开发功能： 
web前端提交上传文件 —> a服务器接收 —> 转发到b服务器进行文件处理 
下面是简单实现的代码，具体细节优化根本自己的需求更改。

    public String handleResponse(HttpServletRequest request, HttpServletResponse response)
            throws UnsupportedEncodingException, IOException {
        String method = request.getMethod();
        String url = "b服务器的api url";
        if (method.equals("POST")) {
            String contentType = "application/json; charset=UTF-8";
            if (request.getContentType() != null)
                contentType = request.getContentType();// 会获取到空指针
            Map<String, String[]> tmp = new HashMap(request.getParameterMap());
            if (contentType.toLowerCase().startsWith("multipart/")) {
                MultipartHttpServletRequest multipartRequest = WebUtils.getNativeRequest(request,
                        MultipartHttpServletRequest.class);
                MultipartFile file = multipartRequest.getFile("file");
                return httpClientUpload(url, file, tmp);
            }
        }
        return  null;
    }

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public String httpClientUpload(String url, MultipartFile file, Map<String, String[]> params)
            throws ClientProtocolException, IOException {
        HttpClient httpclient = new DefaultHttpClient();
        // 请求处理页面
        HttpPost httppost = new HttpPost(url);
        // 创建待处理的文件
        String fileName = file.getOriginalFilename();
        ContentBody files = new ByteArrayBody(file.getBytes(), fileName);
        // 对请求的表单域进行填充
        MultipartEntity reqEntity = new MultipartEntity();
        reqEntity.addPart("file", files);

        if (params != null) {//这里草草处理values[]
            for (String key : params.keySet()) {
                String[] values = params.get(key);
                for (int i = 0; i < values.length; i++) {
                    String value = http://www.mamicode.com/values[i];"hljs-keyword">try {
                        value = http://www.mamicode.com/URLEncoder.encode(value, "UTF-8");
                        reqEntity.addPart(key, new StringBody(value));
                    } catch (UnsupportedEncodingException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }
        }
        // 设置请求
        httppost.setEntity(reqEntity);
        // 执行
        HttpResponse response = httpclient.execute(httppost);
        if (HttpStatus.SC_OK == response.getStatusLine().getStatusCode()) {
            HttpEntity entity = response.getEntity();
            return EntityUtils.toString(entity, Charset.forName("UTF-8"));
        }
        return null;
    }

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