在Hibernate中，Session的get()和load()方法有何不同，附带示例

Hibernate Session提供了从数据库中获取数据的不同方法。其中两个方法是get()和load()。针对这两个方法，还有很多重载方法可以在不同情况下使用。乍一看，get()和load()似乎相似，因为它们都用于从数据库中获取数据，然而它们之间有一些区别。让我们通过一个简单的例子来看一下。

package com.Olivia.hibernate.main;

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

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

public class HibernateGetVsLoad {

	public static void main(String[] args) {
		
		//Prep Work
		SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
		Session session = sessionFactory.openSession();
		Transaction tx = session.beginTransaction();
		
		//Get Example
		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");
		
		//load Example
		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");
		
		//Close resources
		tx.commit();
		sessionFactory.close();
	}
}

当我执行上述代码时，它会产生以下输出结果。

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}

从输出结果来看，很明显get()通过从数据库或Hibernate缓存中获取对象返回该对象，而load()只返回一个可能不存在的对象的引用，只有当您访问对象的其他属性时，它才从数据库或缓存中加载数据。现在让我们尝试获取数据库中不存在的数据。

//Get Example
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();
}

//load Example
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.Olivia.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.Olivia.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.Olivia.hibernate.main.HibernateExample.main(HibernateExample.java:36)

仔细看输出结果，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，因为它在调用时尝试加载数据。而使用load()，我们能够打印出id，但是试图访问其他字段时，它会触发数据库查询，并在给定标识符找不到记录时抛出org.hibernate.ObjectNotFoundException异常。这是一个特定于Hibernate的运行时异常，所以我们不需要显式地捕捉它。让我们也来看一些方法的重载。以上的get()和load()方法也可以按照下面的方式编写。

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

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

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

在LockOptions参数中还有其他方法，但我没用过它们。需要注意的是，我们需要把完整的类名作为参数传递。根据上述解释，get()和load()之间有以下几点区别。

1. get()调用时会立即加载数据，而load()返回一个代理对象，在需要时才加载数据，因此load()更好，因为它支持延迟加载。

1. 由于load()在找不到数据时会抛出异常，我们应该只在确定数据存在时使用它。

当我们想确保数据库中存在数据时，应该使用get()。

关于Hibernate的获取和加载方法就介绍到这里，希望能够解答一些疑问，并帮助你在不同的情况下决定使用哪种方法。