hibernate之4.延遲加載

延遲加載:

只有當使用以實體對象的屬性（除主鍵屬性外）時，纔會發送查詢語句到數據庫

get不支持延遲加載

@Test
	public 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:3
name:ddddd

在調用get方法時，就已經發出查詢語句

load支持延遲加載

@Test
	public 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:3
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=?
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,當需要使用studentName屬性時，再發出查詢語句

關閉session後

get:

@Test
	public 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:3
name:ddddd

與在session作用域中的結果一樣

load

@Test
	public 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:3
org.hibernate.LazyInitializationException: could not initialize proxy - no Session

拋異常了,可見延時加載只在session的作用域內有效

小結:

1.只有當使用以實體對象的屬性（除主鍵屬性外）時，纔會發送查詢語句到數據庫

2.延時加載只在session的作用域內有效