1.一级缓存: 基于PerpetualCache 的 HashMap本地缓存， 其存储作用域为 Session ，当 Session flush 或 close 之后，该Session中的所有 Cache 就将清空。
2. 二级缓存与一级缓存其机制相同，默认也是采用 PerpetualCache，HashMap存储，不同在于其 存储作用域为 Mapper(Namespace)，并且可自定义存储源 ，如 Ehcache。
3. 对于缓存数据更新机制，当某一个作用域(一级缓存Session/二级缓存Namespaces)的进行了 C/U/D 操作后，默认该作用域下所有 select 中的缓存将被clear。
准备数据库
CREATE TABLE c_user(
id INT PRIMARY KEY AUTO_INCREMENT,
NAME VARCHAR(20),
age INT
);
INSERT INTO c_user(NAME, age) VALUES('Tom', 12);
INSERT INTO c_user(NAME, age) VALUES('Jack', 11);
![一、二级缓存（[尚硅谷]_张晓飞_Mybatis 学习笔记六）](http://img.it610.com/image/product/9fa1cfcb3d0a4b9ebe01c3f1e60d250e.png)
与数据库表对应的javabean， 需要序列化（用于二级缓存）
package com.hb.bean;
import java.io.Serializable;
public class User implements Serializable{
private int id;
private String name;
private int age;
private int minAge;
private int maxAge;
public int getMinAge() {
return minAge;
}
public void setMinAge(int minAge) {
this.minAge = minAge;
}
public int getMaxAge() {
return maxAge;
}
public void setMaxAge(int maxAge) {
this.maxAge = maxAge;
}
//get,set方法
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public User(int id, String name, int age) {
super();
this.id = id;
this.name = name;
this.age = age;
}
public User() {
}
@Override
public String toString() {
return "User [id=" + id + ", name=" + name + ", age=" + age + "]";
}
}
开启二级缓存使用<cache/>标签
<?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">
<!-- 这里namespace必须是“包名 + xml文件名”，不然要运行的时候要报错 “is not known to the MapperRegistry” -->
<mapper namespace="com.hb.bean.userMapper">
<cache/>
<select id="getUser" parameterType="int" resultType="com.hb.bean.User">
select *
from user where id=#{id}
</select>
<update id="updateUser" parameterType="com.hb.bean.User">
update user set
name=#{name}, age=#{age} where id=#{id}S
</update>
</mapper>
package com.hb.test5;
import java.io.InputStream;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;
import com.hb.bean.User;
/**
* 一级缓存测试
* @author Think
*
*/
public class Test5 {
public static SqlSessionFactory sqlSessionFactory;
public static SqlSession sqlSession;
@Before
public void init(){
//"/"表示是在src根目录下
String source = "config.xml";
InputStream inputStream = Test5.class.getClassLoader().getResourceAsStream(source);
try {
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
// sqlSession = sqlSessionFactory.openSession();
//是否自动提交，默认不是自动提交的
sqlSession = sqlSessionFactory.openSession(true);
} catch (Exception e) {
e.printStackTrace();
}
}
@Test
public void selectByCondition(){
/*
1. 必须是同一个Session,如果session对象已经close()过了就不可能用了
*/
String stament = "com.hb.bean.userMapper.getUser";
User user = sqlSession.selectOne(stament, 8);
System.out.println(user);
User user1 = sqlSession.selectOne(stament, 8);
System.out.println(user1);
//2015-02-24 14:21:18,717 [main] DEBUG [com.hb.bean.userMapper.getUser] - ooo Using Connection [com.mysql.jdbc.Connection@1f54031]
//2015-02-24 14:21:18,717 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Preparing: select * from user where id=?
//2015-02-24 14:21:18,780 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Parameters: 8(Integer)
//User [id=8, name=Tom, age=12]
//User [id=8, name=Tom, age=12]
}
@Test
public void selectByCondition1(){
/*
1. 必须是同一个Session,如果session对象已经close()过了就不可能用了
*/
String stament = "com.hb.bean.userMapper.getUser";
User user = sqlSession.selectOne(stament, 8);
System.out.println(user);
sqlSession.clearCache();
User user1 = sqlSession.selectOne(stament, 8);
System.out.println(user1);
//2015-02-24 14:21:40,560 [main] DEBUG [com.hb.bean.userMapper.getUser] - ooo Using Connection [com.mysql.jdbc.Connection@10cb191]
//2015-02-24 14:21:40,560 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Preparing: select * from user where id=?
//2015-02-24 14:21:40,623 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Parameters: 8(Integer)
//User [id=8, name=Tom, age=12]
//2015-02-24 14:21:40,670 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Parameters: 8(Integer)
//User [id=8, name=Tom, age=12]
}
@Test
public void selectByCondition2(){
/*
4. 没有执行过增删改的操作(这些操作都会清理缓存)
*/
String statement = "com.hb.bean.userMapper.getUser";
User user = sqlSession.selectOne(statement, 10);
System.out.println(user);
sqlSession.update("com.hb.bean.userMapper.updateUser",
new User(10, "user", 23));
User user1 = sqlSession.selectOne(statement, 10);
System.out.println(user1);
//2015-02-24 14:20:37,788 [main] DEBUG [com.hb.bean.userMapper.getUser] - ooo Using Connection [com.mysql.jdbc.Connection@5dae4f]
//2015-02-24 14:20:37,788 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Preparing: select * from user where id=?
//2015-02-24 14:20:37,851 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Parameters: 10(Integer)
//User [id=10, name=user, age=18]
//2015-02-24 14:20:37,882 [main] DEBUG [com.hb.bean.userMapper.updateUser] - ooo Using Connection [com.mysql.jdbc.Connection@5dae4f]
//2015-02-24 14:20:37,882 [main] DEBUG [com.hb.bean.userMapper.updateUser] - ==> Preparing: update user set name=?, age=? where id=?
//2015-02-24 14:20:37,882 [main] DEBUG [com.hb.bean.userMapper.updateUser] - ==> Parameters: user(String), 23(Integer), 10(Integer)
//2015-02-24 14:20:37,914 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Parameters: 10(Integer)
//User [id=10, name=user, age=23]
}
}
从selectByCondition()结果中可以看出， 两次调用getUserById方法，但是只有一次查询数据库的过程，这种现象产生的原因就是mybatis的一级缓存，并且一级缓存是默认开启的 。
package com.hb.test5;
import java.io.InputStream;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;
import com.hb.bean.User;
/**
* 二级缓存测试
* @author Think
*
*/
public class Test5_1 {
public static SqlSessionFactory sqlSessionFactory;
public static SqlSession sqlSession;
@Before
public void init(){
//"/"表示是在src根目录下
String source = "config.xml";
InputStream inputStream = Test5_1.class.getClassLoader().getResourceAsStream(source);
try {
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
// sqlSession = sqlSessionFactory.openSession();
//是否自动提交，默认不是自动提交的
sqlSession = sqlSessionFactory.openSession(true);
} catch (Exception e) {
e.printStackTrace();
}
}
@Test
public void selectByCondition(){
String statement = "com.hb.bean.userMapper.getUser";
SqlSession session = sqlSessionFactory.openSession();
User user = session.selectOne(statement, 8);
session.commit();
System.out.println("user="+user);
session.close();
SqlSession session2 = sqlSessionFactory.openSession();
user = session2.selectOne(statement, 8);
session2.commit();
System.out.println("user2="+user);
session2.close();
//2015-02-24 14:42:19,606 [main] DEBUG [com.hb.bean.userMapper.getUser] - ooo Using Connection [com.mysql.jdbc.Connection@11666b7]
//2015-02-24 14:42:19,606 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Preparing: select * from user where id=?
//2015-02-24 14:42:19,670 [main] DEBUG [com.hb.bean.userMapper.getUser] - ==> Parameters: 8(Integer)
//user=User [id=8, name=Tom, age=12]
//user2=User [id=8, name=Tom, age=12]
}
}
在mybatis配置文件中可以 开启全局二级缓存 。
<?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>
<!-- changes from the defaults for testing
<settings>
<setting name="cacheEnabled" value="false" />
<setting name="useGeneratedKeys" value="true" />
<setting name="defaultExecutorType" value="REUSE" />
</settings>
-->
<environments default="development">
<environment id="development">
<transactionManager type="jdbc" />
<dataSource type="POOLED">
<!--
<property name="driver" value="oracle.jdbc.driver.OracleDriver"/>
<property name="url" value="jdbc:oracle:thin:@192.168.56.201:1521:system"/>
-->
<property name="driver" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://localhost:3306/mybatis" />
<property name="username" value="root" />
<property name="password" value="123456" />
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/hb/bean/userMapper.xml" />
</mappers>
</configuration>
实体类实现可序列化接口:
public class User implements Serializable {...}
如果实体类不实现可序列化接口，使用二级缓存，那么会报下列异常:
org.apache.ibatis.cache.CacheException: Error serializing object. Cause: java.io.NotSerializableException:
