response.reset()和response.resetBuffer()
getResponse的getWriter()方法
连续两次输出流到页面的时候，第二次的流会包括第一次的流，所以可以使用将response.reset或者resetBuffer的方法。
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reset():
Clears any data that exists in the buffer as well as the status code and headers. If the response has been committed, this method throws an IllegalStateException.
resetBuffer():
Clears the content of the underlying buffer in the response without clearing headers or status code. If the response has been committed, this method throws an IllegalStateException.
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response.reset()的使用有一个条件受限：response的任何打开流关闭之后都不能再reset?.
