HttpServletRequest的getInputStream方法和getParameter方法

根据Servlet规范，如果同时满足下列条件，则请求体(Entity)中的表单数据，将被填充到request的parameter集合中（request.getParameter系列方法可以读取相关数据）：

1 这是一个HTTP/HTTPS请求

2 请求方法是POST（querystring无论是否POST都将被设置到parameter中）

3 请求的类型（Content-Type头）是application/x-www-form-urlencoded

4 Servlet调用了getParameter系列方法

如果上述条件没有同时满足，则相关的表单数据不会被设置进request的parameter集合中，相关的数据可以通过request.getInputStream()来访问。反之，如果上述条件均满足，相关的表单数据将不能再通过request.getInputStream()来读取。

Servlet Specifiaction 3.0:

引用

3.1.1 When Parameters Are Available
The following are the conditions that mustbe met before post form data will be
populated to the parameter set:
1. The request is an HTTP or HTTPS request.
2.  The HTTP method is POST.
3.  The content type is application/x-www-form-urlencoded.
4.  The servlet has made an initial call of any of the getParameterfamily of methods
on the request object.
If the conditions are not met and the post form data is not included in the parameter
set, the post data must still be available to the servlet via the request object’s input
stream. If the conditions are met, post form data will no longer be available for
reading directly from the request object’s input stream.