hibernate学习笔记第12讲-lazy策略

Hibernatelazy策略:延迟加载、懒加载。

代理的概念:只有真正使用该对象的时候才会创建。

Lazy在hibernate:在真正使用的时候才会发出sql查询。

实现原理:采用第三方库cglib,对类生成代理类。代理可以控制源对象,并且可以对源对象的功能进行增强。

Cglib和jdk动态代理的区别:

1,cglib能够对类进行代理,jdk动态代理只能对实现了接口的类进行代理。

2,Jdk动态代理类和原类实现相同的接口

3,Cglib 代理类继承原类,生成一个子类。

 

Lazy在hibernate中的使用地方:可以取值true或false

* <class>标签,可以取值true或false。默认为true。

* <property> 标签,可以取值true或false。很少用。

* 集合<set><list>,可以取值true、fasle或extra。默认为true。

* <one-to-one> 、<many-to-one> 单端关联上,可以取值false、proxy或noproxy。默认proxy

Load支持lazy,get不支持lazy。并且Load的时候只会加载普通属性,集合不会加载

Lazy的生命周期跟session一样,session关闭,lazy也失效。

 

Class上的lazy策略:影响的范围是普通属性的加载,对集合不起作用。

session.beginTransaction();

                            //不会发出sql

                            Group group =(Group)session.load(Group.class, 1);

                            //不会发出sql

                            System.out.println("group.id="+ group.getId());

                            System.out.println("group.name="+ group.getName());

Group group = null;

                   try {

                            session =HibernateUtils.getSession();

                            session.beginTransaction();

                            group =(Group)session.load(Group.class, 1);

                            session.getTransaction().commit();

                   }catch(Exceptione) {

                            e.printStackTrace();

                            session.getTransaction().rollback();

                   }finally {

                            HibernateUtils.closeSession(session);

                   }

                  

                  //不能正确输出,抛出LazyInitializationException 异常,因为session已经关闭

                   //hibernate支持lazy策略只有在session打开状态下有效

                   System.out.println("group.name="+ group.getName());

集合上的lazy:建议使用extra,因为会发出智能sql

session.beginTransaction();

                            //不会发出sql

                            Classes classes = (Classes)session.load(Classes.class,1);

                            //会发出sql

                            System.out.println("classes.name="+ classes.getName());

                            Set students =classes.getStudents();

                            //会发出sql

                            for (Iteratoriter=students.iterator(); iter.hasNext();) {

                                     Student student= (Student)iter.next();

                                     System.out.println("student.name="+ student.getName());

                            session.getTransaction().commit();

设置<class>标签上的lazy=false

session.beginTransaction();     

                            //会发出sql

                            Classes classes =(Classes)session.load(Classes.class, 1);

                            //不会发出sql

                            System.out.println("classes.name="+ classes.getName());

                            Set students =classes.getStudents();

                            //会发出sql

                            for (Iteratoriter=students.iterator(); iter.hasNext();) {

                                     Studentstudent = (Student)iter.next();

                                     System.out.println("student.name="+ student.getName());

                            session.getTransaction().commit();

设置集合上的lazy=false,默认为true。

session.beginTransaction();

                            //不会发出sql

                            Classes classes =(Classes)session.load(Classes.class, 1);

                            //会发出sql,会发出两条sql分别加载Classes和Student

                            System.out.println("classes.name="+ classes.getName());

                            Set students =classes.getStudents();

                            //不会发出sql

                            for (Iteratoriter=students.iterator(); iter.hasNext();) {

                                     Studentstudent = (Student)iter.next();

                                     System.out.println("student.name="+ student.getName());

                            session.getTransaction().commit();

设置集合上的lazy=extra

1,跟lazy=true效果一样。

session.beginTransaction();

                            //不会发出sql

                            Classes classes =(Classes)session.load(Classes.class, 1);

                            //会发出sql

                            System.out.println("classes.name="+ classes.getName());

                            Set students =classes.getStudents();

                            //会发出sql

                            for (Iteratoriter=students.iterator(); iter.hasNext();) {

                                     Studentstudent = (Student)iter.next();

                                     System.out.println("student.name="+ student.getName());

                            session.getTransaction().commit();

2,发出智能sql

session.beginTransaction();

                            //不会发出sql

                            Classes classes =(Classes)session.load(Classes.class, 1);

                            //会发出sql

                            System.out.println("classes.name="+ classes.getName());

                            Set students =classes.getStudents();

                            //会发出sql,发出一条比较智能的sql,直接发出count()sql

                            System.out.println("student.count="+ students.size());

                            session.getTransaction().commit();

单端关联上的lazy:可以取值false,proxy,noproxy,默认proxy,noproxy采用字节码,需要增强工具支持。

单端关联上默认lazy属性:跟集合一样

session.beginTransaction();

                            //不会发出sql

                            Useruser = (User)session.load(User.class, 1);

                            //会发出sql

                            System.out.println("user.name="+ user.getName());

                            Groupgroup = user.getGroup();

                            //会发出sql

                            System.out.println("group.name="+ group.getName());

单端关联上lazy=false:跟集合一样

session.beginTransaction();

                            //不会发出sql

                            User user =(User)session.load(User.class, 1);

                            //会发出sql,发出两条sql分别加载User和Group

                            System.out.println("user.name="+ user.getName());

                            Group group =user.getGroup();

                            //不会发出sql

                            System.out.println("group.name="+ group.getName());

class上lazy=fasle,单端关联上lazy=proxy

class上的lazy 不会影响到单端关联上的lazy。

                           session.beginTransaction();

                            //会发出sql

                            User user =(User)session.load(User.class, 1);

                            //不会发出sql

                            System.out.println("user.name="+ user.getName());

                            Group group =user.getGroup();

                            //会发出sql

                            System.out.println("group.name="+ group.getName());

 

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