session get和load方法对比

get测试代码如下：

public class Test {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        ApplicationContext ac=new ClassPathXmlApplicationContext("applicationContext.xml");
        SessionFactory sessionFactory=(SessionFactory) ac.getBean("sessionFactory");
        Session session=sessionFactory.openSession();
    
        User user1=(User) session.get(User.class, 3);
        System.out.println("-----------");
        System.out.println(user1);

    }

}

输出结果是：

Hibernate:
select
user0_.id as id0_0_,
user0_.name as name0_0_
from
mydb.user user0_
where
user0_.id=?
-----------
User [id=3, name=sara3]

load测试代码

public class Test {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        ApplicationContext ac=new ClassPathXmlApplicationContext("applicationContext.xml");
        SessionFactory sessionFactory=(SessionFactory) ac.getBean("sessionFactory");
        Session session=sessionFactory.openSession();

        User user2=(User) session.load(User.class,2);
        System.out.println("********");
        System.out.println(user2);
        session.close();

    }

}

输出结果：

********
Hibernate:
select
user0_.id as id0_0_,
user0_.name as name0_0_
from
mydb.user user0_
where
user0_.id=?
User [id=2, name=sara2]

可以看出get方法直接发出sql语句，从数据库加载,没有延迟。load方法只有真正用到对象的内容时才会发出sql语句

接下来再用两个测试用例

get测试用例

public class Test {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        ApplicationContext ac=new ClassPathXmlApplicationContext("applicationContext.xml");
        SessionFactory sessionFactory=(SessionFactory) ac.getBean("sessionFactory");
        Session session=sessionFactory.openSession();
    
        User user1=(User) session.get(User.class, 3);
        System.out.println("-----------");
        System.out.println(user1.getClass());

        session.close();

    }

}

输出结果是：

Hibernate:
select
user0_.id as id0_0_,
user0_.name as name0_0_
from
mydb.user user0_
where
user0_.id=?
-----------
class com.db.entity.User

load测试代码：

public class Test {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        ApplicationContext ac=new ClassPathXmlApplicationContext("applicationContext.xml");
        SessionFactory sessionFactory=(SessionFactory) ac.getBean("sessionFactory");
        Session session=sessionFactory.openSession();
    
        User user2=(User) session.load(User.class,2);
        System.out.println("********");
        System.out.println(user2.getClass());
        session.close();
    }
}

输出结果是：

********
class com.db.entity.User_$$_javassist_0

结论：

　　get方法会在调用之后立即向数据库发出sql语句（不考虑缓存的情况下），返回持久化对象；而load方法会在调用后返回一个代理对象，该代理对象只保存了实体对象的id，直到使用对象的非主键属性时才会发出sql语句