读mybatis源码之六：执行器Executor之基类BaseExecutor

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在sqlsession中主要是执行都是通过executor来处理的，具体执行逻辑，大部分在BaseExecutor里面，使用的模板模式，

一、query：

public <E> List<E> 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); }

BoundSql ：sql语句，参数等

CacheKey ：缓存key，通过mapperID,sql、参数等产生缓存key

 @SuppressWarnings("unchecked")  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();      }      deferredLoads.clear(); // issue #601      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {        clearLocalCache(); // issue #482      }    }    return list;  }

可以看见：

1、先查本地缓存，没有再去查数据库，注意这里的本地缓存是同一个session内的缓存，也就是同一个opensession内。

2、通过configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT来看，可以设置参数，将本地缓存去掉，不使用本地缓存。

 private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {    List<E> list;    localCache.putObject(key, EXECUTION_PLACEHOLDER);    try {      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);    } finally {      localCache.removeObject(key);    }    localCache.putObject(key, list);    if (ms.getStatementType() == StatementType.CALLABLE) {      localOutputParameterCache.putObject(key, parameter);    }    return list;  }

本地缓存先占位，后更新。

使用不同的执行器去做查询动作，具体的doQuery由不同的执行去具体实现

二、update

 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);  }

清除本地缓存，做更新操作，具体更新操作在具体执行器中。

各大执行器主要需要写自己的：

 protected abstract int doUpdate(MappedStatement ms, Object parameter)      throws SQLException;  protected abstract List<BatchResult> doFlushStatements(boolean isRollback)      throws SQLException;  protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)      throws SQLException;

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