Hibernate Session get() vs load() 的区别以及示例

Hibernate 会话 提供了从数据库中获取数据的不同方法。其中两种方法是 – get() 和 load()。这两种方法都有很多重载方法，可以根据不同情况使用。乍一看，get() 和 load() 似乎很相似，因为它们都从数据库中获取数据，但是它们之间确实有一些区别，让我们通过一个简单的例子来看看。

package com.journaldev.hibernate.main;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;

import com.journaldev.hibernate.model.Employee;
import com.journaldev.hibernate.util.HibernateUtil;

public class HibernateGetVsLoad {

	public static void main(String[] args) {
		
		//预备工作
		SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
		Session session = sessionFactory.openSession();
		Transaction tx = session.beginTransaction();
		
		//获取示例
		Employee emp = (Employee) session.get(Employee.class, new Long(2));
		System.out.println("Employee get called");
		System.out.println("Employee ID= "+emp.getId());
		System.out.println("Employee Get Details:: "+emp+"\n");
		
		//加载示例
		Employee emp1 = (Employee) session.load(Employee.class, new Long(1));
		System.out.println("Employee load called");
		System.out.println("Employee ID= "+emp1.getId());
		System.out.println("Employee load Details:: "+emp1+"\n");
		
		//关闭资源
		tx.commit();
		sessionFactory.close();
	}
}

当我执行上述代码时，它产生以下输出。

Hibernate: select employee0_.emp_id as emp_id1_1_0_, employee0_.emp_name as emp_name2_1_0_, employee0_.emp_salary as emp_sala3_1_0_, address1_.emp_id as emp_id1_0_1_, address1_.address_line1 as address_2_0_1_, address1_.city as city3_0_1_, address1_.zipcode as zipcode4_0_1_ from EMPLOYEE employee0_ left outer join ADDRESS address1_ on employee0_.emp_id=address1_.emp_id where employee0_.emp_id=?
Employee get called
Employee ID= 2
Employee Get Details:: Id= 2, Name= David, Salary= 200.0, {Address= AddressLine1= Arques Ave, City=Santa Clara, Zipcode=95051}

Employee load called
Employee ID= 1
Hibernate: select employee0_.emp_id as emp_id1_1_0_, employee0_.emp_name as emp_name2_1_0_, employee0_.emp_salary as emp_sala3_1_0_, address1_.emp_id as emp_id1_0_1_, address1_.address_line1 as address_2_0_1_, address1_.city as city3_0_1_, address1_.zipcode as zipcode4_0_1_ from EMPLOYEE employee0_ left outer join ADDRESS address1_ on employee0_.emp_id=address1_.emp_id where employee0_.emp_id=?
Employee load Details:: Id= 1, Name= Pankaj, Salary= 100.0, {Address= AddressLine1= Albany Dr, City=San Jose, Zipcode=95129}

从输出中可以清楚地看出，get() 通过从数据库或 Hibernate 缓存 中获取对象来返回对象，而 load() 只是返回对象的引用，该对象实际上可能不存在，它仅在访问对象的其他属性时从数据库或缓存加载数据。现在让我们尝试获取数据库中不存在的数据。

//获取示例
try{
Employee emp = (Employee) session.get(Employee.class, new Long(200));
System.out.println("Employee get called");
if(emp != null){
System.out.println("Employee GET ID= "+emp.getId());
System.out.println("Employee Get Details:: "+emp+"\n");
}
}catch(Exception e){
	e.printStackTrace();
}

//加载示例
try{
Employee emp1 = (Employee) session.load(Employee.class, new Long(100));
System.out.println("Employee load called");
System.out.println("Employee LOAD ID= "+emp1.getId());
System.out.println("Employee load Details:: "+emp1+"\n");
}catch(Exception e){
	e.printStackTrace();
}

上述代码产生以下输出。

Hibernate: select employee0_.emp_id as emp_id1_1_0_, employee0_.emp_name as emp_name2_1_0_, employee0_.emp_salary as emp_sala3_1_0_, address1_.emp_id as emp_id1_0_1_, address1_.address_line1 as address_2_0_1_, address1_.city as city3_0_1_, address1_.zipcode as zipcode4_0_1_ from EMPLOYEE employee0_ left outer join ADDRESS address1_ on employee0_.emp_id=address1_.emp_id where employee0_.emp_id=?
Employee get called
Employee load called
Employee LOAD ID= 100
Hibernate: select employee0_.emp_id as emp_id1_1_0_, employee0_.emp_name as emp_name2_1_0_, employee0_.emp_salary as emp_sala3_1_0_, address1_.emp_id as emp_id1_0_1_, address1_.address_line1 as address_2_0_1_, address1_.city as city3_0_1_, address1_.zipcode as zipcode4_0_1_ from EMPLOYEE employee0_ left outer join ADDRESS address1_ on employee0_.emp_id=address1_.emp_id where employee0_.emp_id=?
org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [com.journaldev.hibernate.model.Employee#100]
	at org.hibernate.internal.SessionFactoryImpl$1$1.handleEntityNotFound(SessionFactoryImpl.java:253)
	at org.hibernate.proxy.AbstractLazyInitializer.checkTargetState(AbstractLazyInitializer.java:262)
	at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:176)
	at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:286)
	at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:185)
	at com.journaldev.hibernate.model.Employee_$$_jvst407_1.toString(Employee_$$_jvst407_1.java)
	at java.lang.String.valueOf(String.java:2847)
	at java.lang.StringBuilder.append(StringBuilder.java:128)
	at com.journaldev.hibernate.main.HibernateExample.main(HibernateExample.java:36)

看着输出，当我们使用get()来检索不存在的数据时，它会返回null。这是有道理的，因为它尝试在调用时加载数据。使用load()，我们能够打印出ID，但是一旦我们尝试访问其他字段，它会触发数据库查询，并在找不到具有给定标识符的记录时抛出org.hibernate.ObjectNotFoundException。这是Hibernate特定的运行时异常，所以我们不需要显式地捕获它。让我们也看一下一些重载的方法。上面的get()和load()方法也可以写成下面这样。

Employee emp = (Employee) session.get("com.journaldev.hibernate.model.Employee", new Long(2));

Employee emp1 = (Employee) session.load("com.journaldev.hibernate.model.Employee", new Long(1));

Employee emp2 = new Employee();
session.load(emp1, new Long(1));

还有一些带有LockOptions参数的其他方法，但我没有使用它们。请注意，我们需要将完整的类名作为参数传递。根据上述解释，get() vs load()之间有以下差异：

1. get()在调用时加载数据，而load()返回代理对象，并仅在实际需要时加载数据，因此load()更好，因为它支持延迟加载。
2. 由于load()在找不到数据时会抛出异常，我们应该只在知道数据存在时使用它。
3. 当我们想确保数据存在于数据库中时，应该使用get()。

关于Hibernate的get()和load()方法就介绍到这里，希望能够解答一些疑问，并帮助您在不同场景下决定使用哪个。