Second Level Cache In Hibernate

Second Level Cache:

First level cache will be enabled by default, but for enable second level cache we need to follow some settings, let us see few points regarding this..

·         Second level cache was introduced in hibernate 3.0

·         Whenever we are loading any object from the database,  then hibernate verify whether that object is available in the local cache memory of that particular session [ means first level cache ], if not available then hibernate verify whether the object is available in global cache or factory cache [second level cache ], if not available then hibernate will hit the database and loads the object from there, and then first stores in the local cache of the session [ first level ] then in the global cache [ second level cache ]

·         When another session need to load the same object from the database,  then hibernate copies that object from global cache [ second level cache ] into the local cache of this new session

Second level cache in the hibernate is of  from 4 vendors…

·         Easy Hibernate [EHCache] Cache from hibernate framework.

·         Open Symphony [OS] cache from Open Symphony.

·         SwarmCache.

·         TreeCache from JBoss.

How to enable second level cache in hibernate

We need one provider class, here we are going to see hibernate provider class that is EHCache.

Example of Second Level Cache using EH Cache

To understand the second level cache through example, we need to create following pages:

1. Employee.java
2. employee.hbm.xml
3. hibernate.cfg.xml
4. ehcache.xml
5. FetchTest.java

Here, we are assuming, there is emp1012 table in the oracle database containing some records.

File: Employee.java

1.    package com.javatpoint;  

2.      

3.    public class Employee {  

4.    private int id;  

5.    private String name;  

6.    private float salary;  

7.      

8.    public Employee() {}  

9.    public Employee(String name, float salary) {  

10.     super();  

11.     this.name = name;  

12.     this.salary = salary;  

13. }  

14. //setters and getters  

15. }  

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File: employee.hbm.xml

1.    <?xml version='1.0' encoding='UTF-8'?>  

2.    <!DOCTYPE hibernate-mapping PUBLIC  

3.              "-//Hibernate/Hibernate Mapping DTD 3.0//EN"  

4.              "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">  

5.      

6.              <hibernate-mapping>  

7.              <class name="com.javatpoint.Employee" table="emp1012">  

8.              <cache usage="read-only" />  

9.              <id name="id">  

10.           <generator class="native"></generator>  

11.           </id>  

12.           <property name="name"></property>  

13.           <property name="salary"></property>  

14.           </class>  

15.             

16.           </hibernate-mapping>  

Here, we are using read-only cache usage for the class. The cache usage can also be used in collection.

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File: hibernate.cfg.xml

1.    <?xml version='1.0' encoding='UTF-8'?>  

2.    <!DOCTYPE hibernate-configuration PUBLIC  

3.              "-//Hibernate/Hibernate Configuration DTD 3.0//EN"  

4.              "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">  

5.      

6.    <!-- Generated by MyEclipse Hibernate Tools.                   -->  

7.    <hibernate-configuration>  

8.      

9.        <session-factory>  

10.         <property name="show_sql">true</property>  

11.         <property name="hbm2ddl.auto">update</property>  

12.         <property name="dialect">org.hibernate.dialect.Oracle9Dialect</property>  

13.         <property name="connection.url">jdbc:oracle:thin:@localhost:1521:xe</property>  

14.         <property name="connection.username">system</property>  

15.         <property name="connection.password">oracle</property>  

16.         <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>  

17.         

18.         <property name="cache.provider_class">org.hibernate.cache.EhCacheProvider</property>  

19.          <property name="hibernate.cache.use_second_level_cache">true</property>  

20.       

21.     <mapping resource="employee.hbm.xml"/>  

22.     </session-factory>  

23.   

24. </hibernate-configuration>  

To implement second level cache, we need to define cache.provider_class property in the configuration file.

---

File: ehcache.xml

1.    <?xml version="1.0"?>  

2.    <ehcache>  

3.    <defaultCache   

4.    maxElementsInMemory="100"   

5.    eternal="false"   

6.    timeToIdleSeconds="120"   

7.    timeToLiveSeconds="200" />  

8.      

9.    <cache name="com.javatpoint.Employee"   

10. maxElementsInMemory="100"   

11. eternal="false"   

12. timeToIdleSeconds="5"   

13. timeToLiveSeconds="200" />  

14. </ehcache>  

You need to create ehcache.xml file to define the cache property.

defaultCache will be used for all the persistent classes. We can also define persistent class explicitely by using the cache element.

eternal If we specify eternal="true", we don't need to define timeToIdleSeconds and timeToLiveSeconds attributes because it will be handled by hibernate internally. Specifying eternal="false" gives control to the programmer, but we need to define timeToIdleSeconds and timeToLiveSeconds attributes.

timeToIdleSeconds It defines that how many seconds object can be idle in the second level cache.

timeToLiveSeconds It defines that how many seconds object can be stored in the second level cache whether it is idle or not.

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File: FetchTest.java

1.    package com.javatpoint;  

2.      

3.    import org.hibernate.Session;  

4.    import org.hibernate.SessionFactory;  

5.    import org.hibernate.cfg.Configuration;  

6.      

7.    public class FetchTest {  

8.    public static void main(String[] args) {  

9.    Configuration cfg=new Configuration().configure("hibernate.cfg.xml");  

10. SessionFactory factory=cfg.buildSessionFactory();  

11.       

12.     Session session1=factory.openSession();  

13.     Employee emp1=(Employee)session1.load(Employee.class,121);  

14.     System.out.println(emp1.getId()+" "+emp1.getName()+" "+emp1.getSalary());  

15.     session1.close();  

16.       

17.     Session session2=factory.openSession();  

18.     Employee emp2=(Employee)session2.load(Employee.class,121);  

19.     System.out.println(emp2.getId()+" "+emp2.getName()+" "+emp2.getSalary());  

20.     session2.close();  

21.       

22. }  

23. }  

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Output:

As we can see , hibernate does not fire query twice. If you don't use second level cache, hibernate will fire query twice because both query uses different session objects.