報錯原因: 

當使用 javax.servlet.ServletResponse.getOutputStream() 方法獲取  ServletOutputStream 物件;

又再使用 javax.servlet.ServletResponse.getWriter() 方法獲取  PrintWriter 物件時就會報錯.

原始碼片段: org.apache.catalina.connector.Response

/** Using output stream flag. **/
protected boolean usingOutputStream = false;

@Override
public ServletOutputStream getOutputStream()throws IOException {
	if (usingWriter) {
		throw new IllegalStateException(sm.getString("coyoteResponse.getOutputStream.ise"));
	}
    // 當獲取 ServletOutputStream 物件時, 會把 usingOutputStream 成員變數設定為 true.
	usingOutputStream = true;
	if (outputStream == null) {
		outputStream = new CoyoteOutputStream(outputBuffer);
	}
	return outputStream;
}

@Override
public PrintWriter getWriter()throws IOException {
	// 如果已經通過 getOutputStream() 方法獲取 ServletOutputStream 物件了, usingOutputStream = true
	if (usingOutputStream) {
		throw new IllegalStateException(sm.getString("coyoteResponse.getWriter.ise"));
	}
	
	if (ENFORCE_ENCODING_IN_GET_WRITER) {
		/*
		* If the response's character encoding has not been specified as
		* described in <code>getCharacterEncoding</code> (i.e., the method
		* just returns the default value <code>ISO-8859-1</code>),
		* <code>getWriter</code> updates it to <code>ISO-8859-1</code>
		* (with the effect that a subsequent call to getContentType() will
		* include a charset=ISO-8859-1 component which will also be
		* reflected in the Content-Type response header, thereby satisfying
		* the Servlet spec requirement that containers must communicate the
		* character encoding used for the servlet response's writer to the
		* client).
		*/
		setCharacterEncoding(getCharacterEncoding());
	}
	usingWriter = true;
	outputBuffer.checkConverter();
	if (writer == null) {
		writer = new CoyoteWriter(outputBuffer);
	}
	return writer;
}