[Hibernate]

Hibernate中使用session的load(...)和get(...)区别：

1、load()方法是在返回的类被后期调用时，才会去提交SQL查询数据库。

2、get()方法是在调用get()时就即时提交SQL查询数据库。

3、load()方法如果查找不到数据，会抛出ObjectNotFoundException的异常。

4、get()方法如果查找不到数据，不会抛异常，但会返回一个null对象。

5、load()和get()，都会先到session的缓存中查找，如果找到，直接缓存返回对象，如果找不到才去数据库查找对象。

6、均可以使用session.evict()方法清除缓存。

示例：

package com.my.init;

import org.hibernate.ObjectNotFoundException;
import org.hibernate.Session;
import org.hibernate.Transaction;

import com.my.bean.User;
import com.my.dao.util.HibernateUtil;

public class Test {

    public static void main(String[] args) {
        Session session = HibernateUtil.getSessionFactory().openSession();
        Transaction tx = session.beginTransaction();

        try {
            // ---------------------------------------------------------
            // Load class
            // ---------------------------------------------------------
            User user1 = (User) session.load(User.class, (long) 1);
            System.out.println("Load class");
            System.out.println("User name:" + user1.getUserName());

            // ---------------------------------------------------------
            // Get class
            // ---------------------------------------------------------
            User user2 = (User) session.get(User.class, (long) 2);
            System.out.println("Get class");
            System.out.println("User name:" + user2.getUserName());

            // ---------------------------------------------------------
            // Load cache class
            // ---------------------------------------------------------
            User user3 = (User) session.load(User.class, (long) 1);
            System.out.println("Load cache class");
            System.out.println("User name:" + user3.getUserName());

            // ---------------------------------------------------------
            // Get cache class
            // ---------------------------------------------------------
            User user4 = (User) session.get(User.class, (long) 2);
            System.out.println("Get cache class");
            System.out.println("User name:" + user4.getUserName());

            try {
                // ---------------------------------------------------------
                // Load class throw ObjectNotFoundException
                // ---------------------------------------------------------
                User user5 = (User) session.load(User.class, (long) 3);
                System.out.println("User name:" + user5.getUserName());
            } catch (ObjectNotFoundException e) {
                System.out.println("Load class throw ObjectNotFoundException");
            }

            // ---------------------------------------------------------
            // Get class return null
            // ---------------------------------------------------------
            User user6 = (User) session.get(User.class, (long) 3);
            if (user6 == null) {
                System.out.println("Get class return null");
            }

            // ---------------------------------------------------------
            // Remove User cache
            // ---------------------------------------------------------
            session.evict(user1);
            User user7 = (User) session.load(User.class, (long) 1);
            System.out.println("Load class");
            System.out.println("User name:" + user7.getUserName());

            // ---------------------------------------------------------
            // Remove User cache
            // ---------------------------------------------------------
            session.evict(user2);
            User user8 = (User) session.get(User.class, (long) 2);
            System.out.println("Get class");
            System.out.println("User name:" + user8.getUserName());

            // ---------------------------------------------------------
            // Remove session all cache
            // ---------------------------------------------------------
            session.clear();

            tx.commit();
        } catch (Exception e) {
            tx.rollback();
            e.printStackTrace();
        }

        session.close();
    }

}

上面代码的执行结果：