = #{id}

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    </select>

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    <select id="findBookByAuthorId" parameterType="long" resultType="dulk.learn.mybatis.pojo.Book">

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        SELECT *

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        FROM book

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        WHERE author_id = #{authorId}

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    </select>

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</mapper>

• 使用select方式加载内联属性的数据（而非联表查询）
• 设置全局延迟加载，或者局部设置 fetchType="lazy"
• column列为关联查询的列

写个单元测试看看：

public class TestAuthor {

    @org.junit.Test
    public void testMyBatis() throws IOException {
        //读取配置文件
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        //获取工厂类
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
        //获取SqlSession数据库会话对象
        SqlSession sqlSession = factory.openSession();
        //获取Dao
        AuthorDao authorDao = sqlSession.getMapper(AuthorDao.class);

        Author author = authorDao.findAuthorById(2);
        System.out.println("-------------------------------------");
        List<Book> bookList = author.getBookList();
    }

}

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public class TestAuthor {

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    @org.junit.Test

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    public void testMyBatis() throws IOException {

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        //读取配置文件

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        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");

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        //获取工厂类

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        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);

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        //获取SqlSession数据库会话对象

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        SqlSession sqlSession = factory.openSession();

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        //获取Dao

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        AuthorDao authorDao = sqlSession.getMapper(AuthorDao.class);

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        Author author = authorDao.findAuthorById(2);

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        System.out.println("-------------------------------------");

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        List<Book> bookList = author.getBookList();

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    }

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}

以下输出日志，能够看到bookList在调用时才执行了SQL语句去查询：

Opening JDBC Connection
Created connection 1685232414.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@64729b1e]
==>  Preparing: SELECT * FROM author WHERE id = ? 
==> Parameters: 2(Long)
<==    Columns: id, name, age
<==        Row: 2, 李四, 18
<==      Total: 1
-------------------------------------
==>  Preparing: SELECT * FROM book WHERE author_id = ? 
==> Parameters: 2(Long)
<==    Columns: id, name, price, author_id
<==        Row: 1, 小帽子历险记, 23, 2
<==        Row: 2, 红帽子探险记, 12, 2
<==      Total: 2

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Opening JDBC Connection

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Created connection 1685232414.

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Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@64729b1e]

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==> Preparing: SELECT * FROM author WHERE id = ? 

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==> Parameters: 2(Long)

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<==   Columns: id, name, age

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<==       Row: 2, 李四, 18

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<==     Total: 1

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

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==> Preparing: SELECT * FROM book WHERE author_id = ? 

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==> Parameters: 2(Long)

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<==   Columns: id, name, price, author_id

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<==       Row: 1, 小帽子历险记, 23, 2

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<==       Row: 2, 红帽子探险记, 12, 2

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<==     Total: 2