Mybatis的缓存

Mybatis的缓存氛围二级缓存和一级缓存，其中一级缓存默认开启，二级缓存默认不开启。

一级缓存的具体实现在了BaseExecutor类里面。

在执行一条查询语句的时候，第一次设计到一级缓存的时候是在BaseExecutor的query()方法里面，这里为这次查询生成了相应的key用来在缓存中定位。

@Override
public  List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
  BoundSql boundSql = ms.getBoundSql(parameter);
  CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
  return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}

在这里，调用了createCacheKey()方法生成了缓存的key。

@Override
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
  if (closed) {
    throw new ExecutorException("Executor was closed.");
  }
  CacheKey cacheKey = new CacheKey();
  cacheKey.update(ms.getId());
  cacheKey.update(rowBounds.getOffset());
  cacheKey.update(rowBounds.getLimit());
  cacheKey.update(boundSql.getSql());
  List parameterMappings = boundSql.getParameterMappings();
  TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
  // mimic DefaultParameterHandler logic
  for (ParameterMapping parameterMapping : parameterMappings) {
    if (parameterMapping.getMode() != ParameterMode.OUT) {
      Object value;
      String propertyName = parameterMapping.getProperty();
      if (boundSql.hasAdditionalParameter(propertyName)) {
        value = boundSql.getAdditionalParameter(propertyName);
      } else if (parameterObject == null) {
        value = null;
      } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
        value = parameterObject;
      } else {
        MetaObject metaObject = configuration.newMetaObject(parameterObject);
        value = metaObject.getValue(propertyName);
      }
      cacheKey.update(value);
    }
  }
  if (configuration.getEnvironment() != null) {
    // issue #176
    cacheKey.update(configuration.getEnvironment().getId());
  }
  return cacheKey;
}

其中CacheKey的update()方法主要是根据传入的对象的hashcode，更新这个key的hashcode，可以看到这里的缓存key的生成涉及到了mapperStatement的id，具体的sql，sql的分页情况以及涉及到的参数和配置环境的id，根据上述成员完成对于单次查询唯一性的确定，保证之后缓存结果的获取可以准确无误。

在完成cacheKey的获取之后，将会在query()具体对一级缓存进行操作。

@SuppressWarnings("unchecked")
@Override
public  List 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 list;
  try {
    queryStack++;
    list = resultHandler == null ? (List) 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;
}

 

在这里可以看到，如果能够在localCache中通过CacheKey得到相应的结果，那么直接会从这里的缓存得到结果并返回，反之，则将正式执行查询的sql语句。一级缓存就在这里发挥了作用。

 

这里可以看到，一级缓存的容器实则是BaseExecutor的map，而可以看到DefaultSqlSessionFactory生成session的方法。

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    final Environment environment = configuration.getEnvironment();
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    final Executor executor = configuration.newExecutor(tx, execType);
    return new DefaultSqlSession(configuration, executor, autoCommit);
  } catch (Exception e) {
    closeTransaction(tx); // may have fetched a connection so lets call close()
    throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

 

 

每一个session是与一个executor对应的，那么由此可见一级缓存是不同的session之间隔离的。

 

 

然而，一级缓存会随着每一次update()而清空。

@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);
}

可见BaseExecutor中的update()方法，每次执行前都会清空本地的缓存，所以一级缓存会随着更新操作而清楚。

 

 

二级缓存默认不开启，二级缓存的开启需要在配置中显示的配置CacheEnable参数，当这个参数配置为true的时候，在创建新的executor的时候，会对executor加上一层包装类CachingExecutor达到实现二级缓存的目的。

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
  executorType = executorType == null ? defaultExecutorType : executorType;
  executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
  Executor executor;
  if (ExecutorType.BATCH == executorType) {
    executor = new BatchExecutor(this, transaction);
  } else if (ExecutorType.REUSE == executorType) {
    executor = new ReuseExecutor(this, transaction);
  } else {
    executor = new SimpleExecutor(this, transaction);
  }
  if (cacheEnabled) {
    executor = new CachingExecutor(executor);
  }
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}

 

二级缓存发生的地方也就是在于CachingExecutor的query()方法里面。

 

@Override
public  List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
    throws SQLException {
  Cache cache = ms.getCache();
  if (cache != null) {
    flushCacheIfRequired(ms);
    if (ms.isUseCache() && resultHandler == null) {
      ensureNoOutParams(ms, boundSql);
      @SuppressWarnings("unchecked")
      List list = (List) tcm.getObject(cache, key);
      if (list == null) {
        list = delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
        tcm.putObject(cache, key, list); // issue #578 and #116
      }
      return list;
    }
  }
  return delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

 

用这里可以看到，这里的缓存的获取是针对一个MappedStatement的，而一个MappedStatement所对应的一般是一条sql语句，也就是说，二级缓存对应的是一条sql语句，再根据生成的CacheKey在二级缓存中寻找结果，并且可以看到，开启了二级缓存之后，只有二级缓存找不到才会接续下一层的query()方法的调用，由此可见，二级缓存的优先级高于一级缓存。

 

如果开启了二级缓存没有查到，那么在经过查到结果之后，会调用putObject()方法来提交结果。

public void putObject(Cache cache, CacheKey key, Object value) {
  getTransactionalCache(cache).putObject(key, value);
}

 

这里可以看到，这里根据这个MappedStatement的cache更新的缓存只是还是针对单个session的，此时，并没有保证整个MappedStatement的共享。只有在调用了commit()方法才会保证二级缓存在同一个sql下面的共享。

 

private void flushPendingEntries() {
  for (Map.Entry entry : entriesToAddOnCommit.entrySet()) {
    delegate.putObject(entry.getKey(), entry.getValue());
  }
  for (Object entry : entriesMissedInCache) {
    if (!entriesToAddOnCommit.containsKey(entry)) {
      delegate.putObject(entry, null);
    }
  }
}