mybatis源码-一级缓存

流程图：

1、进入query方法

queryStack嵌套查询使用到的层级

@Override
public Object getObject(Object key) {
  return cache.get(key);
}

private final Map<Object, Object> cache = new HashMap<>();

可以看到一级缓存底层就是一个hashmap作为缓存

缓存的key定义：

由6个部分组成：

1、statementId

2、分页条件：Integer的最小值0

3、分页条件：Interget的最大值2147483647

4、sql语句

5、参数

6、环境变量

第一次查询，缓存为null，那么就走doQuery查询数据库并填充缓存

第二次查询，直接获取缓存中的数据

验证上一节刷新缓存场景：mybatis源码-一级缓存命中的场景

1、update后会清空缓存

@Override
public int update(MappedStatement ms, Object parameter) throws SQLException {
  ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
  if (closed) {
    throw new ExecutorException("Executor was closed.");
  }
  clearLocalCache();
  return doUpdate(ms, parameter);
}

2、配置flushCache=true会清空缓存

@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.");
  }
  if (queryStack == 0 && ms.isFlushCacheRequired()) {
    clearLocalCache();
  }
  List<E> list;
  try {
    queryStack++;
    list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
    if (list != null) {
      handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
    } else {
      list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }
  } finally {
    queryStack--;
  }
  if (queryStack == 0) {
    for (DeferredLoad deferredLoad : deferredLoads) {
      deferredLoad.load();
    }
    // issue #601
    deferredLoads.clear();
    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
      // issue #482
      clearLocalCache();
    }
  }
  return list;
}

if (queryStack == 0 && ms.isFlushCacheRequired()) {
    clearLocalCache();
  }

这段代码表示：如果查询是第一次（不是子查询），并且配置了flushCache=true，那么清空缓存

3、配置localCacheScope=STATEMENT会清空缓存

if (queryStack == 0) {
  for (DeferredLoad deferredLoad : deferredLoads) {
    deferredLoad.load();
  }
  // issue #601
  deferredLoads.clear();
  if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
    // issue #482
    clearLocalCache();
  }
}

这段代码表示：如果查询是第一次（不是子查询），并且配置了localCacheScope=STATEMENT，那么清空缓存

4、commit、rollback操作会清空缓存

@Override
public void commit(boolean required) throws SQLException {
  if (closed) {
    throw new ExecutorException("Cannot commit, transaction is already closed");
  }
  clearLocalCache();
  flushStatements();
  if (required) {
    transaction.commit();
  }
}

@Override
public void rollback(boolean required) throws SQLException {
  if (!closed) {
    try {
      clearLocalCache();
      flushStatements(true);
    } finally {
      if (required) {
        transaction.rollback();
      }
    }
  }
}