hibernate的抓取策略

hibernate抓取策略（单端代理的批量抓取）

保持默认，同fetch="select",如：
<many-to-one name="classes" column="classesid" fetch="select"/>
fetch="select",另外发送一条select语句抓取当前对象关联实体或集合

//fetch1,fetch2public void testFetch1() {Session session = null;try {session = HibernateSessionFactory.getSession();session.beginTransaction();Student student = (Student)session.load(Student.class, 1);System.out.println("student.name=" + student.getName());System.out.println("classes.name=" + student.getClasses().getName());session.getTransaction().commit();}catch(Exception e) {e.printStackTrace();session.getTransaction().rollback();}finally {HibernateSessionFactory.closeSession(session);}}

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设置fetch="join",如：
<many-to-one name="classes" column="classesid" fetch="join"/>
fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合
此时lazy会失效，同上

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保持默认，同fetch="select",如：
<set name="students" inverse="true" cascade="all" fetch="select">
fetch="select",另外发送一条select语句抓取当前对象关联实体或集合

//fetch3,fetch4public void testFetch2() {Session session = null;try {session = HibernateSessionFactory.getSession();session.beginTransaction();Classes classes = (Classes)session.load(Classes.class, 1);System.out.println("classes.name=" + classes.getName());for (Iterator iter=classes.getStudents().iterator(); iter.hasNext();) {Student student = (Student)iter.next();System.out.println("student.name=" + student.getName());}session.getTransaction().commit();}catch(Exception e) {e.printStackTrace();session.getTransaction().rollback();}finally {HibernateSessionFactory.closeSession(session);}}

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设置fetch="join",如：
<set name="students" inverse="true" cascade="all" fetch="join">
fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合
此时lazy会失效

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设置fetch="subselect",如：
<set name="students" inverse="true" cascade="all" fetch="subselect">
fetch="subselect",另外发送一条select语句抓取在前面查询到的所有实体对象的关联集合

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//fetch5public void testFetch3() {Session session = null;try {session = HibernateSessionFactory.getSession();session.beginTransaction();List classesList = session.createQuery("select c from Classes c where c.id in(1, 2, 3)").list();for (Iterator iter=classesList.iterator(); iter.hasNext();) {Classes classes = (Classes)iter.next();System.out.println("classes.name=" + classes.getName());for (Iterator iter1=classes.getStudents().iterator(); iter1.hasNext();) {Student student = (Student)iter1.next();System.out.println("student.name=" + student.getName());}}session.getTransaction().commit();}catch(Exception e) {e.printStackTrace();session.getTransaction().rollback();}finally {HibernateSessionFactory.closeSession(session);}}

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batch-size属性，可以批量加载实体类，参见：Classes.hbm.xml
<class name="Classes" table="t_classes" batch-size="3">

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List students = session.createQuery("select s from Student s where s.id in(:ids)").setParameterList("ids", new Object[]{1, 11, 21, 31, 41, 51, 61, 71, 81, 91}).list();for (Iterator iter=students.iterator(); iter.hasNext();) {Student  student = (Student)iter.next();System.out.println("student.name=" + student.getName());System.out.println("classes.name=" + student.getClasses().getName());}

batch-size属性，可以批量加载实体类，参见：Classes.hbm.xml
<set name="students" inverse="true" cascade="all" batch-size="5">

List classesList = session.createQuery("select c from Classes c").list();for (Iterator iter=classesList.iterator(); iter.hasNext();) {Classes classes = (Classes)iter.next();System.out.println("classes.name=" + classes.getName());for (Iterator iter1=classes.getStudents().iterator(); iter1.hasNext();) {Student student = (Student)iter1.next();System.out.println("student.name=" + student.getName());}}

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