MyBatis的缓存

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;

    @Override
    public 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 mapper
        PUBLIC "-//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;
    @Before
    public void init() throws IOException {
        //加载配置文件
        String resource = "mybatis-config.xml";
        inputStream = Resources.getResourceAsStream(resource);
        //创建SessionFactory
        sqlSessionFactory= 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)
    @Test
    public 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)
    @Test
    public void testNoGoCache(){
        //SqlSession sqlSession = sessionFactory.openSession();
        SqlSession sqlSession1 =sqlSessionFactory.openSession();
        SqlSession sqlSession2 =sqlSessionFactory.openSession();
        //拥有不同的sqlsession
        UserDao 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)
    @Test
    public void testNoGoCache2(){
        //SqlSession sqlSession = sessionFactory.openSession();
        //拥有同一个sqlsession
        UserDao 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);
    }

        @After
    public 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;
        @Before
        public void init() throws IOException {
            //加载配置文件
            String resource = "mybatis-config.xml";
            inputStream = Resources.getResourceAsStream(resource);
            //创建SessionFactory
            sqlSessionFactory= new SqlSessionFactoryBuilder().build(inputStream);
            //使用数据的会话实例
            sqlSession = sqlSessionFactory.openSession();
        }
        //二级缓存不执行sql
    @Test
    public void testGoCache(){
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        SqlSession sqlSession2 = sqlSessionFactory.openSession();
        //拥有相同的sqlSessionFactrory
        UserDao 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 = UserDao2.findUserById(41);//执行sql？不执行sql
        System.out.println(user2);
    }
    //二级缓存执行sql(不同的sqlSessionFactrory)
    @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
        UserDao 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.findUserById(41);//执行sql？执行sql
        System.out.println(user2);
    }
    //二级缓存执行sql(相同的sqlSessionFactrory)
    @Test
    public void testNoGoCache2(){
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        SqlSession sqlSession2 = sqlSessionFactory.openSession();
        //拥有相同的sqlSessionFactrory
        UserDao 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.findUserById(41);//执行sql？执行sql
        System.out.println(user2);
    }
        @After
        public void close() throws IOException {
            sqlSession.close();
            inputStream.close();
        }
    }

二级缓存不执行sql