hibernate fetch

HQL实现多对多关联查询（无关系表的映射）

1. 应用场景
2. 演示

[一]、应用场景

在用注解的方式（@ManyToMany @JoinTable）实现多对多映射时，并没有具体的多对多关系表的实体类，那么我们如何利用HQL实现关联查询呢？比如：学生和课程表之间的多对多关系。

[二]、演示

1.实体类

IdEntity.java

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package com . micmiu . hibernate . anno . entity ;   import javax . persistence . Column ; import javax . persistence . GeneratedValue ; import javax . persistence . Id ; import javax . persistence . MappedSuperclass ;   /** * 统一定义id的entity基类. * * 基类统一定义id的属性名称、数据类型、列名映射及生成策略. <br> * 子类可重载getId()函数重定义id的列名映射和生成策略. * * @author <a href="http://www.micmiu.com">Michael Sun</a> */ @ MappedSuperclass public abstract class IdEntity {   protected Long id ;   @ Id @ Column ( name = "ID" ) @ GeneratedValue // @GeneratedValue(strategy = GenerationType.SEQUENCE) // @GeneratedValue(generator = "system-uuid") // @GenericGenerator(name = "system-uuid", strategy = "uuid") public Long getId ( ) { return id ; }   public void setId ( Long id ) { this . id = id ; } }

学生实体类：Student.java

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package com . micmiu . hibernate . anno . entity ;   import java . util . Collection ; import java . util . Date ;   import javax . persistence . Column ; import javax . persistence . Entity ; import javax . persistence . JoinColumn ; import javax . persistence . JoinTable ; import javax . persistence . ManyToMany ; import javax . persistence . OrderBy ; import javax . persistence . Table ;   import org . hibernate . annotations . Fetch ; import org . hibernate . annotations . FetchMode ;   /** * * Description: * * @author <a href="http://www.micmiu.com">Michael</a> * @see <a href="http://www.micmiu.com">http://www.micmiu.com</a> * @time Create on 2013-6-6 下午2:40:08 * @version 1.0 */ @ Entity @ Table ( name = "DEMO_T_STUDENT" ) public class Student extends IdEntity {   @ Column ( name = "NAME" ) private String name ;   @ Column ( name = "BIRTHDAY" ) private Date birthday ;   private Collection & lt ; Course & gt ; courses ;   public String getName ( ) { return name ; }   public void setName ( String name ) { this . name = name ; }   public Date getBirthday ( ) { return birthday ; }   public void setBirthday ( Date birthday ) { this . birthday = birthday ; }   @ ManyToMany @ JoinTable ( name = "DEMO_R_S2C" , joinColumns = { @ JoinColumn ( name = "SID" ) } , inverseJoinColumns = { @ JoinColumn ( name = "CID" ) } ) @ Fetch ( FetchMode . SUBSELECT ) @ OrderBy ( "id" ) public Collection & lt ; Course & gt ; getCourses ( ) { return courses ; }   public void setCourses ( Collection & lt ; Course & gt ; courses ) { this . courses = courses ; }   @ Override public String toString ( ) { return "Student [name=" + name + ", birthday=" + birthday + "]" ; }   }

课程实体类：Course.java

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package com . micmiu . hibernate . anno . entity ;   import javax . persistence . Column ; import javax . persistence . Entity ; import javax . persistence . Table ;   /** * * Description: * * @author <a href="http://www.micmiu.com">Michael</a> * @see <a href="http://www.micmiu.com">http://www.micmiu.com</a> * @time Create on 2013-6-6 下午2:52:31 * @version 1.0 */ @ Entity @ Table ( name = "DEMO_T_COURSE" ) public class Course extends IdEntity {   @ Column ( name = "NAME" ) private String name ;   public String getName ( ) { return name ; }   public void setName ( String name ) { this . name = name ; }   @ Override public String toString ( ) { return "Course [name=" + name + "]" ; }   }

2.初始化数据：

利用dbunit初始化演示数据：

XHTML

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<? xml version = '1.0' encoding = "utf-8" ?> <dataset> <demo_t_student id = "101" name = "micmiu.com" birthday = "2012-12-12" /> <demo_t_student id = "102" name = "[email protected]" birthday = "2008-08-08" /> <demo_t_student id = "103" name = "ctosun.com" birthday = "2013-01-04" />   <demo_t_course id = "901" name = "chinese" /> <demo_t_course id = "902" name = "english" /> <demo_t_course id = "903" name = "math" />   <demo_r_s2c sid = "101" cid = "901" /> <demo_r_s2c sid = "101" cid = "902" /> <demo_r_s2c sid = "101" cid = "903" /> <demo_r_s2c sid = "102" cid = "902" /> <demo_r_s2c sid = "102" cid = "903" /> <demo_r_s2c sid = "103" cid = "903" />   </dataset>

3.演示关联查询

在Student中配置了多对多关系，在Course没有配置多对多关系，下面将分别演示关联查询这两个实体的方法：

测试基础类 HibernateBaseTest.java

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package com . micmiu . hibernate ;   import org . hibernate . SessionFactory ; import org . hibernate . cfg . Configuration ; import org . hibernate . service . ServiceRegistry ; import org . hibernate . service . ServiceRegistryBuilder ; import org . junit . AfterClass ; import org . junit . BeforeClass ; import org . junit . Test ;   /** * * Description: * * @author <a href="http://www.micmiu.com">Michael</a> * @see <a href="http://www.micmiu.com">http://www.micmiu.com</a> * @time Create on 2013-6-6 下午7:31:02 * @version 1.0 */ public abstract class HibernateBaseTest {   protected static SessionFactory sessionFactory ;   @ BeforeClass public static void beforeClass ( ) { Configuration configuration = new Configuration ( ) . configure ( ) ; ServiceRegistry serviceRegistry = new ServiceRegistryBuilder ( ) . applySettings ( configuration . getProperties ( ) ) . buildServiceRegistry ( ) ; sessionFactory = configuration . buildSessionFactory ( serviceRegistry ) ;   }   @ AfterClass public static void afterClass ( ) { sessionFactory . close ( ) ; }   @ Test public abstract void testMethod ( ) ;   }

测试HQL查询：ManyToManyTest.java

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package com . micmiu . hibernate ;   import java . util . List ;   import org . hibernate . Query ; import org . hibernate . Session ; import org . junit . Assert ; import org . junit . Test ;   import com . micmiu . hibernate . anno . entity . Course ; import com . micmiu . hibernate . anno . entity . Student ;   /** * * Description:测试 注解配置多对多关系的表之间的联合查询 * * @author <a href="http://www.micmiu.com">Michael</a> * @see <a href="http://www.micmiu.com">http://www.micmiu.com</a> * @time Create on 2013-6-7 上午11:38:49 * @version 1.0 */ public class ManyToManyTest extends HibernateBaseTest {   @ Override @ Test public void testMethod ( ) { // 测试查询有配置多对多关系的对象 test1 ( ) ; // 测试查询无配置多对多关系的对象 test2 ( ) ;   }   /** * 测试查询有配置多对多关系的对象 */ @ SuppressWarnings ( "unchecked" ) public void test1 ( ) { Session session = sessionFactory . openSession ( ) ;   session . beginTransaction ( ) ; String hql = "select s from Student s join s.courses c where s.name like '%micmiu.com%' and c.name ='math'" ; String hql2 = "select s from Student s,Course c where c.id in elements (s.courses) and s.name like '%micmiu.com%' and c.name ='math'" ; Query query = session . createQuery ( hql ) ;   List & lt ; Student & gt ; list = query . list ( ) ; System . out . println ( "----------- size:" + list . size ( ) ) ; Assert . assertEquals ( 1 , list . size ( ) ) ;   for ( Student s : list ) { Assert . assertEquals ( "micmiu.com" , s . getName ( ) ) ; System . out . println ( s ) ; }   session . getTransaction ( ) . commit ( ) ; session . close ( ) ; }   /** * 测试查询无配置多对多关系的对象 */ @ SuppressWarnings ( "unchecked" ) public void test2 ( ) { Session session = sessionFactory . openSession ( ) ;   session . beginTransaction ( ) ; String hql = "select distinct c from Student s,Course c where c.id in elements (s.courses) and s.name like '%micmiu.com%' and c.name ='math'" ;   Query query = session . createQuery ( hql ) ;   List & lt ; Course & gt ; list = query . list ( ) ; System . out . println ( "----------- size:" + list . size ( ) ) ; Assert . assertEquals ( 1 , list . size ( ) ) ; for ( Course c : list ) { Assert . assertEquals ( "math" , c . getName ( ) ) ; System . out . println ( c ) ; }   session . getTransaction ( ) . commit ( ) ; session . close ( ) ; } }

在我们查询Parent对象的时候，默认只有Parent的内容，并不包含childs的信息，如果在Parent.hbm.xml里设置lazy="false"的话才同时取出关联的所有childs内容.
     问题是我既想要hibernate默认的性能又想要临时的灵活性该怎么办？  这就是fetch的功能。我们可以把fetch与lazy="true"的关系类比为事务当中的编程式事务与声明式事务,不太准确，但是大概是这个意思。
总值，fetch就是在代码这一层给你一个主动抓取得机会.

 Parent parent = (Parent)hibernateTemplate.execute(new HibernateCallback() {
 public Object doInHibernate(Session session) throws HibernateException, SQLException {
                 Query q = session.createQuery(
 "from Parent as parent "+
 " left outer join fetch parent.childs " +
 " where parent.id = :id"
                 );
                 q.setParameter("id",new Long(15));
 return (Parent)q.uniqueResult();
             }
         });
         Assert.assertTrue(parent.getChilds().size() > 0);

    你可以在lazy="true"的情况下把fetch去掉，就会报异常. 当然，如果lazy="false"就不需要fetch了。

 

    fetch设置为lazy，查询Parent对象的时候，默认只有Parent的内容，并不包含child的信息，这样可以避免n+1问题。

    但又想查询parent时把children得到，就可以用fetch。

    fetch主动将child得到并set到children列表中。

    HQL语句：from Parent p left join fetch p.children

 

   fetch关键字强制Hibernate进行集合填充操作

   如:from TUser user inner join  fetch user.addresses

   这里的"fetch"关键字表明TAddress对象读出以后立即填充到对应的TUser对象中。