MyBatis的缓存-拓冰建站

为什么使用缓存？

首次访问时，查询数据库，并将数据存储到内存中；再次访问时直接访问缓存，减少IO、硬盘读写次数、提高效率
Mybatis中的一级缓存和二级缓存？
- 一级缓存:
  
  它指的是mybatis中的SqlSession对象的缓存。当我们执行完查询之后，查询的结果会同时存在在SqlSession为我们提供的一块区域中。当我们再次查询同样的数据，mybatis会先去SqlSession中查询是否有，有的话直接拿出来使用。当SqlSession对象消失时，Mybatis的一级缓存也就消失了。
- 二级缓存：
  
  它指的是Mybatis中SqlSessionFactory对象的缓存，由同一个SqlSessioFactory对象创建的SqlSession共享其缓存。

项目结构

User类

public class User implements Serializable {private Integer id;private String username;private Date birthday;private String password;private String sex;private String address;@Overridepublic String toString() {return "User{" +"id=" + id +", username='" + username + '\'' +", birthday=" + birthday +", password='" + password + '\'' +", sex='" + sex + '\'' +", address='" + address + '\'' +", accountList=" + accountList +'}';}public String getPassword() {return password;}public void setPassword(String password) {this.password = password;}private List<Account> accountList;public List<Account> getAccountList() {return accountList;}public void setAccountList(List<Account> accountList) {this.accountList = accountList;}public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getUsername() {return username;}public void setUsername(String username) {this.username = username;}public Date getBirthday() {return birthday;}public void setBirthday(Date birthday) {this.birthday = birthday;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public String getAddress() {return address;}public void setAddress(String address) {this.address = address;}}

一级缓存

一级缓存是SqlSession范围的缓存，当调用SqlSession的commit()，close()等方法时，就会清空一级缓存。

一级缓存结构图：

第一次发起查询用户id为 1 的用户信息，先去找缓存中是否有id为 1 的用户信息，如果没有，从数据库查询用户信息。得到用户信息，将用户信息存储到一级缓存中。
如果sqlSession去执行 commit操作（执行插入、更新、删除），清空 SqlSession 中的一级缓存，这样做的目的为了让缓存中存储的是最新的信息，避免脏读。
第二次发起查询用户id为1的用户信息，先去找缓存中是否有id为1的用户信息，缓存中有，直接从缓存中获取用户信息。

在UserDao接口中

public interface UserDao {//根据id查询用户信息public User findUserById(Integer id);void deleteUserById(Integer id);
}

在UserDao.xml文件中

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.by.dao.UserDao"><select id="findUserById" resultType="User" parameterType="int">select * from user where id=#{id}</select>
</mapper>

在测试类

public class MyBatisTset {private SqlSession sqlSession;private InputStream inputStream;private SqlSessionFactory sqlSessionFactory;@Beforepublic void init() throws IOException {//加载配置文件String resource = "mybatis-config.xml";inputStream = Resources.getResourceAsStream(resource);//创建SessionFactorysqlSessionFactory= new SqlSessionFactoryBuilder().build(inputStream);//使用数据的会话实例sqlSession = sqlSessionFactory.openSession();}//一级缓存读取缓存sql(同一个sqlsession)@Testpublic void testGoCache1(){UserDao userDao = sqlSession.getMapper(UserDao.class);System.out.println("================第一次查询================");User userById1 = userDao.findUserById(41);System.out.println(userById1);System.out.println("================第二次查询================");UserDao userDao2 = sqlSession.getMapper(UserDao.class);User userById2 = userDao2.findUserById(41);System.out.println(userById2);}//一级缓存不读取缓存sql(不同一个sqlsession)@Testpublic void testNoGoCache(){//SqlSession sqlSession = sessionFactory.openSession();SqlSession sqlSession1 =sqlSessionFactory.openSession();SqlSession sqlSession2 =sqlSessionFactory.openSession();//拥有不同的sqlsessionUserDao UserDao1 = sqlSession1.getMapper(UserDao.class);UserDao UserDao2 = sqlSession2.getMapper(UserDao.class);System.out.println("================第一次查询================");User userById1 = UserDao1.findUserById(41);System.out.println(userById1);System.out.println("================第二次查询================");User userById2 = UserDao2.findUserById(41);System.out.println(userById2);}
//一级缓存不读取缓存sql(同一个sqlsession，执行CRUD操作)@Testpublic void testNoGoCache2(){//SqlSession sqlSession = sessionFactory.openSession();//拥有同一个sqlsessionUserDao UserDao1 = sqlSession.getMapper(UserDao.class);UserDao UserDao2 = sqlSession.getMapper(UserDao.class);System.out.println("=============第一次查询============");User user1 = UserDao1.findUserById(41); //执行查询System.out.println(user1);System.out.println("=============两次查询之间执行增删改=============");UserDao1.deleteUserById(1);sqlSession.commit();System.out.println("=============第二次查询============");User user2 = UserDao2.findUserById(41);//执行查询System.out.println(user2);}@Afterpublic void close() throws IOException {sqlSession.close();inputStream.close();}
}

一级缓存读取缓存sql(同一个sqlsession)

一级缓存不读取缓存sql(不同一个sqlsession)

一级缓存不读取缓存sql(同一个sqlsession，执行CRUD操作)

二级缓存

二级缓存是mapper映射级别的缓存，多个SqlSession去操作同一个Mapper映射的sql语句，多个SqlSession可以共用二级缓存，二级缓存是跨SqlSession的。

二级缓存结构图：

UserDao和UserDao.xml同上

测试类

public class MybatisSecondCache {private SqlSession sqlSession;private InputStream inputStream;private SqlSessionFactory sqlSessionFactory;@Beforepublic void init() throws IOException {//加载配置文件String resource = "mybatis-config.xml";inputStream = Resources.getResourceAsStream(resource);//创建SessionFactorysqlSessionFactory= new SqlSessionFactoryBuilder().build(inputStream);//使用数据的会话实例sqlSession = sqlSessionFactory.openSession();}//二级缓存不执行sql@Testpublic void testGoCache(){SqlSession sqlSession1 = sqlSessionFactory.openSession();SqlSession sqlSession2 = sqlSessionFactory.openSession();//拥有相同的sqlSessionFactroryUserDao UserDao1 = sqlSession1.getMapper(UserDao.class);UserDao UserDao2 = sqlSession2.getMapper(UserDao.class);System.out.println("=============第一次查询============");User user1 = UserDao1.findUserById(41); //执行sqlSystem.out.println(user1);sqlSession1.commit(); //第一次查询session执行commit或close，才会把数据写到二级缓存System.out.println("=============第二次查询============");User user2 = UserDao2.findUserById(41);//执行sql？不执行sqlSystem.out.println(user2);}//二级缓存执行sql(不同的sqlSessionFactrory)@Testpublic void testNoGoCache() throws IOException {//加载mybatis-config.xmlString resource = "mybatis-config.xml";InputStream inputStream1 = Resources.getResourceAsStream(resource);SqlSessionFactory sessionFactory1 = new SqlSessionFactoryBuilder().build(inputStream1);InputStream inputStream2 = Resources.getResourceAsStream(resource);SqlSessionFactory sessionFactory2 = new SqlSessionFactoryBuilder().build(inputStream2);SqlSession sqlSession1 = sessionFactory1.openSession();SqlSession sqlSession2 = sessionFactory2.openSession();//拥有不相同的sqlSessionFactroryUserDao userMapper1 = sqlSession1.getMapper(UserDao.class);UserDao userMapper2 = sqlSession2.getMapper(UserDao.class);System.out.println("=============第一次查询============");User user1 = userMapper1.findUserById(41); //执行sqlSystem.out.println(user1);sqlSession1.commit(); //第一次查询session执行commit或close，才会把数据写到二级缓存System.out.println("=============第二次查询============");User user2 = userMapper2.findUserById(41);//执行sql？执行sqlSystem.out.println(user2);}//二级缓存执行sql(相同的sqlSessionFactrory)@Testpublic void testNoGoCache2(){SqlSession sqlSession1 = sqlSessionFactory.openSession();SqlSession sqlSession2 = sqlSessionFactory.openSession();//拥有相同的sqlSessionFactroryUserDao userMapper1 = sqlSession1.getMapper(UserDao.class);UserDao userMapper2 = sqlSession2.getMapper(UserDao.class);System.out.println("=============第一次查询============");User user1 = userMapper1.findUserById(41); //执行sqlSystem.out.println(user1);sqlSession1.commit(); //第一次查询session执行commit或close，才会把数据写到二级缓存System.out.println("=============两次查询之间执行增删改查=============");userMapper1.deleteUserById(1);sqlSession1.commit();System.out.println("=============第二次查询============");User user2 = userMapper2.findUserById(41);//执行sql？执行sqlSystem.out.println(user2);}@Afterpublic void close() throws IOException {sqlSession.close();inputStream.close();}}

二级缓存不执行sql