ServletResponse的java.lang.IllegalStateException错误

java.lang.IllegalStateException: STREAM
	at org.eclipse.jetty.server.Response.getWriter(Response.java:707)

java.lang.IllegalStateException: WRITER
	at org.eclipse.jetty.server.Response.getOutputStream(Response.java:681)

今天在写一个框架的时候遇到一个问题，记录一下，报错如上，查看javax.servlet.ServletResponse接口中的定义如下：

    /**
     * Returns a {@link ServletOutputStream} suitable for writing binary 
     * data in the response. The servlet container does not encode the
     * binary data.  
     
     * <p> Calling flush() on the ServletOutputStream commits the response.
     
     * Either this method or {@link #getWriter} may 
     * be called to write the body, not both.
     *
     * @return				a {@link ServletOutputStream} for writing binary data	
     *
     * @exception IllegalStateException if the <code>getWriter</code> method
     * 					has been called on this response
     *
     * @exception IOException 		if an input or output exception occurred
     *
     * @see 				#getWriter
     *
     */

    public ServletOutputStream getOutputStream() throws IOException;

    /**
     * Returns a <code>PrintWriter</code> object that
     * can send character text to the client.
     * The <code>PrintWriter</code> uses the character
     * encoding returned by {@link #getCharacterEncoding}.
     * 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>.
     * <p>Calling flush() on the <code>PrintWriter</code>
     * commits the response.
     * <p>Either this method or {@link #getOutputStream} may be called
     * to write the body, not both.
     *
     * 
     * @return 		a <code>PrintWriter</code> object that 
     *			can return character data to the client 
     *
     * @exception UnsupportedEncodingException
     *			if the character encoding returned
     *			by <code>getCharacterEncoding</code> cannot be used
     *
     * @exception IllegalStateException
     *			if the <code>getOutputStream</code>
     * 			method has already been called for this 
     *			response object
     *
     * @exception IOException
     *			if an input or output exception occurred
     *
     * @see 		#getOutputStream
     * @see 		#setCharacterEncoding
     *
     */

    public PrintWriter getWriter() throws IOException;

注意这两句：
Either this method or {@link #getWriter} may be called to write the body, not both.
Either this method or {@link #getOutputStream} may be called to write the body, not both.

再看jetty的源码，jetty.server.Response的源码：

    public ServletOutputStream getOutputStream() throws IOException
    {
        if (_outputState!=NONE && _outputState!=STREAM)
            throw new IllegalStateException("WRITER");

        ServletOutputStream out = _connection.getOutputStream();
        _outputState=STREAM;
        return out;
    }

   public PrintWriter getWriter() throws IOException
    {
        if (_outputState!=NONE && _outputState!=WRITER)
            throw new IllegalStateException("STREAM");

        /* if there is no writer yet */
        if (_writer==null)
        {
            /* get encoding from Content-Type header */
            String encoding = _characterEncoding;

            if (encoding==null)
            {
                /* implementation of educated defaults */
                if(_cachedMimeType != null)
                   encoding = MimeTypes.getCharsetFromContentType(_cachedMimeType);

                if (encoding==null)
                    encoding = StringUtil.__ISO_8859_1;

                setCharacterEncoding(encoding);
            }

            /* construct Writer using correct encoding */
            _writer = _connection.getPrintWriter(encoding);
        }
        _outputState=WRITER;
        return _writer;
    }

从源码的第3-4行可以得以验证，也就是说对于同一个response实例你只能使用其中的一个方法，不能两者混用。