hibernate之4.延遲載入

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延遲載入:

僅僅有當使用以實體物件的屬性（除主鍵屬性外）時，才會發送查詢語句到資料庫

get不支援延遲載入

@Testpublic void getTest(){Session session=null;Student student=null;try{session=HibernateUtil.openSession();student=(Student) session.get(Student.class, 3);System.out.println("id:"+student.getStudentId());System.out.println("name:"+student.getStudentName());}finally{session.close();}}

結果:

Hibernate: select student0_.student_id as student1_0_0_, student0_.student_name as student2_0_0_, student0_.age as age0_0_ from t_student student0_ where student0_.student_id=?id:3name:ddddd

在調用get方法時，就已經發出查詢語句

load支援延遲載入

@Testpublic void loadTest(){Session session=null;Student student=null;try{session=HibernateUtil.openSession();student=(Student) session.load(Student.class, 3);System.out.println("id:"+student.getStudentId());System.out.println("name:"+student.getStudentName());}finally{session.close();}}

結果:

id:3Hibernate: select student0_.student_id as student1_0_0_, student0_.student_name as student2_0_0_, student0_.age as age0_0_ from t_student student0_ where student0_.student_id=?name:ddddd

在調用load方法，返回一個代理對象,此時並沒有發出查詢語句,當須要使用studentName屬性時，再發出查詢語句

關閉session後

get:

@Testpublic void get2Test(){Session session=null;Student student=null;try{session=HibernateUtil.openSession();student=(Student) session.get(Student.class, 3);}finally{session.close();}System.out.println("id:"+student.getStudentId());System.out.println("name:"+student.getStudentName());}

結果:

Hibernate: select student0_.student_id as student1_0_0_, student0_.student_name as student2_0_0_, student0_.age as age0_0_ from t_student student0_ where student0_.student_id=?
id:3name:ddddd

與在session範圍中的結果一樣

load

@Testpublic void load2Test(){Session session=null;Student student=null;try{session=HibernateUtil.openSession();student=(Student) session.load(Student.class, 3);}finally{session.close();}System.out.println("id:"+student.getStudentId());System.out.println("name:"+student.getStudentName());}

結果:

id:3org.hibernate.LazyInitializationException: could not initialize proxy - no Session

拋異常了,可見延時載入僅僅在session的範圍內有效

小結:

1.僅僅有當使用以實體物件的屬性（除主鍵屬性外）時，才會發送查詢語句到資料庫

2.延時載入僅僅在session的範圍內有效

hibernate之4.延遲載入