getWriter() has already been called for this response的解决办法

Servlet规范说明，不能既调用 response.getOutputStream()，又调用response.getWriter()，不管先调用哪个，在调用第二个时候应会抛出 IllegalStateException 对于getOutputStream()，api里是这样说的：Either this method or getWriter() may be called to write t

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