ServletResponse的java.lang.IllegalStateException错误

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

java.lang.IllegalStateException: WRITERat 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.     *     * @returna {@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。