hibernate检索的使用-优快云博客

本文介绍了Hibernate中懒加载(lazy)与急加载(fetch)的使用方法及效果对比。通过具体的XML配置示例，展示了不同设置下SQL查询的执行情况。

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1.hibernate中检索主要有以下：
   lazy=true|false|extra
   fetch=select subselect join迫切的左外链接
   batch-size 检索的数量

cascade="all" 级联操作

2.第一种lazy的使用：

2.1 Classes.hbm.xml

<set name="stus" lazy="false" >
           <key column="class_id"></key>
           <one-to-many class="Student"/>
        </set>
     </class>
</hibernate-mapping>
2.2 使用lazy="false"时效果如下;
Hibernate:
    select
        student0_.ID as ID1_1_0_,
        student0_.NAME as NAME2_1_0_,
        student0_.class_id as class_id3_1_0_
    from
        STUDENT student0_
    where
        student0_.ID=?
Hibernate:
    select
        classes0_.ID as ID1_0_0_,
        classes0_.NAME as NAME2_0_0_
    from
        CLASSES classes0_
    where
        classes0_.ID=?
Hibernate:
    select
        stus0_.class_id as class_id3_0_1_,
        stus0_.ID as ID1_1_1_,
        stus0_.ID as ID1_1_0_,
        stus0_.NAME as NAME2_1_0_,
        stus0_.class_id as class_id3_1_0_
    from
        STUDENT stus0_
    where
        stus0_.class_id=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]

2.2 使用lazy="true"时，效果如下;

Hibernate:
    select
        student0_.ID as ID1_1_0_,
        student0_.NAME as NAME2_1_0_,
        student0_.class_id as class_id3_1_0_
    from
        STUDENT student0_
    where
        student0_.ID=?
Hibernate:
    select
        classes0_.ID as ID1_0_0_,
        classes0_.NAME as NAME2_0_0_
    from
        CLASSES classes0_
    where
        classes0_.ID=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]

2.3 使用lazy="extra"时，效果如下;

Hibernate:
    select
        student0_.ID as ID1_1_0_,
        student0_.NAME as NAME2_1_0_,
        student0_.class_id as class_id3_1_0_
    from
        STUDENT student0_
    where
        student0_.ID=?
Hibernate:
    select
        classes0_.ID as ID1_0_0_,
        classes0_.NAME as NAME2_0_0_
    from
        CLASSES classes0_
    where
        classes0_.ID=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]

3 第二种 fetch的使用

3.1 Classes.hbm.xml

3.2 fetch="join"时：

Hibernate:
    select
        student0_.ID as ID1_1_0_,
        student0_.NAME as NAME2_1_0_,
        student0_.class_id as class_id3_1_0_
    from
        STUDENT student0_
    where
        student0_.ID=?
Hibernate:
    select
        classes0_.ID as ID1_0_1_,
        classes0_.NAME as NAME2_0_1_,
        stus1_.class_id as class_id3_0_3_,
        stus1_.ID as ID1_1_3_,
        stus1_.ID as ID1_1_0_,
        stus1_.NAME as NAME2_1_0_,
        stus1_.class_id as class_id3_1_0_
    from
        CLASSES classes0_
    left outer join
        STUDENT stus1_
            on classes0_.ID=stus1_.class_id
    where
        classes0_.ID=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]

3.2 fetch="select"时：

ibernate:
    select
        student0_.ID as ID1_1_0_,
        student0_.NAME as NAME2_1_0_,
        student0_.class_id as class_id3_1_0_
    from
        STUDENT student0_
    where
        student0_.ID=?
Hibernate:
    select
        classes0_.ID as ID1_0_0_,
        classes0_.NAME as NAME2_0_0_
    from
        CLASSES classes0_
    where
        classes0_.ID=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]