public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String url = request.getParameter("url"); //得到要下载的资源的URL.

        if (!StringUtils.isNullOrEmpty(url)) {
            System.out.println(url);
            URL u = new URL(url);
            URLConnection conn = u.openConnection();

            try { ///把响应头设置成一样的.
                for (Map.Entry<String, List<String>> m : conn.getHeaderFields()
                        .entrySet()) {
                    if (m != null && m.getKey() != null && m.getValue() != null
                            && m.getValue().size() > 0) {
                        response.setHeader(m.getKey(), m.getValue().get(0));
                    }
                }
            } catch (Exception e) {
                // TODO: handle exception
                e.printStackTrace();
                CommonLog.log.warn(e);
            }

            InputStream in = conn.getInputStream(); //从 connection得到inputstream
            OutputStream out = response.getOutputStream(); //得到向客户端的输出流
            try {
                IOUtils
                        .write(conn.getInputStream(), response
                                .getOutputStream()); //写数据.
            } catch (Exception e) {
                e.printStackTrace();
                // TODO: handle exception
                CommonLog.log.warn(e);
                if (in != null) {
                    in.close();
                }
                if (out != null) {
                    out.close();
                }
            }
        }
    }

    /**
     * 从input里面读取数据然后写入output，读完后自动关闭流。
     * @param input inputStream
     * @param output outputStream
     * */
    public static void write(InputStream input, OutputStream output)
            throws IOException {
        write(input,output,true);
    }

    /**
     * 自动从inputstream里面读数据，然后写到outputstream里面去。
     * @param input inputstream
     * @param output outputstream
     * @param close 读完后是否自动关闭流。
     * */
    public static void write(InputStream input, OutputStream output,boolean close)
            throws IOException {
        byte[] b = new byte[1024];
        int len = input.read(b);
        while (len != -1) {
            output.write(b, 0, len);
            len = input.read(b);
        }
        
        output.flush();
        if (close) {
            input.close();    
            output.close();
        }
    }

output.write(b, 0, len);//执行这行代码的时候.

ClientAbortException:  java.net.SocketException: Connection reset by peer: socket write error 
 	at org.apache.catalina.connector.OutputBuffer.realWriteBytes(OutputBuffer.java:366) 
 	at org.apache.tomcat.util.buf.ByteChunk.flushBuffer(ByteChunk.java:432) 
 	at org.apache.tomcat.util.buf.ByteChunk.append(ByteChunk.java:347) 
 	at org.apache.catalina.connector.OutputBuffer.writeBytes(OutputBuffer.java:392)