mybatis 源码分析（四）一二级缓存分析

本篇博客主要讲了 mybatis 一二级缓存的构成，以及一些容易出错地方的示例分析；

一、mybatis 缓存体系

mybatis 的一二级缓存体系大致如下：

• 首先当一二级缓存同时开启的时候，首先命中二级缓存；
• 一级缓存位于 BaseExecutor 中不能关闭，但是可以指定范围 STATEMENT、SESSION；
• 整个二级缓存虽然经过了很多事务相关的组件，但是最终是落地在 MapperStatement 的 Cache 中（Cache 的具体实例类型可以在 mapper xml 的 cache type 标签中指定，默认 PerpetualCache），而 MapperStatement 和 namespace 一一对应，所以二级缓存的作用域是 mapper namespace；
• 在使用二级缓存的时候，如果 cache 没有命中则向后查找，然后查询的结果不是直接放到 cache 中，而是首先放到 TransactionCache 的本地缓存中，这里区分 entriesToAddOnCommit、entriesMissedInCache 是为了统计命令率，最后在 sqlSession commit 的时候，才会将 TransactionCache 的本地缓存提交到 cache 中，此时 cache 才是对其他 sqlSession 可见的；
• 此外当需要分布式缓存的时候，就需要将二级缓存放到 JVM 之外，这里可以实现 cache 接口编写自己的 cache，此时在实现的 cache 中就可以使用 ehcache、redis 等外部缓存进行操作；

以上就大致是 mybatis 缓存的整体结构，下面将分模块拆分测试一二级缓存；

二、一级缓存

mybatis 的一级缓存一般情况很少使用，其原因主要有两个：

• 一级缓存的生命周期同 SqlSession，所以容易出现脏读；
• 一级缓存的 cache 的实现只能是 PerpetualCache，所以不能指定容量等设置；

1. 脏读测试

指定一级缓存范围为 SESSION：

<setting name="localCacheScope" value="SESSION"/>

@Test
public void test01() {
  SqlSessionFactory sqlSessionFactory = DBUtils.getSessionFactory();
  try (
    SqlSession sqlSession1 = sqlSessionFactory.openSession(true);
    SqlSession sqlSession2 = sqlSessionFactory.openSession(true);
  ) {
    UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
    UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
    log.info("---get: {}", userMapper1.getUser(1L));
    log.info("---get: {}", userMapper2.getUser(1L));
    log.info("---update: {}", userMapper1.setNameById(1L, "LiSi"));
    log.info("---get: {}", userMapper1.getUser(1L));
    log.info("---get: {}", userMapper2.getUser(1L));
  }
}

结果如下：

[DEBUG] sanzao.db.UserMapper.getUser - ==>  Preparing: select * from user where id = ?
[DEBUG] sanzao.db.UserMapper.getUser - ==> Parameters: 1(Long)
[TRACE] sanzao.db.UserMapper.getUser - <==    Columns: id, username, password, address
[TRACE] sanzao.db.UserMapper.getUser - <==        Row: 1, ZhangSan, 123456, TT
[DEBUG] sanzao.db.UserMapper.getUser - <==      Total: 1
[INFO] sanzao.Test01 - ---get: User{id=1, user_name='ZhangSan', password='123456', address='TT'}
[DEBUG] org.apache.ibatis.transaction.jdbc.JdbcTransaction - Opening JDBC Connection
[DEBUG] org.apache.ibatis.datasource.pooled.PooledDataSource - Created connection 61073295.
[DEBUG] sanzao.db.UserMapper.getUser - ==>  Preparing: select * from user where id = ?
[DEBUG] sanzao.db.UserMapper.getUser - ==> Parameters: 1(Long)
[TRACE] sanzao.db.UserMapper.getUser - <==    Columns: id, username, password, address
[TRACE] sanzao.db.UserMapper.getUser - <==        Row: 1, ZhangSan, 123456, TT
[DEBUG] sanzao.db.UserMapper.getUser - <==      Total: 1
[INFO] sanzao.Test01 - ---get: User{id=1, user_name='ZhangSan', password='123456', address='TT'}
[DEBUG] sanzao.db.UserMapper.setNameById - ==>  Preparing: update user set username = ? where id = ?
[DEBUG] sanzao.db.UserMapper.setNameById - ==> Parameters: LiSi(String), 1(Long)
[DEBUG] sanzao.db.UserMapper.setNameById - <==    Updates: 1
[INFO] sanzao.Test01 - ---update: 1
[DEBUG] sanzao.db.UserMapper.getUser - ==> Parameters: 1(Long)
[TRACE] sanzao.db.UserMapper.getUser - <==    Columns: id, username, password, address
[TRACE] sanzao.db.UserMapper.getUser - <==        Row: 1, LiSi, 123456, TT
[DEBUG] sanzao.db.UserMapper.getUser - <==      Total: 1
[INFO] sanzao.Test01 - ---get: User{id=1, user_name='LiSi', password='123456', address='TT'}
[INFO] sanzao.Test01 - ---get: User{id=1, user_name='ZhangSan', password='123456', address='TT'}

可以看到当 sqlSession1 更新的时候，sqlSession2 的缓存仍然有效所以出现了脏读；所以通常都设置一级缓存的范围为：STATEMENT；

2. 源码分析

mybatis 的一级缓存主要和 Executor 整合比较多，所以建议先查看我上一篇博客 Executor 详解 ，详细了解缓存命中的整体流程；这里一级缓存的源码也很简单：

• 查询的时候，首先查缓存，命中则返回，未命中就查数据库，然后填充缓存；
• 更新、提交等操作情况缓存；

@SuppressWarnings("unchecked")
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
  ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
  if (closed) { throw new ExecutorException("Executor was closed."); }

  // 查询的时候一般不清楚缓存，但是可以通过 xml配置或者注解强制清除，queryStack == 0 是为了防止递归调用
  if (queryStack == 0 && ms.isFlushCacheRequired()) { clearLocalCache(); }
  List<E> list;
  try {
    queryStack++;
    // 首先查看一级缓存
    list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
    if (list != null) {
      handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
    } else {
      // 没有查到的时候直接到数据库查找
      list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }
  } finally {
    queryStack--;
  }
  if (queryStack == 0) {
    // 延迟加载队列
    for (DeferredLoad deferredLoad : deferredLoads) {
      deferredLoad.load();
    }
    deferredLoads.clear();
    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
  	  // 一级缓存本身不能关闭，但是可以设置作用范围 STATEMENT，每次都清除缓存
      clearLocalCache();
    }
  }
  return list;
}

三、二级缓存

mybatis 二级缓存要稍微复杂一点，中间多了一步事务缓存：

• 首先无论是查询还是更新，都会按要求清空缓存 flushCacheIfRequired，默认更新清空，查询不清空，也可以在 xml 或者注解中指定；
• 查询的时候，先查缓存，命中返回，未命中查一级缓存、数据库，然后回填事务缓存，注意这里不是直接填充到缓存中；此时的事务缓存对任何的 SqlSession 都是不可见的，因为自己查询的时候也是直接查询的目标缓存；
• 更新就直接委托给目标 Executor 执行；
• 最后 SqlSession 执行commit 的时候，将事务缓存刷新到目标缓存中；

1. 事务缓存测试

设置二级缓存：

<setting name="cacheEnabled" value="true"/>

<mapper namespace="***">
  <cache/>
</mapper>

@Test
public void test02() {
  SqlSessionFactory sqlSessionFactory = DBUtils.getSessionFactory();
  try (
    SqlSession sqlSession1 = sqlSessionFactory.openSession(true);
    SqlSession sqlSession2 = sqlSessionFactory.openSession(true);
  ) {
    UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
    UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);

    User u1 = userMapper1.getUser(1L);
    System.out.println("---get u1: " + u1);

    User u2 = userMapper2.getUser(1L);
    System.out.println("---get u2: " + u2);

    User u3 = userMapper1.getUser(1L);
    System.out.println("---get u3: " + u3);
  }
}

打印：

DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.0
DEBUG [main] - ==>  Preparing: select * from user where id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <==      Total: 1
---get u1: User{id=1, user_name='sanzao', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.0
DEBUG [main] - Opening JDBC Connection
DEBUG [main] - Created connection 1613095350.
DEBUG [main] - ==>  Preparing: select * from user where id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <==      Total: 1
---get u2: User{id=1, user_name='sanzao', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.0
---get u3: User{id=1, user_name='sanzao', password='123456', address='TT'}

可以看到：

• SqlSession1 为提交事务缓存，所以 SqlSession2 又从数据库中查了一次；
• 当SqlSession1 再次查询的时候，二级缓存未命中 Cache Hit Ratio 为 0，但是命中了一级缓存，所以并未再查数据库；

2. 二级缓存测试

这次我们提交缓存看看是否命中：

@Test
public void test03() {
  SqlSessionFactory sqlSessionFactory = DBUtils.getSessionFactory();
  try (
    SqlSession sqlSession1 = sqlSessionFactory.openSession(true);
    SqlSession sqlSession2 = sqlSessionFactory.openSession(true);
  ) {
    UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
    UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);

    User u1 = userMapper1.getUser(1L);
    System.out.println("---get u1: " + u1);
    sqlSession1.commit();

    User u2 = userMapper2.getUser(1L);
    System.out.println("---get u2: " + u2);

    int i = userMapper1.setNameById(1L, "LiSi");
    System.out.println("---update user: " + i);
    sqlSession1.commit();

    User u3 = userMapper1.getUser(1L);
    System.out.println("---get u3: " + u3);
    sqlSession1.commit();

    User u4 = userMapper2.getUser(1L);
    System.out.println("---get u4: " + u4);
  }
}

打印：

DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.0
DEBUG [main] - ==>  Preparing: select * from user where id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <==      Total: 1
---get u1: User{id=1, user_name='sanzao', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.5
---get u2: User{id=1, user_name='sanzao', password='123456', address='TT'}
DEBUG [main] - ==>  Preparing: update user set username = ? where id = ?
DEBUG [main] - ==> Parameters: LiSi(String), 1(Long)
DEBUG [main] - <==    Updates: 1
---update user: 1
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.3333333333333333
DEBUG [main] - ==>  Preparing: select * from user where id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <==      Total: 1
---get u3: User{id=1, user_name='LiSi', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.5
---get u4: User{id=1, user_name='LiSi', password='123456', address='TT'}

这次就能看到当 SqlSession1 提交事务缓存后，SqlSession2 就能看到了；

3. 缓存配置测试

此外还可以配置各种二级缓存策略，比如大小，刷新间隔时间，淘汰策略等，这里主要就是使用了 Cache 接口的装饰者模式：

• LRU – 最近最少使用：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：基于垃圾回收器状态和软引用规则移除对象。
• WEAK – 弱引用：更积极地基于垃圾收集器状态和弱引用规则移除对象。

但是需要注意的是这里的策略也能用户本地缓存，对于分布式缓存有些策略还是有问题；比如：

<cache eviction="FIFO" flushInterval="60000" size="2" readOnly="true"/>

这里主要定义了缓存大小2，使用 FIFO 策略更新；

@Test
public void test04() {
  SqlSessionFactory sqlSessionFactory = DBUtils.getSessionFactory();
  try (
    SqlSession sqlSession1 = sqlSessionFactory.openSession(true);
    SqlSession sqlSession2 = sqlSessionFactory.openSession(true);) {
    UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
    UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);

    System.out.println("---get user: " + userMapper1.getUser(1L));
    sqlSession1.commit();

    System.out.println("---get user: " + userMapper1.getUser(2L));
    sqlSession1.commit();

    System.out.println("---get user: " + userMapper1.getUser(3L));
    sqlSession1.commit();

    System.out.println("---get user: " + userMapper2.getUser(1L));

    System.out.println("---get user: " + userMapper2.getUser(2L));

    System.out.println("---get user: " + userMapper1.getUser(1L));
    sqlSession2.commit();

    System.out.println("------------");
    System.out.println("---get user: " + userMapper1.getUser(1L));
    System.out.println("---get user: " + userMapper1.getUser(2L));
    System.out.println("---get user: " + userMapper1.getUser(3L));
  }
}

打印：

DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.0
DEBUG [main] - ==>  Preparing: select * from user where id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <==      Total: 1
---get user: User{id=1, user_name='s1', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.0
DEBUG [main] - ==>  Preparing: select * from user where id = ?
DEBUG [main] - ==> Parameters: 2(Long)
DEBUG [main] - <==      Total: 1
---get user: User{id=2, user_name='s2', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.0
DEBUG [main] - ==>  Preparing: select * from user where id = ?
DEBUG [main] - ==> Parameters: 3(Long)
DEBUG [main] - <==      Total: 1
---get user: User{id=3, user_name='s3', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.0
DEBUG [main] - ==>  Preparing: select * from user where id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <==      Total: 1
---get user: User{id=1, user_name='s1', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.2
---get user: User{id=2, user_name='s2', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.16666666666666666
DEBUG [main] - ==>  Preparing: select * from user where id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <==      Total: 1
---get user: User{id=1, user_name='s1', password='123456', address='TT'}
------------
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.2857142857142857
---get user: User{id=1, user_name='s1', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.25
DEBUG [main] - ==> Parameters: 2(Long)
DEBUG [main] - <==      Total: 1
---get user: User{id=2, user_name='s2', password='123456', address='TT'}
DEBUG [main] - Cache Hit Ratio [sanzao.db.UserMapper]: 0.3333333333333333
---get user: User{id=3, user_name='s3', password='123456', address='TT'}

从日志中可以看到对于 SqlSession1，大小2，FIFO 是生效的，但是 SqlSession2 提交了之后，就发现缓存 s1，s2，s3 都命中了；

至于源码太多了就不一次分析了，对于上面说的使用装饰者模式，可以在 CacheBuilder 中看到；

public Cache build() {
  setDefaultImplementations();
  Cache cache = newBaseCacheInstance(implementation, id);
  setCacheProperties(cache);
  // issue #352, do not apply decorators to custom caches
  if (PerpetualCache.class.equals(cache.getClass())) {
    for (Class<? extends Cache> decorator : decorators) {
      cache = newCacheDecoratorInstance(decorator, cache);
      setCacheProperties(cache);
    }
    cache = setStandardDecorators(cache);
  } else if (!LoggingCache.class.isAssignableFrom(cache.getClass())) {
    cache = new LoggingCache(cache);
  }
  return cache;
}

总结

• mybatis 一级缓存的生命周期和 SqlSession 是一样的，通常情况下不建议使用一级缓存，通常将一级缓存范围设置为 STATEMENT；
• 使用 mybatis 二级的时候，务必记得 SqlSession.commit ，否则二级缓存是不生效的；
• 在配置 mybatis 分布式二级缓存的时候，要确保缓存淘汰等策略是可以用于分布式缓存的；