模板方法模式的应用之 MyBatis BaseExecutor

Executor 接口是 MyBatis 最核心的接口之一，它定义了执行 SQL 语句的方法，包括查询、更新等操作：

public interface Executor {

  ResultHandler NO_RESULT_HANDLER = null;

  int update(MappedStatement ms, Object parameter) throws SQLException;

  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;

  // ...
}

BaseExecutor 类是 Executor 接口的抽象实现类，它定义了 Executor 接口需要实现的大部分方法，包括对 SQL 语句的更新和查询等操作，并提供了抽象方法 doUpdate() 和 doQuery()，供子类实现：

public abstract class BaseExecutor implements Executor {
  @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();
    // ⭐ 执行 update 语句（子类实现）
    return doUpdate(ms, parameter);
  }

  /**
   * ⭐ 执行 update 语句（子类实现）
   */
  protected abstract int doUpdate(MappedStatement ms, Object parameter)
      throws SQLException;

  @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 {
        // ⭐ 缓存未命中，从数据库中查询数据
        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;
  }

  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 {
      // ⭐ 从数据库中查询数据（子类实现）
      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;
  }

  /**
   * ⭐ 从数据库中查询数据（子类实现）
   */
  protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
      throws SQLException;
}

Executor 的实现类 SimpleExecutor、ClosedExecutor、ReuseExecutor、BatchExecutor 都继承自 BaseExecutor，并提供了具体的实现方法 doUpdate() 和 doQuery()。比如 SimpleExecutor 的 doUpdate 方法如下：

@Override
public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
  Statement stmt = null;
  try {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
    stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.update(stmt);
  } finally {
    closeStatement(stmt);
  }
}

可以看到，最终调用的是 StatementHandler 的 update() 方法。