mybatis多表关联查询 - N+1次查询+延迟加载

•  对象一对一关联

实体类：

public class Student {
	private int id;
	private String name;
	private int cid;
	private Classroom classroom;
        //略
}

public class Classroom {
	private int id;
	private String name;
        //略
}

ClassroomMapper.xml

	<select id="selById" resultType="Classroom" parameterType="int">
		select * from classroom where id=#{0}
	</select>

StudentMapper.xml

<resultMap type="Student" id="myresultmap">
		<id column="id" property="id"/>
		<result column="name" property="name"/>
		<result column="cid" property="cid"/>
		<!-- 在最终返回Student对象时,同时帮助去查询以下所关联的对象 -->
		<association property="classroom" select="a.b.selById" column="cid"></association>
</resultMap>
	<select id="selAll" resultMap="myresultmap">
		select * from student
	</select>

• 关联集合对象

 实体类：

public class Classroom {
	private int id;
	private String name;
	private List<Student> list;
        //略
}

public class Student {
	private int id;
	private String name;
	private int cid;
        //略
}

StudentMapper.xml

	<select id="selByCid" resultType="Student" parameterType="int">
		select * from student where cid=#{0}
	</select>

ClassroomMapper.xml

	<resultMap type="Classroom" id="myresultmap1">
		<id column="id" property="id"/>
		<result column="name" property="name"/>
		<collection property="list" select="a.c.selByCid" column="id" ></collection>
	</resultMap>
	<select id="selAll3" resultMap="myresultmap1">
		select * from classroom
	</select>

N+1次查询执行sql命令多次，效率低。

解决办法：1.添加缓存；

                2.开启延时加载。

• 延时加载配置：

mybatis默认没有开启延迟加载，需要在SqlMapConfig.xml中setting配置。

在mybatis核心配置文件中配置：lazyLoadingEnabled、aggressiveLazyLoading

设置项	描述	允许值	默认值
lazyLoadingEnabled	全局性设置懒加载。如果设为‘false’，则所有相关联的都会被初始化加载。	true | false	false
aggressiveLazyLoading	当设置为‘true’的时候，懒加载的对象可能被任何懒属性全部加载。否则，每个属性都按需加载。	true | false	true

在SqlMapConfig.xml中配置：

<settings>
    <!-- 打开延迟加载的开关 -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 将积极加载改为消极加载，即延迟加载 -->
    <setting name="aggressiveLazyLoading" value="false"/>
</settings>