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1、二級緩存的知識
Hibernate提供的緩存：有一級緩存、二級緩存。 目的是為了減少對數據庫的訪問次數，提升程序執行效率！
一級緩存：基于Session的緩存，緩存內容只在當前session有效，session關閉，緩存內容失效！
特點：作用范圍較小！ 緩存的時間短。緩存效果不明顯。
二級緩存：
Hibernate提供了基于應用程序級別的緩存， 可以跨多個session，即不同的session都可以訪問緩存數據。 這個緩存也叫二級緩存。
Hibernate提供的二級緩存有默認的實現，且是一種可插配的緩存框架！如果用戶想用二級緩存，只需要在hibernate.cfg.xml中配置即可；不想用，直接移除，不影響代碼。
如果用戶覺得hibernate提供的框架不好用，可以換其他的緩存框架或自己實現緩存框架都可以。
下面的配置位于%hibernate%/project/etc/hibernate.properties中
########################## ### Second-level Cache ### ########################## ## disable the second-level cache 二級緩存默認不開啟，需要手動開啟 #hibernate.cache.use_second_level_cache false ## enable the query cache #hibernate.cache.use_query_cache true 開啟查詢緩存 ## choose a cache implementation 二級緩存框架的實現 #hibernate.cache.provider_class org.hibernate.cache.EhCacheProvider #hibernate.cache.provider_class org.hibernate.cache.EmptyCacheProvider hibernate.cache.provider_class org.hibernate.cache.HashtableCacheProvider #hibernate.cache.provider_class org.hibernate.cache.TreeCacheProvider #hibernate.cache.provider_class org.hibernate.cache.OSCacheProvider #hibernate.cache.provider_class org.hibernate.cache.SwarmCacheProvider
緩存的并發策略
<class-cache usage="read-only"/> 放入二級緩存的對象，只讀; <class-cache usage="nonstrict-read-write"/> 非嚴格的讀寫 <class-cache usage="read-write"/> 讀寫； 放入二級緩存的對象可以讀、寫； <class-cache usage="transactional"/> (基于事務的策略)
2、使用二級緩存
二級緩存，使用步驟
1)開啟二級緩存
<property name="hibernate.cache.use_second_level_cache">true</property>
2)指定緩存框架
<property name="hibernate.cache.provider_class">org.hibernate.cache.HashtableCacheProvider</property>
3)指定那些類加入二級緩存
<!-- 指定哪一些類，需要加入二級緩存 --> <class-cache usage="read-write" class="com.rk.hibernate.cache.Department"/> <class-cache usage="read-write" class="com.rk.hibernate.cache.Employee"/> <!-- 集合緩存[集合緩存的元素對象，也加加入二級緩存] --> <collection-cache usage="read-write" collection="com.rk.hibernate.cache.Department.emps"/>
4)測試二級緩存！
示例代碼和配置
hibernate.cfg.xml
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <!-- 通常，一個session-factory節點代表一個數據庫 --> <session-factory> <!-- 1. 數據庫連接配置 --> <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property> <property name="hibernate.connection.url">jdbc:mysql:///test</property> <property name="hibernate.connection.username">root</property> <property name="hibernate.connection.password">root</property> <!-- 數據庫方言配置， hibernate在運行的時候，會根據不同的方言生成符合當前數據庫語法的sql --> <property name="hibernate.dialect">org.hibernate.dialect.MySQL5Dialect</property> <!-- 2. 其他相關配置 --> <!-- 2.1 顯示hibernate在運行時候執行的sql語句 --> <property name="hibernate.show_sql">true</property> <!-- 2.2 格式化sql --> <property name="hibernate.format_sql">false</property> <!-- 2.3 自動建表 --> <property name="hibernate.hbm2ddl.auto">update</property> <!--****************** 【二級緩存配置】****************** --> <!-- a. 開啟二級緩存 --> <property name="hibernate.cache.use_second_level_cache">true</property> <!-- b. 指定使用哪一個緩存框架(默認提供的) --> <property name="hibernate.cache.provider_class">org.hibernate.cache.HashtableCacheProvider</property> <!-- 開啟查詢緩存 --> <property name="hibernate.cache.use_query_cache">true</property> <!-- c. 指定哪一些類，需要加入二級緩存 --> <class-cache usage="read-write" class="com.rk.hibernate.cache.Department"/> <class-cache usage="read-write" class="com.rk.hibernate.cache.Employee"/> <!-- 集合緩存[集合緩存的元素對象，也加加入二級緩存] --> <collection-cache usage="read-write" collection="com.rk.hibernate.cache.Department.emps"/> </session-factory> </hibernate-configuration>
Department.java
package com.rk.hibernate.cache;
import java.util.Set;
public class Department
{
private int deptId;
private String deptName;
private Set<Employee> emps;
private int version;
public int getVersion()
{
return version;
}
public void setVersion(int version)
{
this.version = version;
}
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;
}
@Override
public String toString()
{
return "Department [deptId=" + deptId + ", deptName=" + deptName + "]";
}
}Department.hbm.xml
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.rk.hibernate.cache" auto-import="true"> <class name="Department" table="T_Department"> <!-- <cache usage="read-only"/> --> <id name="deptId" column="id"> <generator class="native"></generator> </id> <version name="version" column="dept_version"></version> <property name="deptName" column="name" type="string"></property> <set name="emps" table="T_Employee"> <!-- <cache usage="read-only"/> --> <key column="deptId"></key> <one-to-many class="Employee"/> </set> </class> </hibernate-mapping>
Employee.java
package com.rk.hibernate.cache;
public class Employee
{
private int empId;
private String empName;
private int salary;
private Department dept;
private int version;
public int getVersion()
{
return version;
}
public void setVersion(int version)
{
this.version = version;
}
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 int getSalary()
{
return salary;
}
public void setSalary(int salary)
{
this.salary = salary;
}
public Department getDept()
{
return dept;
}
public void setDept(Department dept)
{
this.dept = dept;
}
@Override
public String toString()
{
return "Employee [empId=" + empId + ", empName=" + empName + ", salary=" + salary + "]";
}
}Employee.hbm.xml
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.rk.hibernate.cache" auto-import="true"> <class name="Employee" table="T_Employee"> <!-- <cache usage="read-only"/> --> <id name="empId" column="id"> <generator class="native"></generator> </id> <version name="version" column="emp_version"></version> <property name="empName" column="name" type="string"></property> <property name="salary" column="salary" type="int"></property> <many-to-one name="dept" column="deptId" class="Department"></many-to-one> </class> </hibernate-mapping>
App.java
package com.rk.hibernate.cache;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import org.hibernate.Query;
import org.hibernate.SQLQuery;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import org.junit.Test;
public class App
{
private static SessionFactory sf;
static
{
sf = new Configuration()
.configure()
.addClass(Department.class)
.addClass(Employee.class)
.buildSessionFactory();
}
// 1. 測試二級緩存的使用
//思路：sesion本身提供了一級緩存，它是mandatory，不能關閉
//二級緩存，是optional，通過配置文件可以開啟，可以關閉。
//如果不開啟二級緩存，兩個session查詢同一個id的數據，會發送兩個SQL語句
//如果開啟二級緩存，兩個session查詢同一個id的數據，會發送一條SQL語句
//通過開啟和關閉二級緩存，來查看執行的SQL數目
@Test
public void testSecondLevelCache()
{
//第1次查詢，第一個session
Session session1 = sf.openSession();
session1.beginTransaction();
Department dept1 = (Department) session1.get(Department.class, 3);
Set<Employee> emps1 = dept1.getEmps();
System.out.println(dept1);
System.out.println(emps1);
session1.getTransaction().commit();
session1.close();
System.out.println("-------------------------------");
//第2次查詢，第二個session
Session session2 = sf.openSession();
session2.beginTransaction();
Department dept2 = (Department) session2.get(Department.class, 3);
Set<Employee> emps2 = dept2.getEmps();
System.out.println(dept2);
System.out.println(emps2);
session2.getTransaction().commit();
session2.close();
}
@Test
public void testQueryCache()
{
// 第1次查詢，第一個session
Session session1 = sf.openSession();
session1.beginTransaction();
// HQL查詢 【setCacheable 指定從二級緩存找，或者是放入二級緩存】
Query q1 = session1.createQuery("from Department").setCacheable(true);
List<Department> list1 = q1.list();
System.out.println(list1);
session1.getTransaction().commit();
session1.close();
System.out.println("-------------------------------");
// 第2次查詢，第二個session
Session session2 = sf.openSession();
session2.beginTransaction();
// HQL查詢 【setCacheable 指定從二級緩存找，或者是放入二級緩存】
Query q2 = session2.createQuery("from Department").setCacheable(true);
List<Department> list2 = q2.list();
System.out.println(list2);
session2.getTransaction().commit();
session2.close();
}
}當我們開啟二級緩存后，例如在上面的testSecondLevelCache()方法中，第一次從Session讀取數據后，會存儲在二級緩存上；第二次打開Session，程序再進行相同查詢，就不需要再發送SQL語句，因為它會從二級緩存中讀取數據。
有一點需要注意：如果二級緩存中沒有數據，第一次開啟Session讀取數據，并調用session.clear()方法，再讀取數據，會發送兩次SQL語句。
@Test
public void test1()
{
// 第1次查詢，第一個session
Session session1 = sf.openSession();
session1.beginTransaction();
Department dept1 = (Department) session1.get(Department.class, 2);
System.out.println(dept1);
System.out.println(dept1.getEmps());
session1.clear();
dept1 = (Department) session1.get(Department.class, 2);
System.out.println(dept1);
System.out.println(dept1.getEmps());
session1.getTransaction().commit();
session1.close();
}結果如下：
Hibernate: select department0_.id as id0_0_, department0_.dept_version as dept2_0_0_, department0_.name as name0_0_ from T_Department department0_ where department0_.id=? Department [deptId=2, deptName=woqu] Hibernate: select emps0_.deptId as deptId0_1_, emps0_.id as id1_, emps0_.id as id1_0_, emps0_.emp_version as emp2_1_0_, emps0_.name as name1_0_, emps0_.salary as salary1_0_, emps0_.deptId as deptId1_0_ from T_Employee emps0_ where emps0_.deptId=? [Employee [empId=3, empName=TO_T_, salary=4]] Hibernate: select department0_.id as id0_0_, department0_.dept_version as dept2_0_0_, department0_.name as name0_0_ from T_Department department0_ where department0_.id=? Department [deptId=2, deptName=woqu] Hibernate: select emps0_.deptId as deptId0_1_, emps0_.id as id1_, emps0_.id as id1_0_, emps0_.emp_version as emp2_1_0_, emps0_.name as name1_0_, emps0_.salary as salary1_0_, emps0_.deptId as deptId1_0_ from T_Employee emps0_ where emps0_.deptId=? [Employee [empId=3, empName=TO_T_, salary=4]]
如果二級緩存中已經存在數據，第二次開啟Session，調用session.clear()，再讀取數據，并不會發送SQL語句。
@Test
public void test1()
{
Session session2 = sf.openSession();
session2.beginTransaction();
Department dept2 = (Department) session2.get(Department.class, 2);
System.out.println(dept2);
System.out.println(dept2.getEmps());
session2.getTransaction().commit();
session2.close();
System.out.println("-------------------------------");
// 第1次查詢，第一個session
Session session1 = sf.openSession();
session1.beginTransaction();
Department dept1 = (Department) session1.get(Department.class, 2);
System.out.println(dept1);
System.out.println(dept1.getEmps());
session1.clear();
dept1 = (Department) session1.get(Department.class, 2);
System.out.println(dept1);
System.out.println(dept1.getEmps());
session1.getTransaction().commit();
session1.close();
System.out.println("-------------------------------");
}結果如下：
Hibernate: select department0_.id as id0_0_, department0_.dept_version as dept2_0_0_, department0_.name as name0_0_ from T_Department department0_ where department0_.id=? Department [deptId=2, deptName=woqu] Hibernate: select emps0_.deptId as deptId0_1_, emps0_.id as id1_, emps0_.id as id1_0_, emps0_.emp_version as emp2_1_0_, emps0_.name as name1_0_, emps0_.salary as salary1_0_, emps0_.deptId as deptId1_0_ from T_Employee emps0_ where emps0_.deptId=? [Employee [empId=3, empName=TO_T_, salary=4]] ------------------------------- Department [deptId=2, deptName=woqu] [Employee [empId=3, empName=TO_T_, salary=4]] Department [deptId=2, deptName=woqu] [Employee [empId=3, empName=TO_T_, salary=4]] -------------------------------
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