BatchExecutor：顾名思义就是进行批量操作，通过批量操作来提高性能

public class BatchExecutor extends BaseExecutor {

  public static final int BATCH_UPDATE_RETURN_VALUE = Integer.MIN_VALUE + 1002;

  /* Statement链表**/
  private final List<Statement> statementList = new ArrayList<Statement>();

  /* batch结果链表**/
  private final List<BatchResult> batchResultList = new ArrayList<BatchResult>();
  private String currentSql;
  private MappedStatement currentStatement;

  public BatchExecutor(Configuration configuration, Transaction transaction) {
    super(configuration, transaction);
  }

  @Override
  public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
	//获得配置信息
    final Configuration configuration = ms.getConfiguration();
	//获得StatementHandler
    final StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, null, null);
    final BoundSql boundSql = handler.getBoundSql();
	//获得Sql语句
    final String sql = boundSql.getSql();
    final Statement stmt;
	//如果sql语句等于当前sql MappedStatement 等于当前Map碰到Statement
    if (sql.equals(currentSql) && ms.equals(currentStatement)) {

      int last = statementList.size() - 1;
	  //获得最后一个
      stmt = statementList.get(last);
	  handler.parameterize(stmt);//fix Issues 322
	  //有相同的MappedStatement和参数
      BatchResult batchResult = batchResultList.get(last);
      batchResult.addParameterObject(parameterObject);
    } else {
	  //如果不存在就创建一个批处理操作
      Connection connection = getConnection(ms.getStatementLog());
      stmt = handler.prepare(connection);
      handler.parameterize(stmt);    //fix Issues 322
      currentSql = sql;
      currentStatement = ms;
	  //添加批量处理操作
      statementList.add(stmt);
      batchResultList.add(new BatchResult(ms, sql, parameterObject));
    }
  // handler.parameterize(stmt);
    //最终是调用jdbc的批处理操作
    handler.batch(stmt);
    return BATCH_UPDATE_RETURN_VALUE;
  }

  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
      throws SQLException {
    Statement stmt = null;
    try {
      flushStatements();
	  //获得配置信息
      Configuration configuration = ms.getConfiguration();
	  //获得StatementHandler
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);
	  //获得连接
      Connection connection = getConnection(ms.getStatementLog());
      stmt = handler.prepare(connection);
	  //获得Statement
      handler.parameterize(stmt);
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

  /* 刷新Statement，记录执行次数*/
  @Override
  public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
    try {
      List<BatchResult> results = new ArrayList<BatchResult>();
      if (isRollback) {
        return Collections.emptyList();
      }
	  //如果进行了批量处理
      for (int i = 0, n = statementList.size(); i < n; i++) {
        Statement stmt = statementList.get(i);
        BatchResult batchResult = batchResultList.get(i);
        try {
		  //记录批量处理执行操作的条数
          batchResult.setUpdateCounts(stmt.executeBatch());
          MappedStatement ms = batchResult.getMappedStatement();
		  //参数对象集合
          List<Object> parameterObjects = batchResult.getParameterObjects();
		  //生成key
          KeyGenerator keyGenerator = ms.getKeyGenerator();
          if (Jdbc3KeyGenerator.class.equals(keyGenerator.getClass())) {
            Jdbc3KeyGenerator jdbc3KeyGenerator = (Jdbc3KeyGenerator) keyGenerator;
            jdbc3KeyGenerator.processBatch(ms, stmt, parameterObjects);
          } else if (!NoKeyGenerator.class.equals(keyGenerator.getClass())) { //issue #141
            for (Object parameter : parameterObjects) {
              keyGenerator.processAfter(this, ms, stmt, parameter);
            }
          }
        } catch (BatchUpdateException e) {
          StringBuilder message = new StringBuilder();
          message.append(batchResult.getMappedStatement().getId())
              .append(" (batch index #")
              .append(i + 1)
              .append(")")
              .append(" failed.");
          if (i > 0) {
            message.append(" ")
                .append(i)
                .append(" prior sub executor(s) completed successfully, but will be rolled back.");
          }
          throw new BatchExecutorException(message.toString(), e, results, batchResult);
        }
		//记录操作
        results.add(batchResult);
      }
      return results;
    } finally {
      for (Statement stmt : statementList) {
        closeStatement(stmt);
      }
      currentSql = null;
      statementList.clear();
      batchResultList.clear();
    }
  }

}