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mybatis 二级缓存

Mybatis读取缓存次序：

先从二级缓存中获取数据，如果有直接获取，如果没有进行下一步；
从一级缓存中取数据，有直接获取，如果没有进行下一步；
到数据库中进行查询，并保存到一级缓存中；
当sqlSession关闭的时候，把一级缓存中的数据保存在二级缓存中。

二级缓存的使用：

myBatis的二级缓存默认是不开启的。我们需要在mybatis的核心配置文件中配置setting选项和在Mapper的配置文件中加入cache标签

启用二缓存分两步：

1、在mybatis-config.xml中配置全局的参数

2.到Mapper配置文件中书写Cache标签

3、实体bean对象要实现java.io.Serializable接口

public class Clazz implements Serializable {
    private int id;
    private String name;
    private List<Student> stus;

没有实现java.io.Serializable接口的错误提示：

@Test
    public void testSecondCache1() {
        System.out.println("==================testSecondCache1()======================");
        SqlSession session = sqlSessionFactory.openSession();
        ClazzMapper clazzMapper = session.getMapper(ClazzMapper.class);
        Clazz clazz1 = clazzMapper.queryClazzById(1);
        System.out.println("clazz1 = " + clazz1);
        session.close();
        
        SqlSession session2 = sqlSessionFactory.openSession();
        ClazzMapper clazzMapper2 = session2.getMapper(ClazzMapper.class);
        Clazz clazz2 = clazzMapper2.queryClazzById(1);
        System.out.println("clazz2 = " + clazz2);
        session2.close();
    }

开启缓存后运行结果：

==================testSecondCache1()======================
DEBUG [main] - Cache Hit Ratio [com.soyoungboy.onetomany.mapper.ClazzMapper]: 0.0
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Created connection 1791868405.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - ==>  Preparing: select c.*,s.id stu_id,s.name stu_name from t_clazz c left join t_student s on c.id = s.clazz_id where c.id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 3
clazz1 = Clazz [id=1, name=javaEE20170228, stus=[Student [id=1, name=stu0228_张三, clazz=null], Student [id=2, name=stu0228_李四, clazz=null], Student [id=3, name=stu0228_王五, clazz=null]]]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - Returned connection 1791868405 to pool.
DEBUG [main] - Cache Hit Ratio [com.soyoungboy.onetomany.mapper.ClazzMapper]: 0.5
clazz2 = Clazz [id=1, name=javaEE20170228, stus=[Student [id=1, name=stu0228_张三, clazz=null], Student [id=2, name=stu0228_李四, clazz=null], Student [id=3, name=stu0228_王五, clazz=null]]]

常用的属性

userCache

　　userCache默认值是true,表示启用缓存。

　　如果设置useCache为false就表示不启用二级缓存,不影响一级缓存。

<!-- // 根据 id 查询 key的信息 -->
    <!-- public Key queryKeyById(int id); -->
    <select id="queryClazzById" parameterType="int"
        resultMap="queryClazzById_resultMap" useCache="false">
        select c.*,s.id
        stu_id,s.name stu_name from
        t_clazz c left join t_student s
        on
        c.id =
        s.clazz_id
        where
        c.id = #{id}

    </select>

运行结果：

结果：
==================testSecondCache1()======================
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Created connection 394714818.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1786dec2]
DEBUG [main] - ==>  Preparing: select c.*,s.id stu_id,s.name stu_name from t_clazz c left join t_student s on c.id = s.clazz_id where c.id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 3
clazz1 = Clazz [id=1, name=javaEE20170228, stus=[Student [id=1, name=stu0228_张三, clazz=null], Student [id=2, name=stu0228_李四, clazz=null], Student [id=3, name=stu0228_王五, clazz=null]]]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1786dec2]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@1786dec2]
DEBUG [main] - Returned connection 394714818 to pool.
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Checked out connection 394714818 from pool.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1786dec2]
DEBUG [main] - ==>  Preparing: select c.*,s.id stu_id,s.name stu_name from t_clazz c left join t_student s on c.id = s.clazz_id where c.id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 3
clazz2 = Clazz [id=1, name=javaEE20170228, stus=[Student [id=1, name=stu0228_张三, clazz=null], Student [id=2, name=stu0228_李四, clazz=null], Student [id=3, name=stu0228_王五, clazz=null]]]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1786dec2]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@1786dec2]
DEBUG [main] - Returned connection 394714818 to pool.

flushCache

flushCache属性默认值是true,在update标签，delete标签，insert标签中有。

表示当执行插入，修改，删除语句的时候，会清空缓存。

如果设置为false,很有肯能会产生脏数据。

xml文件配置：

<!-- // 根据 id 查询 key的信息 -->
    <!-- public Key queryKeyById(int id); -->
    <select id="queryClazzById" parameterType="int"
        resultMap="queryClazzById_resultMap" useCache="true">
        select c.*,s.id
        stu_id,s.name stu_name from
        t_clazz c left join t_student s
        on
        c.id =
        s.clazz_id
        where
        c.id = #{id}

    </select>

    <update id="updateClazz" parameterType="com.soyoungboy.onetomany.bean.Clazz"
        flushCache="false">
        update t_clazz set name =
        #{name} where id = #{id}
    </update>

测试的java代码：

@Test
    public void testSecondCacheUpdate() {
        System.out.println("======================================================");
        SqlSession session = sqlSessionFactory.openSession();
        ClazzMapper userMapper = session.getMapper(ClazzMapper.class);
         Clazz user = userMapper.queryClazzById(1);
        System.out.println(user);
        session.close();
        
        SqlSession session2 = sqlSessionFactory.openSession();
        ClazzMapper userMapper2 = session2.getMapper(ClazzMapper.class);
        userMapper2.updateClazz(new Clazz(1, "更新了"));
        session2.commit();
        session2.close();
        
        
        SqlSession session1 = sqlSessionFactory.openSession();
        ClazzMapper userMapper1 = session1.getMapper(ClazzMapper.class);
        user = userMapper1.queryClazzById(1);
        System.out.println(user);
        session1.close();
        System.out.println("======================================================");
    }

现在看看是否产生脏数据：

======================================================
DEBUG [main] - Cache Hit Ratio [com.soyoungboy.onetomany.mapper.ClazzMapper]: 0.0
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Created connection 1791868405.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - ==>  Preparing: select c.*,s.id stu_id,s.name stu_name from t_clazz c left join t_student s on c.id = s.clazz_id where c.id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 3
Clazz [id=1, name=javaEE20170228, stus=[Student [id=1, name=stu0228_张三, clazz=null], Student [id=2, name=stu0228_李四, clazz=null], Student [id=3, name=stu0228_王五, clazz=null]]]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - Returned connection 1791868405 to pool.
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Checked out connection 1791868405 from pool.
DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - ==>  Preparing: update t_clazz set name = ? where id = ? 
DEBUG [main] - ==> Parameters: 更新了(String), 1(Integer)
DEBUG [main] - <==    Updates: 1
DEBUG [main] - Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@6acdbdf5]
DEBUG [main] - Returned connection 1791868405 to pool.
DEBUG [main] - Cache Hit Ratio [com.soyoungboy.onetomany.mapper.ClazzMapper]: 0.5
Clazz [id=1, name=javaEE20170228, stus=[Student [id=1, name=stu0228_张三, clazz=null], Student [id=2, name=stu0228_李四, clazz=null], Student [id=3, name=stu0228_王五, clazz=null]]]
如上所示，当usecache为true ,flushcache为false,不清空的时候就会出现脏数据

中间更新了数据为"更新了",但是接下来数据并没有读取更新后的数据，而是读取缓存中原有的数据，即脏数据。

<cache></cache>标签的介绍和说明undefined

默认的<cache/>标签的作用:

1、映射语句文件中的所有 select 语句将会被缓存。

2、射语句文件中的所有 insert,update 和 delete 语句会刷新缓存。

3、缓存会使用 Least Recently Used(LRU,最近最少使用的)算法来收回。

4、根据时间表(比如 no Flush Interval,没有刷新间隔), 缓存不会以任何时间顺序来刷新。

5、缓存会存储列表集合或对象(无论查询方法返回什么)的 1024 个引用。

6、缓存会被视为是 read/write(可读/可写)的缓存,意味着对象检索不是共享的,而且可以安全地被调用者修改,而不干扰其他调用者或线程所做的潜在修改。

cache标签示例解析：

<cache eviction="FIFO" flushInterval="60000" size="512" readOnly="true"/>

eviction 属性表示缓存策略。

LRU – 最近最少使用的:移除最长时间不被使用的对象。
FIFO – 先进先出:按对象进入缓存的顺序来移除它们。
SOFT – 软引用:移除基于垃圾回收器状态和软引用规则的对象。
WEAK – 弱引用:更积极地移除基于垃圾收集器状态和弱引用规则的对象。

默认的是 LRU。

flushInterval 属性表示间隔多长时间刷新一下缓冲区，清理一下溢出的数据。以毫秒为单位。

size 属性表示缓存中可以保存多少个对象。默认是1024。

readOnly 属性表示是否只读。如果设置为true。表示缓存中只有一个对象。如果设置为false（默认）每次取出来都会反序列化拷贝一份。

type 属性表示自定义二级缓存对象。

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原文地址：https://www.cnblogs.com/androidsuperman/p/7327668.html