1. RoutingStatementHandler类

该类的构造函数代码如下所示：

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

这里使用了策略模式，MappedStatement 参数的statementType为必传参数，通过该参数进行实例化策略类。StatementHandler为抽象策略类，SimpleStatementHandler、PreparedStatementHandler、CallableStatementHandler为具体策略类，RoutingStatementHandler为环境类。
除了使用到了策略模式外，还用到了模板方法模式，抽象模板类是BaseStatementHandler，模板方法是prepare，该方法是获取Statement接口的对象的，抽象方法是instantiateStatement，具体方法setStatementTimeout、setFetchSize、closeStatement。

2. 枚举类StatementType

public enum StatementType {
  STATEMENT, PREPARED, CALLABLE
}

枚举类StatementType 分别对应普通的Statement、PreparedStatement、CallableStatement，关联的处理器就是SimpleStatementHandler、PreparedStatementHandler、CallableStatementHandler，当需要执行存储过程时，将会使用CallableStatementHandler。