Hibernate Session get（）vs load（）的区别

Hibernate Session提供了从数据库中获取数据的不同方法。其中两个是 - get（）和load（）。我们还可以在不同的情况下使用很多重载方法。

首先看两者get()并且load()看起来相似，因为它们都从数据库中获取数据，但它们之间几乎没有差别，让我们用一个简单的例子来看一下它们。


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) {//Prep WorkSessionFactory sessionFactory = HibernateUtil.getSessionFactory();Session session = sessionFactory.openSession();Transaction tx = session.beginTransaction();//Get ExampleEmployee 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 ExampleEmployee 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 resourcestx.commit();sessionFactory.close();}
}

当我执行上面的代码时，它会产生以下输出。


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到，get()通过从数据库或从hibernate缓存中获取对象来返回对象，而load()只返回可能实际不存在的对象的引用，只有在访问对象的其他属性时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();
}

上面的代码产生以下输出。


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)

仔细查看输出，当我们get()用来检索不存在的数据时，它返回null。这是有道理的，因为它试图在调用数据后立即加载数据。

有了load()，我们可以打印id，但是一旦我们尝试访问其他字段，它就会触发数据库查询并org.hibernate.ObjectNotFoundException在没有找到给定标识符的记录时抛出。它是特定于hibernate的运行时异常，所以我们不需要明确地捕获它。

我们来看看一些重载方法。上面的get（）和load（）方法也可以编写如下。


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));

还有其他方法有LockOptions参数，但我没有使用它们。请注意，我们需要传递完整的类名作为参数。

基于以上解释，我们在get（）和load（）之间存在以下差异：

get()一旦调用数据就加载数据，而load()返回代理对象并仅在实际需要时加载数据，因此load()它更好，因为它支持延迟加载。
由于load()在找不到数据时抛出异常，我们应该只在知道数据存在时才使用它。
get()当我们想要确保数据库中存在数据时，我们应该使用它们。

这就是hibernate get和load方法的全部内容，我希望它能解决一些疑问并帮助您决定在不同场景中使用哪一个。

转载来源：https://www.journaldev.com/3472/hibernate-session-get-vs-load-difference-with-examples

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