hibernate抓取策略

抓取策略（fetching strategy）是指：当应用程序需要在（Hibernate实体对象图的）关联关系间进行导航的时候，Hibernate如何获取关联对象的策略。抓取策略可以在O/R映射的元数据中声明，也可以在特定的HQL或条件查询（Criteria Query）中重载声明。

Hibernate定义了如下几种抓取策略：

1、连接抓取（Join fetching）--Hibernate通过在select语句中使用outer join（外链接）来获得对象的关联实例或者关联集合。这时lazy无效。

2、查询抓取（select fetching）--另外发送一条select语句抓取当前对象的关联实体或集合。除非你显示的指定lazy=false禁止延迟抓取，否则只有当你真正访问关联关系的时候，才会执行第二条select语句。

3、子查询抓取（Subselect fetching）--另外发送一条select语句抓取在前面查询到（或者抓取到）的所有实体对象的关联集合。除非你显示的指定lazy=false禁止延迟抓取，否则只有当你真正访问关联关系的时候，才会执行第二条select语句。

4、批量抓取（Batch fetching）--对查询抓取的优化方案，通过指定一个主键或外键列表，Hibernate使用单条select语句获取一批对象实例或集合。

下面我们通过项目案例来简单了解下这几种策略

java项目结构如下：

该项目所有代码跟上一节《hibernate加载策略之lazy》一样，可以参考。

查询抓取（select fetching）

在多的一端设置fetch

Book.hbm.xml配置文件

<hibernate-mapping package="com.test.pojo">      <class name="Book" table="book">          <id name="id">              <generator class="identity" />          </id>          <many-to-one name="category" class="Category" column="category_id"            cascade="save-update" lazy="proxy" fetch="select"/>          <property name="author" />          <property name="name" column="book_name" />          <property name="price" />          <property name="pubDate" />      </class>  </hibernate-mapping>

在HibernateTest类中添加代码

@Testpublic void testLoad1(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();Book book=(Book) session.get(Book.class, 1);System.out.println(book.getName());System.out.println(book.getCategory().getName());tx.commit();HibernateUtil.closeSession();}

断点执行，当执行到

System.out.println(book.getName());

打印sql语句如下

Hibernate:     select        book0_.id as id1_1_0_,        book0_.category_id as category2_1_0_,        book0_.author as author3_1_0_,        book0_.book_name as book_nam4_1_0_,        book0_.price as price5_1_0_,        book0_.pubDate as pubDate6_1_0_     from        book book0_     where        book0_.id=?

当执行到

System.out.println(book.getCategory().getName());

打印sql语句如下

Hibernate:     select        category0_.id as id1_0_0_,        category0_.name as name2_0_0_     from        Category category0_     where        category0_.id=?

当我们把Book.hbm.xml中的lazy改为false，如下

<many-to-one name="category" class="Category" column="category_id"            cascade="save-update" lazy="false" fetch="select"/>

断点执行testLoad1，当执行到

Book book=(Book) session.get(Book.class, 1);

打印sql语句如下

Hibernate:     select        book0_.id as id1_1_0_,        book0_.category_id as category2_1_0_,        book0_.author as author3_1_0_,        book0_.book_name as book_nam4_1_0_,        book0_.price as price5_1_0_,        book0_.pubDate as pubDate6_1_0_     from        book book0_     where        book0_.id=?Hibernate:     select        category0_.id as id1_0_0_,        category0_.name as name2_0_0_     from        Category category0_     where        category0_.id=?

下面我们在一的一端设置fetch

Category.hbm.xml代码

<hibernate-mapping package="com.test.pojo">      <class name="Category" >          <id name="id" >              <generator class="identity" />          </id>          <property name="name" />          <set name="books" inverse="true" lazy="extra" fetch="select">              <key>                  <column name="category_id" />              </key>              <one-to-many class="Book" />          </set>      </class>    </hibernate-mapping>

断点执行HibernateTest类中的testGet方法

@Testpublic void testGet(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();Category category=(Category) session.get(Category.class, 1);System.out.println("分类名:"+category.getName());for(Iterator<Book> iter=category.getBooks().iterator();iter.hasNext();){System.out.println(iter.next().getName());}tx.commit();HibernateUtil.closeSession();}

断点执行到

Category category=(Category) session.get(Category.class, 1);

打印sql语句如下

Hibernate:     select        category0_.id as id1_0_0_,        category0_.name as name2_0_0_     from        Category category0_     where        category0_.id=?

执行到

System.out.println(iter.next().getName());

打印sql语句如下

Hibernate:     select        books0_.category_id as category2_0_0_,        books0_.id as id1_1_0_,        books0_.id as id1_1_1_,        books0_.category_id as category2_1_1_,        books0_.author as author3_1_1_,        books0_.book_name as book_nam4_1_1_,        books0_.price as price5_1_1_,        books0_.pubDate as pubDate6_1_1_     from        book books0_     where        books0_.category_id=?

从上面可以看出来，get方法，当时就查询了Category表的sql语句，当需要查询Book的数据时，才会执行book的数据库表信息。

连接抓取（Join fetching）

先看在多的一端设置fetch

将Book.hbm.xml中的fetch设为join，lazy设为proxy，如下

<many-to-one name="category" class="Category" column="category_id"            cascade="save-update" lazy="proxy" fetch="join"/>

执行testLoad1，打印信息如下：

Hibernate:     select        book0_.id as id1_1_0_,        book0_.category_id as category2_1_0_,        book0_.author as author3_1_0_,        book0_.book_name as book_nam4_1_0_,        book0_.price as price5_1_0_,        book0_.pubDate as pubDate6_1_0_,        category1_.id as id1_0_1_,        category1_.name as name2_0_1_     from        book book0_     left outer join        Category category1_             on book0_.category_id=category1_.id     where        book0_.id=?读者文学

由上面可以看到，在需要book的信息时，sql语句使用 left outer join 把Category表中的信息也查询了出来。

再来从一的一端来查询

将Category.hbm.xml中的fetch设为join，lazy默认为true

<set name="books" inverse="true" fetch="join"> 

执行testLoad代码

@Testpublic void testLoad(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();Category category=(Category) session.get(Category.class, 1);System.out.println("分类名:"+category.getName());System.out.println("对应的书大小："+category.getBooks().size());tx.commit();HibernateUtil.closeSession();}

打印信息如下

Hibernate:     select        category0_.id as id1_0_0_,        category0_.name as name2_0_0_,        books1_.category_id as category2_0_1_,        books1_.id as id1_1_1_,        books1_.id as id1_1_2_,        books1_.category_id as category2_1_2_,        books1_.author as author3_1_2_,        books1_.book_name as book_nam4_1_2_,        books1_.price as price5_1_2_,        books1_.pubDate as pubDate6_1_2_     from        Category category0_     left outer join        book books1_             on category0_.id=books1_.category_id     where        category0_.id=?分类名:文学对应的书大小：1

从一的一端也是使用left outer join方法把book表中的数据一起查询出来了。

接下来我们把Book.hbm.xml和Category.hbm.xml中的lazy和fetch都去掉，执行testLoad2，代码如下

@Testpublic void testLoad2(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();List<Category> list=session.createCriteria(Category.class).list();System.out.println("类型个数："+list.size());for(Category cate:list){System.out.println(cate.getName()+"---书籍本数："+cate.getBooks().size());}tx.commit();HibernateUtil.closeSession();}

打印sql语句如下

Hibernate:     select        this_.id as id1_0_0_,        this_.name as name2_0_0_     from        Category this_类型个数：6Hibernate:     select        books0_.category_id as category2_0_0_,        books0_.id as id1_1_0_,        books0_.id as id1_1_1_,        books0_.category_id as category2_1_1_,        books0_.author as author3_1_1_,        books0_.book_name as book_nam4_1_1_,        books0_.price as price5_1_1_,        books0_.pubDate as pubDate6_1_1_     from        book books0_     where        books0_.category_id=?文学---书籍本数：1Hibernate:     select        books0_.category_id as category2_0_0_,        books0_.id as id1_1_0_,        books0_.id as id1_1_1_,        books0_.category_id as category2_1_1_,        books0_.author as author3_1_1_,        books0_.book_name as book_nam4_1_1_,        books0_.price as price5_1_1_,        books0_.pubDate as pubDate6_1_1_     from        book books0_     where        books0_.category_id=?科幻---书籍本数：1Hibernate:     select        books0_.category_id as category2_0_0_,        books0_.id as id1_1_0_,        books0_.id as id1_1_1_,        books0_.category_id as category2_1_1_,        books0_.author as author3_1_1_,        books0_.book_name as book_nam4_1_1_,        books0_.price as price5_1_1_,        books0_.pubDate as pubDate6_1_1_     from        book books0_     where        books0_.category_id=?言情---书籍本数：1Hibernate:     select        books0_.category_id as category2_0_0_,        books0_.id as id1_1_0_,        books0_.id as id1_1_1_,        books0_.category_id as category2_1_1_,        books0_.author as author3_1_1_,        books0_.book_name as book_nam4_1_1_,        books0_.price as price5_1_1_,        books0_.pubDate as pubDate6_1_1_     from        book books0_     where        books0_.category_id=?历史---书籍本数：1Hibernate:     select        books0_.category_id as category2_0_0_,        books0_.id as id1_1_0_,        books0_.id as id1_1_1_,        books0_.category_id as category2_1_1_,        books0_.author as author3_1_1_,        books0_.book_name as book_nam4_1_1_,        books0_.price as price5_1_1_,        books0_.pubDate as pubDate6_1_1_     from        book books0_     where        books0_.category_id=?漫画---书籍本数：1Hibernate:     select        books0_.category_id as category2_0_0_,        books0_.id as id1_1_0_,        books0_.id as id1_1_1_,        books0_.category_id as category2_1_1_,        books0_.author as author3_1_1_,        books0_.book_name as book_nam4_1_1_,        books0_.price as price5_1_1_,        books0_.pubDate as pubDate6_1_1_     from        book books0_     where        books0_.category_id=?悬疑---书籍本数：1

从上面的sql语句可以看出sql语句重复

子查询抓取（Subselect fetching）

我们在Category.hbm.xml中加上fetch=subselect

<set name="books" inverse="true" fetch="subselect">  

继续执行testLoad2，打印sql语句如下

Hibernate:     select        this_.id as id1_0_0_,        this_.name as name2_0_0_     from        Category this_类型个数：6Hibernate:     select        books0_.category_id as category2_0_1_,        books0_.id as id1_1_1_,        books0_.id as id1_1_0_,        books0_.category_id as category2_1_0_,        books0_.author as author3_1_0_,        books0_.book_name as book_nam4_1_0_,        books0_.price as price5_1_0_,        books0_.pubDate as pubDate6_1_0_     from        book books0_     where        books0_.category_id in (            select                this_.id             from                Category this_        )文学---书籍本数：1科幻---书籍本数：1言情---书籍本数：1历史---书籍本数：1漫画---书籍本数：1悬疑---书籍本数：1

一次性将所有的分类Category的数据全部查询出来。

假设现在我想查询Category的id是1,3,5的数据，那么方法testLoad2的代码如下

@Testpublic void testLoad2(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();List<Category> list=session.createCriteria(Category.class).add(Restrictions.in("id", new Integer[]{1,3,5})).list();System.out.println("类型个数："+list.size());for(Category cate:list){System.out.println(cate.getName()+"---书籍本数："+cate.getBooks().size());}tx.commit();HibernateUtil.closeSession();}

打印sql语句如下

Hibernate:     select        this_.id as id1_0_0_,        this_.name as name2_0_0_     from        Category this_     where        this_.id in (            ?, ?, ?        )类型个数：3Hibernate:     select        books0_.category_id as category2_0_1_,        books0_.id as id1_1_1_,        books0_.id as id1_1_0_,        books0_.category_id as category2_1_0_,        books0_.author as author3_1_0_,        books0_.book_name as book_nam4_1_0_,        books0_.price as price5_1_0_,        books0_.pubDate as pubDate6_1_0_     from        book books0_     where        books0_.category_id in (            select                this_.id             from                Category this_             where                this_.id in (                    ?, ?, ?                )        )文学---书籍本数：1言情---书籍本数：1漫画---书籍本数：1

批量抓取（Batch fetching）

数据量大的时候采用此方法有意义，数据量小时，采用此方法效果不明显。