Hibernate提供了基于应用程序级别(进程)的缓存， 可以跨多个session，即不同的session都可以访问缓存数据。 这个换存也叫二级缓存。

Hibernate提供的二级缓存有默认的实现，且是一种可插配的缓存框架！如果用户想用二级缓存，只需要在hibernate.cfg.xml中配置即可； 不想用，直接移除，不影响代码。

hibernate.cfg.xml

<!--****************** 【二级缓存配置】****************** --><!-- a.  开启二级缓存 --><property name="hibernate.cache.use_second_level_cache">false</property> <!--未开启情况--><!-- b. 指定使用哪一个缓存框架(默认提供的实现) --><property name="hibernate.cache.provider_class">org.hibernate.cache.HashtableCacheProvider</property><!-- 开启查询缓存,放在class代码之前 --><property name="hibernate.cache.use_query_cache">true</property><!-- c. 指定哪一些类，需要加入二级缓存,一般在总配置中写，便于维护 --><class-cache usage="read-write" class="cn.itcast.b_second_cache.Dept"/><class-cache usage="read-only" class="cn.itcast.b_second_cache.Employee"/><!-- 集合缓存[集合缓存的元素对象，也加加入二级缓存] --><collection-cache usage="read-write" collection="cn.itcast.b_second_cache.Dept.emps"/>

Dept.java

package cn.itcast.b_second_cache;import java.util.HashSet;
import java.util.Set;public class Dept {private int deptId;private String deptName;// 【一对多】 部门对应的多个员工private Set<Employee> emps = new HashSet<Employee>();public Dept(int deptId, String deptName) {super();this.deptId = deptId;this.deptName = deptName;}public Dept() {super();}public int getDeptId() {return deptId;}public void setDeptId(int deptId) {this.deptId = deptId;}public String getDeptName() {return deptName;}public void setDeptName(String deptName) {this.deptName = deptName;}public Set<Employee> getEmps() {return emps;}public void setEmps(Set<Employee> emps) {this.emps = emps;}@Overridepublic String toString() {return "Dept [deptId=" + deptId + ", deptName=" + deptName + "]";}}

Employee.java

package cn.itcast.b_second_cache;public class Employee {private int empId;private String empName;private double salary;// 【多对一】员工与部门private Dept dept;;public int getEmpId() {return empId;}public void setEmpId(int empId) {this.empId = empId;}public String getEmpName() {return empName;}public void setEmpName(String empName) {this.empName = empName;}public double getSalary() {return salary;}public void setSalary(double salary) {this.salary = salary;}public Dept getDept() {return dept;}public void setDept(Dept dept) {this.dept = dept;}
}

Dept.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.itcast.b_second_cache"><class name="Dept" table="t_dept" ><id name="deptId"><generator class="native"></generator></id>	<property name="deptName" length="20"></property><set name="emps"><key column="dept_id"></key><one-to-many class="Employee"/></set></class><!-- 存放sql语句 --><query name="getAllDept"><![CDATA[from Dept d where deptId < ?]]></query>
</hibernate-mapping>

Employee.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.itcast.b_second_cache"><class name="Employee" table="t_employee"><id name="empId"><generator class="native"></generator></id>	<property name="empName" length="20"></property><property name="salary" type="double"></property><many-to-one name="dept" column="dept_id" class="Dept"></many-to-one></class></hibernate-mapping>

App.java

public class App {private static SessionFactory sf;static {sf = new Configuration().configure().addClass(Dept.class)   .addClass(Employee.class)   // 测试时候使用.buildSessionFactory();}// 1. 测试二级缓存的使用// 没有/有用 二级缓存@Testpublic void testCache() {Session session1 = sf.openSession();session1.beginTransaction();// a. 查询一次Dept dept = (Dept) session1.get(Dept.class, 3);dept.getEmps().size();// 集合session1.getTransaction().commit();session1.close();System.out.println("------");// 第二个sessionSession session2 = sf.openSession();session2.beginTransaction();// a. 查询一次dept = (Dept) session2.get(Dept.class, 3);  // 二级缓存配置好； 这里不查询数据库//dept.setDeptName("市场行政部");dept.getEmps().size();session2.getTransaction().commit();session2.close();}}	

二级缓存未配置sql查询情况：

Hibernate: select dept0_.deptId as deptId0_0_, dept0_.deptName as deptName0_0_ from t_dept dept0_ where dept0_.deptId=?
Hibernate: select emps0_.dept_id as dept4_0_1_, emps0_.empId as empId1_, emps0_.empId as empId1_0_, emps0_.empName as empName1_0_, emps0_.salary as salary1_0_, emps0_.dept_id as dept4_1_0_ from t_employee emps0_ where emps0_.dept_id=?
------
Hibernate: select dept0_.deptId as deptId0_0_, dept0_.deptName as deptName0_0_ from t_dept dept0_ where dept0_.deptId=?
Hibernate: select emps0_.dept_id as dept4_0_1_, emps0_.empId as empId1_, emps0_.empId as empId1_0_, emps0_.empName as empName1_0_, emps0_.salary as salary1_0_, emps0_.dept_id as dept4_1_0_ from t_employee emps0_ where emps0_.dept_id=?

<!--****************** 【二级缓存配置】****************** -->
<!-- a.  开启二级缓存 -->
<property name="hibernate.cache.use_second_level_cache">true</property> 

二级缓存配置sql查询情况：

Hibernate: select dept0_.deptId as deptId0_0_, dept0_.deptName as deptName0_0_ from t_dept dept0_ where dept0_.deptId=?
Hibernate: select emps0_.dept_id as dept4_0_1_, emps0_.empId as empId1_, emps0_.empId as empId1_0_, emps0_.empName as empName1_0_, emps0_.salary as salary1_0_, emps0_.dept_id as dept4_1_0_ from t_employee emps0_ where emps0_.dept_id=?
------