Java笔记之hibernate(十一):get和load的区别

0.说在前面

　　基于Hibernate(十):关系延迟加载项目

1.新建GetAndLoadTest类

package com.hibernate.demo.test;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import org.hibernate.classic.Session;

import com.hibernate.demo.bean.Employee;

public class GetAndLoadTest {

    public static void main(String[] args) {
        //加载配置文件,创建会话工厂对象
        SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
        //创建会话对象
        Session session = sessionFactory.openSession();
        Session session2 = sessionFactory.openSession();
        
        System.out.println("log1");
        //使用get方法获取对象
        Employee employee = (Employee) session.get(Employee.class, 2);
        System.out.println("log2");
        System.out.println(employee.getEmpName());
        System.out.println("log3");
        //使用load方法获取对象
        Employee employee2 = (Employee) session2.load(Employee.class, 2);
        System.out.println("log4");
        System.out.println(employee2.getEmpName());
        System.out.println("log5");
        
        System.out.println("======华丽的分割线======");
        
        Employee employee3 = (Employee) session.get(Employee.class, 100);
        System.out.println(employee3);
        System.out.println("log6");
        Employee employee4= (Employee) session2.load(Employee.class, 100);
        System.out.println(employee4);
        
        //关闭会话对象
        session.close();
        //关闭会话工厂对象
        sessionFactory.close();
    }

}

2.运行GetAndLoadTest类

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
log1
Hibernate: select employee0_.emp_id as emp1_0_0_, employee0_.emp_name as emp2_0_0_, employee0_.dept_id as dept3_0_0_ from t_employee employee0_ where employee0_.emp_id=?
Hibernate: select department0_.dept_id as dept1_2_0_, department0_.dept_name as dept2_2_0_ from t_department department0_ where department0_.dept_id=?
Hibernate: select employees0_.dept_id as dept3_2_1_, employees0_.emp_id as emp1_1_, employees0_.emp_id as emp1_0_0_, employees0_.emp_name as emp2_0_0_, employees0_.dept_id as dept3_0_0_ from t_employee employees0_ where employees0_.dept_id=?
log2
李四
log3
log4
Hibernate: select employee0_.emp_id as emp1_0_0_, employee0_.emp_name as emp2_0_0_, employee0_.dept_id as dept3_0_0_ from t_employee employee0_ where employee0_.emp_id=?
Hibernate: select department0_.dept_id as dept1_2_0_, department0_.dept_name as dept2_2_0_ from t_department department0_ where department0_.dept_id=?
Hibernate: select employees0_.dept_id as dept3_2_1_, employees0_.emp_id as emp1_1_, employees0_.emp_id as emp1_0_0_, employees0_.emp_name as emp2_0_0_, employees0_.dept_id as dept3_0_0_ from t_employee employees0_ where employees0_.dept_id=?
李四
log5
======华丽的分割线======
Hibernate: select employee0_.emp_id as emp1_0_0_, employee0_.emp_name as emp2_0_0_, employee0_.dept_id as dept3_0_0_ from t_employee employee0_ where employee0_.emp_id=?
null
log6
Hibernate: select employee0_.emp_id as emp1_0_0_, employee0_.emp_name as emp2_0_0_, employee0_.dept_id as dept3_0_0_ from t_employee employee0_ where employee0_.emp_id=?
Exception in thread "main" org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [com.hibernate.demo.bean.Employee#100]
    at org.hibernate.impl.SessionFactoryImpl$2.handleEntityNotFound(SessionFactoryImpl.java:433)
    at org.hibernate.proxy.AbstractLazyInitializer.checkTargetState(AbstractLazyInitializer.java:189)
    at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:178)
    at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:215)
    at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:190)
    at com.hibernate.demo.bean.Employee_$$_jvst351_0.toString(Employee_$$_jvst351_0.java)
    at java.lang.String.valueOf(String.java:2994)
    at java.io.PrintStream.println(PrintStream.java:821)
    at com.hibernate.demo.test.GetAndLoadTest.main(GetAndLoadTest.java:34)

3.总结

　　(1).从log1与log2之间的信息可以看出,使用get方法查询对象信息,是没有延迟加载的,直接发送SQL语句进行数据库查询;

　　(2).从log3与log4之间可以看出,load方法并没有直接发送SQL语句去数据库查询,而是在log4与log5直接访问对象的属性信息时才发送SQL去数据库查询,是有延迟加载的,也就是属性延迟加载;

　　(3).从log5与log6之间的信息可以看出,get方法在查询不存在的对象信息时返回的是null;

　　(4).从log6之后的信息可以看出,load方法在查询不存在的对象信息时会报异常;