13. Hibernate lazy策略

Hibernate lazy策略可以使用在(hibernate3.0預設為true)：

       * <class>標籤上，可以取值：true/false

              只會影響到類的普通屬性，即使設定為false，類中的集合屬性預設情況下仍然採用延時載入

       * <property>標籤上，可以取值：true/false

              需要類增強工具,通過改變位元組碼來實現，屬性的延時載入，常用於clob和blob大對象的延時載入

       * <set><list>標籤上，可以取值：true/false/extra

       * <one-to-one><many-to-one>單端關聯上，可以取值：false/proxy/noproxy

lazy概念：只有真正使用該對象時，才會建立，對於hibernate而言，真正使用的時候才會發出sql

hibernate支援lazy策略只有在session開啟狀態下有效(OpenSessionInView)

測試案例：

1：

public void testLoad() {

              Session session = null;

              try {

                     session = HibernateUtils.getSession();

                     session.beginTransaction();

                    

                     //不會發出sql

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

                    

                     //不會發出sql

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

                    

                     //會發出sql

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

                    

                     session.getTransaction().commit();

              }catch(Exception e) {

                     e.printStackTrace();

                     session.getTransaction().rollback();

              }finally {

                     HibernateUtils.closeSession(session);

              }

       }           

2.

public void testLoad() {

              Session session = null;

              Group group = null;

              try {

                     session = HibernateUtils.getSession();

                     session.beginTransaction();

                    

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

                    

                     session.getTransaction().commit();

              }catch(Exception e) {

                     e.printStackTrace();

                     session.getTransaction().rollback();

              }finally {

                     HibernateUtils.closeSession(session);

              }

             

                //不能正確輸出，拋出LazyInitializationException 異常，因為session已經關閉

                   //hibernate支援lazy策略只有在session開啟狀態下有效

                   //openSessionInView

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

       }    

3.（Classes和Student的定義見多對多關聯映射）

public void testLoad() {

              Session session = null;

              try {

                     session = HibernateUtils.getSession();

                     session.beginTransaction();

                    

                     //不會發出sql

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

                    

                     //會發出sql

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

                    

                     //不會發出sql，賦值一個persistentSet的代理對象

                     Set students = classes.getStudents();

                    

                     //會發出sql

                     for (Iterator iter=students.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 {

                     HibernateUtils.closeSession(session);

              }