转载:http://blog.csdn.net/zouxucong/article/details/68947052
在实际的项目开发中,通常对数据库的查询性能要求很高,而mybatis提供了查询缓存来缓存数据,从而达到提高查询性能的要求。
mybatis的查询缓存分为一级缓存和二级缓存,一级缓存是SqlSession级别的缓存,二级缓存时mapper级别的缓存,二级缓存是多个SqlSession共享的。mybatis通过缓存机制减轻数据压力,提高数据库性能。
一级缓存:
mybatis的一级缓存是SQLSession级别的缓存,在操作数据库时需要构造SqlSession对象,在对象中有一个HashMap用于存储缓存数据,不同的SqlSession之间缓存数据区域(HashMap)是互相不影响的。
一级缓存的作用域是SqlSession范围的,当在同一个SqlSession中执行两次相同的sql语句时,第一次执行完毕会将数据库中查询的数据写到缓存(内存)中,第二次查询时会从缓存中获取数据,不再去底层进行数据库查询,从而提高了查询效率。需要注意的是:如果SqlSession执行了DML操作(insert、update、delete),并执行commit()操作,mybatis则会清空SqlSession中的一级缓存,这样做的目的是为了保证缓存数据中存储的是最新的信息,避免出现脏读现象。
当一个SqlSession结束后该SqlSession中的一级缓存也就不存在了,Mybatis默认开启一级缓存,不需要进行任何配置。
注意:Mybatis的缓存机制是基于id进行缓存,也就是说Mybatis在使用HashMap缓存数据时,是使用对象的id作为key,而对象作为value保存
例子说明:
工程架构图:
sql语句:
- CREATE TABLE employee(
- id INT(11) PRIMARY KEY AUTO_INCREMENT,
- loginname VARCHAR(18),
- PASSWORD VARCHAR(18),
- NAME VARCHAR(18) DEFAULT NULL,
- sex CHAR(2) DEFAULT NULL,
- age INT(11) DEFAULT NULL,
- phone VARCHAR(21),
- sal DOUBLE,
- state VARCHAR(18)
- );
- INSERT INTO employee (loginname,PASSWORD,NAME,sex,age,phone,sal,state) VALUES('jack','123456','杰克','男',26,'12345678936',9800,'ACTIVE');
- INSERT INTO employee (loginname,PASSWORD,NAME,sex,age,phone,sal,state) VALUES('rose','123456','露丝','女',21,'78965412395',6800,'ACTIVE');
- INSERT INTO employee (loginname,PASSWORD,NAME,sex,age,phone,sal,state) VALUES('tom','123456','汤姆','男',25,'13902017777',8800,'ACTIVE');
- INSERT INTO employee (loginname,PASSWORD,NAME,sex,age,phone,sal,state) VALUES('alice','123456','爱丽丝','女',20,'74185296375',5800,'ACTIVE');
- log4j.rootLogger=ERROR, stdout
- log4j.logger.com.zxc.mapper.EmployeeMapper=DEBUG
- log4j.appender.console=org.apache.log4j.ConsoleAppender
- log4j.appender.console.layout=org.apache.log4j.PatternLayout
- log4j.appender.console.layout.ConversionPattern=[%-12d{HH\:mm\:ss.SS}] [%p] %l %m%n
- log4j.appender.stdout=org.apache.log4j.ConsoleAppender
- log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
- log4j.appender.stdout.layout.ConversionPattern=%d %p [%c] - %m%n
- <?xml version="1.0" encoding="UTF-8"?>
- <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
- "http://mybatis.org/dtd/mybatis-3-config.dtd">
- <configuration>
- <!-- mappers 告诉了Mybatis去哪里找持久化类的映射文件 -->
- <settings>
- <!-- 要使延迟加载生效必须配置下面两个属性 -->
- <setting name="lazyLoadingEnabled" value="true"/>
- <setting name="aggressiveLazyLoading" value="false"/>
- <setting name="logImpl" value="LOG4J" />
- </settings>
- <environments default="mysql">
- <environment id="mysql">
- <!-- 指定事务管理类型,type="JDBC"指直接简单使用了JDBC的提交和回滚设置-->
- <transactionManager type="JDBC"/>
- <!-- dataSource指数据源配置,POOLED是JDBC连接对象的数据源连接池的实现 -->
- <dataSource type="POOLED">
- <property name="driver" value="com.mysql.jdbc.Driver"/>
- <property name="url" value="jdbc:mysql://127.0.0.1:3306/mybatis"/>
- <property name="username" value="root"/>
- <property name="password" value="sfhq1866"/>
- </dataSource>
- </environment>
- </environments>
- <mappers>
- <mapper resource="com/zxc/mapper/EmployeeMapper.xml"/>
- </mappers>
- </configuration>
- package com.zxc.domain;
- import java.io.Serializable;
- public class Employee implements Serializable {
- /**
- *
- */
- private static final long serialVersionUID = 1L;
- private Integer id;
- private String loginname;
- private String password;
- private String name;
- private String sex;
- private String age;
- private String phone;
- private Double sal;
- private String state;
- public Integer getId() {
- return id;
- }
- public void setId(Integer id) {
- this.id = id;
- }
- public String getLoginname() {
- return loginname;
- }
- public void setLoginname(String loginname) {
- this.loginname = loginname;
- }
- public String getPassword() {
- return password;
- }
- public void setPassword(String password) {
- this.password = password;
- }
- public String getName() {
- return name;
- }
- public void setName(String name) {
- this.name = name;
- }
- public String getSex() {
- return sex;
- }
- public void setSex(String sex) {
- this.sex = sex;
- }
- public String getAge() {
- return age;
- }
- public void setAge(String age) {
- this.age = age;
- }
- public String getPhone() {
- return phone;
- }
- public void setPhone(String phone) {
- this.phone = phone;
- }
- public Double getSal() {
- return sal;
- }
- public void setSal(Double sal) {
- this.sal = sal;
- }
- public String getState() {
- return state;
- }
- public void setState(String state) {
- this.state = state;
- }
- }
- <?xml version="1.0" encoding="UTF-8"?>
- <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
- "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
- <mapper namespace="com.zxc.mapper.EmployeeMapper">
- <select id="selectEmployeeById" parameterType="int" resultType="com.zxc.domain.Employee">
- SELECT * FROM employee WHERE id = #{id}
- </select>
- <!-- 查询所有Employee -->
- <select id="selectAllEmployee" parameterType="int" resultType="com.zxc.domain.Employee">
- SELECT * FROM employee
- </select>
- <!-- 根据id删除Employee -->
- <delete id="deleteEmployeeById" parameterType="int">
- DELETE FROM employee WHERE id = #{id}
- </delete>
- </mapper>
- package com.zxc.mapper;
- import java.util.List;
- import com.zxc.domain.Employee;
- public interface EmployeeMapper {
- //根据id查询Employee
- Employee selectEmployeeById(Integer id);
- //查询所有Employee
- List<Employee> selectAllEmployee();
- //根据id删除Employee
- void deleteEmployeeById(Integer id);
- }
- package com.zxc.factory;
- import java.io.IOException;
- import java.io.InputStream;
- import org.apache.ibatis.io.Resources;
- import org.apache.ibatis.session.SqlSession;
- import org.apache.ibatis.session.SqlSessionFactory;
- import org.apache.ibatis.session.SqlSessionFactoryBuilder;
- public class ZSqlSessionFactory {
- private static SqlSessionFactory sqlSessionFactory = null;
- static{
- try {
- InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
- sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
- } catch (IOException e) {
- e.printStackTrace();
- }
- }
- //获取SqlSession对象的静态方法
- public static SqlSession getSqlSession(){
- return sqlSessionFactory.openSession();
- }
- //获取SqlSessionFactory的静态方法
- public static SqlSessionFactory getSessionFactory(){
- return sqlSessionFactory;
- }
- }
- package com.zxc.test;
- import org.apache.ibatis.session.SqlSession;
- import com.zxc.domain.Employee;
- import com.zxc.factory.ZSqlSessionFactory;
- import com.zxc.mapper.EmployeeMapper;
- public class OneLevelCacheTest {
- public void testCache1(){
- //使用工厂类获得SqlSession对象
- SqlSession sqlSession = ZSqlSessionFactory.getSqlSession();
- //获得EmployeeMapping对象
- EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);
- Employee employee1 = employeeMapper.selectEmployeeById(1);
- System.out.println(employee1);
- //再次查询id为1的Employee对象,因为是同一个SqlSession,所以会从之前的一级缓存中查找数据
- Employee employee2 = employeeMapper.selectEmployeeById(1);
- System.out.println(employee2);
- sqlSession.close();
- }
- public static void main(String[] args) {
- OneLevelCacheTest t = new OneLevelCacheTest();
- t.testCache1();
- }
- }
运行之后结果:
- 2017-04-01 18:07:21,577 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?
- 2017-04-01 18:07:21,637 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)
- 2017-04-01 18:07:21,720 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1
- com.zxc.domain.Employee@6a33ee9b
- com.zxc.domain.Employee@6a33ee9b
通过观察结果可以看出,在第一次查询id为1的Employee对象时执行了一条select语句,但是第二次获取id为1的Employee对象时并没有执行select语句,因为此时一级缓存也就是SqlSession缓存中已经缓存了id为1的Employee对象,Mybatis直接从缓存中将对象取出来,并没有再次去查询数据库,所以第二次也就没有执行select语句
执行方法类二:
- public void testCache1(){
- //使用工厂类获得SqlSession对象
- SqlSession sqlSession = ZSqlSessionFactory.getSqlSession();
- //获得EmployeeMapping对象
- EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);
- Employee employee1 = employeeMapper.selectEmployeeById(1);
- System.out.println(employee1);
- //执行delete操作
- employeeMapper.deleteEmployeeById(4);
- //commit提交
- sqlSession.commit();
- //再次查询id为1的Employee对象,因为DML操作会清空sqlSession缓存,所以会再次执行select语句
- Employee employee2 = employeeMapper.selectEmployeeById(1);
- System.out.println(employee2);
- sqlSession.close();
- }
- 10:06:52.691 [main] DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.log4j2.Log4j2Impl' adapter.
- 2017-04-05 10:06:53,129 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?
- 2017-04-05 10:06:53,186 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)
- 2017-04-05 10:06:53,215 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1
- com.zxc.domain.Employee@6a33ee9b
- 2017-04-05 10:06:53,216 DEBUG [com.zxc.mapper.EmployeeMapper.deleteEmployeeById] - ==> Preparing: DELETE FROM employee WHERE id = ?
- 2017-04-05 10:06:53,216 DEBUG [com.zxc.mapper.EmployeeMapper.deleteEmployeeById] - ==> Parameters: 4(Integer)
- 2017-04-05 10:06:53,217 DEBUG [com.zxc.mapper.EmployeeMapper.deleteEmployeeById] - <== Updates: 1
- 2017-04-05 10:06:53,219 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?
- 2017-04-05 10:06:53,219 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)
- 2017-04-05 10:06:53,221 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1
- com.zxc.domain.Employee@1e2fce9c
这里有兴趣的朋友可以试试delete操作之后不commit()操作,看Mybatis是否会再次执行查询语句。
执行方法类三:
- public void testCache1(){
- //使用工厂类获得SqlSession对象
- SqlSession sqlSession = ZSqlSessionFactory.getSqlSession();
- //获得EmployeeMapping对象
- EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);
- Employee employee1 = employeeMapper.selectEmployeeById(1);
- System.out.println(employee1);
- //关闭一级缓存
- sqlSession.close();
- //再次访问,需要再次获取一级缓存,然后才能查找数据,否则会抛出异常
- sqlSession = ZSqlSessionFactory.getSqlSession();
- //再次获得EmployeeMapper对象
- employeeMapper = sqlSession.getMapper(EmployeeMapper.class);
- Employee employee2 = employeeMapper.selectEmployeeById(1);
- System.out.println(employee2);
- sqlSession.close();
- }
- 17:12:03.705 [main] DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.log4j2.Log4j2Impl' adapter.
- 2017-04-05 17:12:04,151 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?
- 2017-04-05 17:12:04,204 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)
- 2017-04-05 17:12:04,230 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1
- com.zxc.domain.Employee@3016f5a0
- 2017-04-05 17:12:04,231 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?
- 2017-04-05 17:12:04,231 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)
- 2017-04-05 17:12:04,233 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1
- com.zxc.domain.Employee@77763895
二级缓存:
二级缓存是mapper级别的缓存,使用二级缓存时,多个SqlSession使用同一个Mapper的sql语句去操作数据库,得到的数据会存在二级缓存区域,它同样是使用HashMapper进行数据存储,相比一级缓存SqlSession,二级缓存的范围更大,多个SqlSession可以共用二级缓存,二级缓存是跨SqlSession的。
二级缓存是多个SqlSession共享的,其作用域是mapper的同一个namespace,不同的SqlSession两次执行相同的namespace下的sql语句,且向sql中传递的参数也相同,即最终执行相同的sql语句,则第一次执行完毕会将数据库中查询的数据写到缓存(内存),第二次查询时会从缓存中获取数据,不再去底层数据库查询,从而提高查询效率。
Mybatis默认没有开启二级缓存,需要在setting全局参数中配置开启二级缓存。
在mybatis-config.xml中配置:
- <settings>
- <setting name="cacheEnabled" value="true"/>
- </settings>
在EmployeeMapper.xml中配置:
- <!-- 开启当前mapper的namespace下的二级缓存 -->
- <cache eviction="LRU" flushInterval="60000" size="512" readOnly="true"/>
cache元素用来开启当前mapper的namespace下的二级缓存,该元素的属性设置如下:
- flushInterval:刷新间隔,可以被设置为任意的正整数,而且它们代表一个合理的毫秒形式的时间段,默认情况下是不设置的,也就是没有刷新间隔,缓存仅仅调用语句时刷新。
- size:缓存数目,可以被设置为任意正整数,要记住你的缓存对象数目和你运行环境可用内存资源数目,默认值是1024.
- readOnly:只读,属性可以被设置为true或false,只读的缓存会给所有调用者返回缓存对象的相同实例,因此这些对象不能被修改。这提供了很重要的性能优势,可读写的缓存会返回缓存对象的拷贝(通过序列化),这会慢一些,但是安全,因此默认是false。
- eviction:收回策略,默认为LRU,有如下几种:
- LRU:最近最少使用的策略,移除最长时间不被使用的对象。
- FIFO:先进先出策略,按对象进入缓存的顺序来移除它们。
- SOFT:软引用策略,移除基于垃圾回收器状态和软引用规则的对象。
- WEAK:弱引用策略,更积极地移除基于垃圾收集器状态和弱引用规则的对象。
执行方法类四:
- public void testCache1(){
- //使用工厂类获得SqlSession对象
- SqlSession sqlSession = ZSqlSessionFactory.getSqlSession();
- //获得EmployeeMapping对象
- EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);
- Employee employee1 = employeeMapper.selectEmployeeById(1);
- System.out.println(employee1);
- //关闭一级缓存
- sqlSession.close();
- //再次访问,需要再次获取一级缓存,然后才能查找数据,否则会抛出异常
- sqlSession = ZSqlSessionFactory.getSqlSession();
- //再次获得EmployeeMapper对象
- employeeMapper = sqlSession.getMapper(EmployeeMapper.class);
- //再次查询id为1的Employee对象,因为DML操作会清空sqlSession缓存,所以会再次执行select语句
- Employee employee2 = employeeMapper.selectEmployeeById(1);
- System.out.println(employee2);
- sqlSession.close();
- }
- 18:18:10.743 [main] DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.log4j2.Log4j2Impl' adapter.
- 2017-04-05 18:18:10,920 DEBUG [com.zxc.mapper.EmployeeMapper] - Cache Hit Ratio [com.zxc.mapper.EmployeeMapper]: 0.0
- 2017-04-05 18:18:11,169 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?
- 2017-04-05 18:18:11,224 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)
- 2017-04-05 18:18:11,252 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1
- com.zxc.domain.Employee@2934ad41
- 2017-04-05 18:18:11,254 DEBUG [com.zxc.mapper.EmployeeMapper] - Cache Hit Ratio [com.zxc.mapper.EmployeeMapper]: 0.5
- com.zxc.domain.Employee@2934ad41
- <select id="selectEmployeeById" parameterType="int" resultType="com.zxc.domain.Employee" useCache="false">
- 18:20:30.418 [main] DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.log4j2.Log4j2Impl' adapter.
- 2017-04-05 18:20:30,861 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?
- 2017-04-05 18:20:30,916 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)
- 2017-04-05 18:20:30,943 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1
- com.zxc.domain.Employee@54d5da26
- 2017-04-05 18:20:30,945 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?
- 2017-04-05 18:20:30,945 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)
- 2017-04-05 18:20:30,947 DEBUG [com.zxc.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1
- com.zxc.domain.Employee@7116778b
小结:本文介绍了Mybatis的缓存机制,包括一级缓存SqlSession和二级缓存mapper,使用缓存可以最大程度低减轻数据查询压力,提高数据库性能。