Hibernate 懒加载

          1）懒加载定义：也就是在用到查询的对象的时候才会执行sql去数据库取数据，如果用不到那就不查了

          2）懒加载作用：可能能够节省查询数据库操作

          3）懒加载原理：也就是使用load方法获得的是类的代理，而不是类本身

          4）使用条件：

               1> 查询的类的lazy属性为true

               2> 查询时候使用的是load方法

               3> 查询的类不是final的（因为final就不能生成子类，所以就不能生成代理类）

          5）实例

public class Employee {
	private Long id;
	private String name;
	private Department department;
        ............
}

  <class name="cn.com.cpf.pojo.Department"   table="t_department" lazy="true">
     <id name="id" column="id">
         <generator class="native"/>
     </id>
     <property name="name"/>
     <set name="employees" inverse="false">
         <key column="departmentId"/>
         <one-to-many class="cn.com.cpf.pojo.Employee" />
     </set>
  </class>

SessionFactory factory = null;
Session session = null;
try 
{
    factory = new Configuration().configure().buildSessionFactory();
    session = factory.openSession();
    Transaction tx = session.beginTransaction();

    Department department = (Department) session.load(Department.class, 2l);

    tx.commit();
}

    结果：因为没有用到Department中任何信息，所以根本没有sql语句

             6）get和load

                  1.load方法是懒加载，在用到类中信息的时候才会去数据库中加载

                  2.get方法是立即执行sql，从数据库中去的数据

                    注意：如果类和其他类有一对多等关联关系，那么其他类是否懒加载，和本类是否懒加载无关系，可以单独设置

                    实例：

 <class name="cn.com.cpf.pojo.Department"   table="t_department" lazy="false">
   <id name="id" column="id">
      <generator class="native"/>
  </id>
  <property name="name"/>
  
  <set name="employees" inverse="false" lazy="true">
      <key column="departmentId"/>
      <one-to-many class="cn.com.cpf.pojo.Employee" />
  </set>
 </class>

factory = new Configuration().configure().buildSessionFactory();
session = factory.openSession();
Transaction tx = session.beginTransaction();

Department department = (Department) session.load(Department.class, 2l);

tx.commit();

      结果为：

Hibernate: select department0_.id as id1_3_0_, department0_.name as name2_3_0_ from t_department department0_ where department0_.id=?

     访问其他类信息：

factory = new Configuration().configure().buildSessionFactory();
session = factory.openSession();
Transaction tx = session.beginTransaction();

Department department = (Department) session.load(Department.class, 2l);
department.getEmployees().size();

     结果为：

Hibernate: select department0_.id as id1_3_0_, department0_.name as name2_3_0_ from t_department department0_ where department0_.id=?
Hibernate: select employees0_.departmentId as departme3_3_0_, employees0_.id as id1_4_0_, employees0_.id as id1_4_1_, employees0_.name as name2_4_1_, employees0_.departmentId as departme3_4_1_ from t_employee employees0_ where employees0_.departmentId=?

             7）本类中其他类的lazy属性还可以设置为extra，也就是根据访问的信息来生成不同的sql，而不是将整个类加载

 <class name="cn.com.cpf.pojo.Department"   table="t_department" lazy="false">
   <id name="id" column="id">
      <generator class="native"/>
  </id>
  <property name="name"/>
  
  <set name="employees" inverse="false" lazy="extra">
      <key column="departmentId"/>
      <one-to-many class="cn.com.cpf.pojo.Employee" />
  </set>
 </class>

factory = new Configuration().configure().buildSessionFactory();
session = factory.openSession();
Transaction tx = session.beginTransaction();

Department department = (Department) session.load(Department.class, 2l);
department.getEmployees().size();

          结果为：

Hibernate: select department0_.id as id1_3_0_, department0_.name as name2_3_0_ from t_department department0_ where department0_.id=?
Hibernate: select count(id) from t_employee where departmentId =?

         因为只是查询数量，所以使用count
                     8） 有可能只有此处需要立即加载，而其他地方都使用懒加载，也就是立即加载懒加载的对象

 <class name="cn.com.cpf.pojo.Department"   table="t_department" lazy="true">
   <id name="id" column="id">
      <generator class="native"/>
  </id>
  <property name="name"/>
  
  <set name="employees" inverse="false" lazy="true">
      <key column="departmentId"/>
      <one-to-many class="cn.com.cpf.pojo.Employee" />
  </set>
 </class>

factory = new Configuration().configure().buildSessionFactory();
session = factory.openSession();
Transaction tx = session.beginTransaction();

Department department = (Department) session.load(Department.class, 2l);
department.getEmployees();

tx.commit();

         结果为：

Hibernate: select department0_.id as id1_3_0_, department0_.name as name2_3_0_ from t_department department0_ where department0_.id=?

factory = new Configuration().configure().buildSessionFactory();
session = factory.openSession();
Transaction tx = session.beginTransaction();

Department department = (Department) session.load(Department.class, 2l);
Hibernate.initialize(department.getEmployees());
tx.commit();

         结果为：

Hibernate: select department0_.id as id1_3_0_, department0_.name as name2_3_0_ from t_department department0_ where department0_.id=?
Hibernate: select employees0_.departmentId as departme3_3_0_, employees0_.id as id1_4_0_, employees0_.id as id1_4_1_, employees0_.name as name2_4_1_, employees0_.departmentId as departme3_4_1_ from t_employee employees0_ where employees0_.departmentId=?

                 9） 懒加载异常

                     1>原因：真正获取数据时，session已经关闭

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

                     2>解决方法：

                          1.不用懒加载

                          2.在session关闭前获得数据

                          3.使用Hibernate.initialize来立即获得需要的对象