MyBatis的缓存
项目文件截图：
一、缓存介绍
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？
1.一级缓存:
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。
2.二级缓存：
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。
二、一级缓存
UserMapper代码：
public interface UserDao {
//根据id查询用户信息
public User findUserById(Integer id);
}UserMapper.xml文件代码：
测试类代码：
package test;
import com.by.mapper.UserMapper;
import com.by.pojo.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;
public class MyBatisFirstCacheTest {
private SqlSession sqlSession;
private InputStream inputStream;
private SqlSessionFactory sessionFactory;
@Before
public void init() throws IOException {
//加载mybatis-config.xml
String resource = "mybatis-config.xml";
inputStream = Resources.getResourceAsStream(resource);
//创建sqlSessionFactory
sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//创建sqlSession
sqlSession = sessionFactory.openSession();
}
@Test
public void testGoCache(){
SqlSession sqlSession = sessionFactory.openSession();
//拥有同一个sqlsession
UserMapper userMapper1 = sqlSession.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession.getMapper(UserMapper.class);
System.out.println("=============第一次查询============");
User user1 = userMapper1.getUserById(41); //执行查询
System.out.println(user1);
System.out.println("=============第二次查询============");
User user2 = userMapper2.getUserById(41);//执行查询？
System.out.println(user2);
}
@Test
public void testNoGoCache(){
SqlSession sqlSession1 = sessionFactory.openSession();
SqlSession sqlSession2 = sessionFactory.openSession();
//拥有不同的sqlsession
UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
System.out.println("=============第一次查询============");
User user1 = userMapper1.getUserById(41); //执行查询
System.out.println(user1);
System.out.println("=============第二次查询============");
User user2 = userMapper2.getUserById(41);//执行查询？不执行
System.out.println(user2);
}
@Test
public void testNoGoCache2(){
SqlSession sqlSession = sessionFactory.openSession();
//拥有同一个sqlsession
UserMapper userMapper1 = sqlSession.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession.getMapper(UserMapper.class);
System.out.println("=============第一次查询============");
User user1 = userMapper1.getUserById(41); //执行查询
System.out.println(user1);
System.out.println("=============两次查询之间执行增删改=============");
userMapper1.deleteUserById(1);
sqlSession.commit();
System.out.println("=============第二次查询============");
User user2 = userMapper2.getUserById(41);//执行查询
System.out.println(user2);
}
@After
public void close() throws IOException {
/* sqlSession.close();
inputStream.close();*/
}
}
分析：
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。
-
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。
二级缓存：
UserMapper代码：
package com.by.mapper;
import com.by.pojo.User;
public interface UserMapper {
User getUserById(Integer id);
void deleteUserById(Integer id);
}
UserMapper.xml文件代码：
delete from mybatis.user where id=#{id}
User类代码：
package com.by.pojo;
import java.io.Serializable;
import java.util.Date;
public class User implements Serializable {
private Integer id;
private String username;
private Date birthday;
private String sex;
private String address;
public void setId(Integer id) {
this.id = id;
}
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;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", username='" + username + '\'' +
", birthday=" + birthday +
", sex='" + sex + '\'' +
", address='" + address + '\'' +
'}';
}
}二级缓存测试类代码：
package test;
import com.by.mapper.UserMapper;
import com.by.pojo.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;
public class MyBatisSecondCacheTest {
private SqlSession sqlSession;
private InputStream inputStream;
private SqlSessionFactory sessionFactory;
@Before
public void init() throws IOException {
//加载mybatis-config.xml
String resource = "mybatis-config.xml";
inputStream = Resources.getResourceAsStream(resource);
//创建sqlSessionFactory
sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//创建sqlSession
sqlSession = sessionFactory.openSession();
}
@Test
public void testGoCache(){
SqlSession sqlSession1 = sessionFactory.openSession();
SqlSession sqlSession2 = sessionFactory.openSession();
//拥有相同的sqlSessionFactrory
UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
System.out.println("=============第一次查询============");
User user1 = userMapper1.getUserById(41); //执行sql
System.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.getUserById(41);//执行sql？不执行sql
System.out.println(user2);
}
@Test
public void testNoGoCache() throws IOException {
//加载mybatis-config.xml
String 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sqlSessionFactrory
UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
System.out.println("=============第一次查询============");
User user1 = userMapper1.getUserById(41); //执行sql
System.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.getUserById(41);//执行sql？执行sql
System.out.println(user2);
}
@Test
public void testNoGoCache2(){
SqlSession sqlSession1 = sessionFactory.openSession();
SqlSession sqlSession2 = sessionFactory.openSession();
//拥有相同的sqlSessionFactrory
UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
System.out.println("=============第一次查询============");
User user1 = userMapper1.getUserById(41); //执行sql
System.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.getUserById(41);//执行sql？执行sql
System.out.println(user2);
}
@After
public void close() throws IOException {
/* sqlSession.close();
inputStream.close();*/
}
}
分析：
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:
