Hibernate 缓存–一级缓存
欢迎使用Hibernate Caching –一级缓存示例教程。
最近，我们研究了Hibernate体系结构，hibernate映射以及如何使用HQL以面向对象的方式触发SQL查询。
今天，我们将研究Hibernate的重要方面之一-Hibernate Cache。
Hibernate 缓存
如果正确使用Hibernate Cache，在获得快速应用程序性能方面将非常有用。
缓存背后的想法是减少数据库查询的数量，从而减少应用程序的吞吐时间。
Hibernate带有不同类型的缓存：
一级缓存：Hibernate一级缓存与Session对象相关联。
默认情况下，Hibernate一级缓存处于启用状态，无法禁用它。
但是hibernate提供了一些方法，通过这些方法我们可以从缓存中删除选定的对象或者完全清除缓存。
会话中缓存的任何对象对其他会话都不可见，并且当会话关闭时，所有缓存的对象也将丢失。二级缓存：默认情况下，Hibernate 二级缓存是禁用的，但我们可以通过配置启用它。
目前，EHCache和Infinispan为Hibernate Second level缓存提供了实现，我们可以使用它们。
我们将在下一个教程中研究Hibernate 缓存。查询缓存：Hibernate也可以缓存查询结果集。
Hibernate Query Cache不会在缓存中缓存实际实体的状态；它仅缓存标识符值和值类型的结果。
因此，应始终将其与二级缓存结合使用。
Hibernate 缓存–一级缓存示例
对于我的Hibernate 一级缓存示例程序，我使用的配置与HQL示例中的配置相同，您可以签出并配置表，并用伪数据填充表。
首先让我们看一下程序，将其输出，然后我们将介绍与Hibernate First Level Cache相关的一些重要信息。
HibernateCacheExample.java
package com.theitroad.hibernate.main;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import com.theitroad.hibernate.model.Employee;
import com.theitroad.hibernate.util.HibernateUtil;
public class HibernateCacheExample {
public static void main(String[] args) throws InterruptedException {
SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
Session session = sessionFactory.getCurrentSession();
Transaction tx = session.beginTransaction();
//Get employee with id=1
Employee emp = (Employee) session.load(Employee.class, new Long(1));
printData(emp,1);
//waiting for sometime to change the data in backend
Thread.sleep(10000);
//Fetch same data again, check logs that no query fired
Employee emp1 = (Employee) session.load(Employee.class, new Long(1));
printData(emp1,2);
//Create new session
Session newSession = sessionFactory.openSession();
//Get employee with id=1, notice the logs for query
Employee emp2 = (Employee) newSession.load(Employee.class, new Long(1));
printData(emp2,3);
//START: evict example to remove specific object from hibernate first level cache
//Get employee with id=2, first time hence query in logs
Employee emp3 = (Employee) session.load(Employee.class, new Long(2));
printData(emp3,4);
//evict the employee object with id=1
session.evict(emp);
System.out.println("Session Contains Employee with id=1?"+session.contains(emp));
//since object is removed from first level cache, you will see query in logs
Employee emp4 = (Employee) session.load(Employee.class, new Long(1));
printData(emp4,5);
//this object is still present, so you won't see query in logs
Employee emp5 = (Employee) session.load(Employee.class, new Long(2));
printData(emp5,6);
//END: evict example
//START: clear example to remove everything from first level cache
session.clear();
Employee emp6 = (Employee) session.load(Employee.class, new Long(1));
printData(emp6,7);
Employee emp7 = (Employee) session.load(Employee.class, new Long(2));
printData(emp7,8);
System.out.println("Session Contains Employee with id=2?"+session.contains(emp7));
tx.commit();
sessionFactory.close();
}
private static void printData(Employee emp, int count) {
System.out.println(count+":: Name="+emp.getName()+", Zipcode="+emp.getAddress().getZipcode());
}
}
当我们在上面的示例中运行时，输出包含很多与Hibernate 相关的信息。
但是我们对特定于代码的输出以及由hibernate触发以加载数据的查询最感兴趣。
输出代码段如下所示。
Hibernate Configuration loaded Hibernate serviceRegistry created 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=? 1:: Name=hyman, Zipcode=95129 2:: Name=hyman, Zipcode=95129 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=? 3:: Name=hymanK, Zipcode=95129 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=? 4:: Name=David, Zipcode=95051 Session Contains Employee with id=1?false 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=? 5:: Name=hyman, Zipcode=95129 6:: Name=David, Zipcode=95051 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=? 7:: Name=hyman, Zipcode=95129 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=? 8:: Name=David, Zipcode=95051 Session Contains Employee with id=2?true
Hibernate 重要点中的一级缓存
可以从上述程序派生的有关Hibernate中一级缓存的要点是：
默认情况下，Hibernate First Level缓存是启用的，不需要任何配置。
Hibernate 一级缓存是特定于会话的，这就是为什么当我们在同一会话中获取相同的数据时，不会触发查询，而在其他会话中，会触发查询来加载数据。
Hibernate一级缓存可能具有旧值，如上所示，我已使程序进入睡眠状态10秒钟，在那段时间内，我更新了数据库中的值(名称从hyman改为hymanK)，但未其中反映出来。
同一会话。
但是在其他会话中，我们获得了更新的值。我们可以使用session evict()方法从Hibernate 的一级缓存中删除单个对象。
我们可以使用session
clear()方法清除缓存，即从缓存中删除所有对象。我们可以使用session
contains()方法来检查Hibernate 缓存中是否存在对象，如果在缓存中找到该对象，则返回true，否则返回false。由于Hibernate 将所有对象缓存到会话一级缓存中，因此在运行批量查询或者批处理更新时，必须按一定的时间间隔清除缓存以避免内存问题。
