Hibernate抓取策略一

最新推荐文章于 2020-09-02 17:44:20 发布

原创最新推荐文章于 2020-09-02 17:44:20 发布 · 121 阅读

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#Hibernate #SQL #XML #Cache #.net

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本文介绍Hibernate中如何通过设置抓取策略(fetch strategy)减少多表关联查询时的SQL语句数量，从而提升查询效率。文章提供了具体的代码示例，展示了在一对多关系中如何在Classes与Student实体间应用抓取策略。

抓取策略指多表关联查询的时候，Hibernate会发出多条sql进行查询，但如果设置了抓取策略，将多条SQL变成一条SQL执行，实际上就是使用join等关联查询。

如在一对多即示例Classes与Student的关联查询中：

代码：

public void testLoad(){
		Session session = null;
		try {
		
			session = HibernateUtil.getSession();
			session.beginTransaction();
			//从classes得到student
			Classes cls = (Classes)session.load(Classes.class, 1);
			System.out.println(cls.getName());
			Set set = cls.getStudents();
			for (Iterator iterator = set.iterator(); iterator.hasNext();) {
				Student object = (Student) iterator.next();
				System.out.println(object.getName());
				
			}
			
			HibernateUtil.commit(session);
		} catch (Exception e) {
			HibernateUtil.roolback(session);
		}finally{
			HibernateUtil.close(session);
		}
	}

可以看到上面要打印出classes.name和classes.student.name那么默认情况下输出的SQL是：

Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from t_classes classes0_ where classes0_.id=?
classes1
Hibernate: select students0_.class_id as class3_0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from t_student students0_ where students0_.class_id=?
stu4
stu2
stu1
stu0
stu3

而我们可以使用抓取策略：

更改Classes.hbm.xml.在set中增加fetch属性

<hibernate-mapping package="com.lwf.hibernate.pojo">
	<class name="Classes" table="t_classes">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<set name="students" fetch="join">
			<key column="class_id"/>
			<one-to-many class="Student"/>
		</set>
	</class>
</hibernate-mapping>

更改完后再测试上面的代码，只发出一条sql，结果为：

Hibernate: select classes0_.id as id0_1_, classes0_.name as name0_1_, students1_.class_id as class3_0_3_, students1_.id as id3_, students1_.id as id1_0_, students1_.name as name1_0_, students1_.class_id as class3_1_0_ from t_classes classes0_ left outer join t_student students1_ on classes0_.id=students1_.class_id where classes0_.id=?
classes1
stu2
stu4
stu1
stu3
stu0

显然上面比较使用抓取策略后执行的SQL语句少了，性能也跟着提高了。。

上面我们是从Classes端得到Student所以在Classes.hbm.xml中的set里面加了fetch属性。

那么如果我们从Student端得到Classes要使用fetch怎么做呢？

测试方法：

public void testLoad1(){
		Session session = null;
		try {
		
			session = HibernateUtil.getSession();
			session.beginTransaction();
			
			Student s = (Student)session.load(Student.class, 1);
			System.out.println(s.getName());
			System.out.println(s.getClasses().getName());
			
			
			HibernateUtil.commit(session);
		} catch (Exception e) {
			HibernateUtil.roolback(session);
		}finally{
			HibernateUtil.close(session);
		}
	}

默认情况下执行：发出两条SQL

Hibernate: select student0_.id as id1_0_, student0_.name as name1_0_, student0_.class_id as class3_1_0_ from t_student student0_ where student0_.id=?
stu0
Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from t_classes classes0_ where classes0_.id=?
classes1

而要执行抓取策略，更改Student.hbm.xml的many-to-one

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.lwf.hibernate.pojo">
	<class name="Student" table="t_student">
	<!-- <cache usage="read-only"/> -->
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<many-to-one name="classes" column="class_id" fetch="join"></many-to-one>
	</class>
</hibernate-mapping>

上面我们增加了fetch="join"属性

结果：

Hibernate: select student0_.id as id1_1_, student0_.name as name1_1_, student0_.class_id as class3_1_1_, classes1_.id as id0_0_, classes1_.name as name0_0_ from t_student student0_ left outer join t_classes classes1_ on student0_.class_id=classes1_.id where student0_.id=?
stu0
classes1

现在只执行一条SQL了。。。

需要提示的是其实默认情况下fetch="select"