/**
     * 测试上传图片
     * 
     */
    public static void testUploadImage(){
        String url = "http://xxxtest/Api/testUploadModelBaking";
        String fileName = "e:/username/textures/antimap_0017.png";
        Map<String, String> textMap = new HashMap<String, String>();
        //可以设置多个input的name，value
        textMap.put("name", "testname");
        textMap.put("type", "2");
        //设置file的name，路径
        Map<String, String> fileMap = new HashMap<String, String>();
        fileMap.put("upfile", fileName);
        String contentType = "";//image/png
        String ret = formUpload(url, textMap, fileMap,contentType);
        System.out.println(ret);
        //{"status":"0","message":"add succeed","baking_url":"group1\/M00\/00\/A8\/CgACJ1Zo-LuAN207AAQA3nlGY5k151.png"}
    }

    /**
     * 上传图片
     * @param urlStr
     * @param textMap
     * @param fileMap
     * @param contentType 没有传入文件类型默认采用application/octet-stream
     * contentType非空采用filename匹配默认的图片类型
     * @return 返回response数据
     */
    @SuppressWarnings("rawtypes")
    public static String formUpload(String urlStr, Map<String, String> textMap,
            Map<String, String> fileMap,String contentType) {
        String res = "";
        HttpURLConnection conn = null;
        // boundary就是request头和上传文件内容的分隔符
        String BOUNDARY = "---------------------------123821742118716"; 
        try {
            URL url = new URL(urlStr);
            conn = (HttpURLConnection) url.openConnection();
            conn.setConnectTimeout(5000);
            conn.setReadTimeout(30000);
            conn.setDoOutput(true);
            conn.setDoInput(true);
            conn.setUseCaches(false);
            conn.setRequestMethod("POST");
            conn.setRequestProperty("Connection", "Keep-Alive");
            // conn.setRequestProperty("User-Agent","Mozilla/5.0 (Windows; U; Windows NT 6.1; zh-CN; rv:1.9.2.6)");
            conn.setRequestProperty("Content-Type","multipart/form-data; boundary=" + BOUNDARY);
            OutputStream out = new DataOutputStream(conn.getOutputStream());
            // text
            if (textMap != null) {
                StringBuffer strBuf = new StringBuffer();
                Iterator iter = textMap.entrySet().iterator();
                while (iter.hasNext()) {
                    Map.Entry entry = (Map.Entry) iter.next();
                    String inputName = (String) entry.getKey();
                    String inputValue = (String) entry.getValue();
                    if (inputValue == null) {
                        continue;
                    }
                    strBuf.append("\r\n").append("--").append(BOUNDARY).append("\r\n");
                    strBuf.append("Content-Disposition: form-data; name=\"" + inputName + "\"\r\n\r\n");
                    strBuf.append(inputValue);
                }
                out.write(strBuf.toString().getBytes());
            }
            // file
            if (fileMap != null) {
                Iterator iter = fileMap.entrySet().iterator();
                while (iter.hasNext()) {
                    Map.Entry entry = (Map.Entry) iter.next();
                    String inputName = (String) entry.getKey();
                    String inputValue = (String) entry.getValue();
                    if (inputValue == null) {
                        continue;
                    }
                    File file = new File(inputValue);
                    String filename = file.getName();
                    
                    //没有传入文件类型，同时根据文件获取不到类型，默认采用application/octet-stream
                    contentType = new MimetypesFileTypeMap().getContentType(file);
                    //contentType非空采用filename匹配默认的图片类型
                    if(!"".equals(contentType)){
                        if (filename.endsWith(".png")) {
                            contentType = "image/png"; 
                        }else if (filename.endsWith(".jpg") || filename.endsWith(".jpeg") || filename.endsWith(".jpe")) {
                            contentType = "image/jpeg";
                        }else if (filename.endsWith(".gif")) {
                            contentType = "image/gif";
                        }else if (filename.endsWith(".ico")) {
                            contentType = "image/image/x-icon";
                        }
                    }
                    if (contentType == null || "".equals(contentType)) {
                        contentType = "application/octet-stream";
                    }
                    StringBuffer strBuf = new StringBuffer();
                    strBuf.append("\r\n").append("--").append(BOUNDARY).append("\r\n");
                    strBuf.append("Content-Disposition: form-data; name=\"" + inputName + "\"; filename=\"" + filename + "\"\r\n");
                    strBuf.append("Content-Type:" + contentType + "\r\n\r\n");
                    out.write(strBuf.toString().getBytes());
                    DataInputStream in = new DataInputStream(new FileInputStream(file));
                    int bytes = 0;
                    byte[] bufferOut = new byte[1024];
                    while ((bytes = in.read(bufferOut)) != -1) {
                        out.write(bufferOut, 0, bytes);
                    }
                    in.close();
                }
            }
            byte[] endData = ("\r\n--" + BOUNDARY + "--\r\n").getBytes();
            out.write(endData);
            out.flush();
            out.close();
            // 读取返回数据
            StringBuffer strBuf = new StringBuffer();
            BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
            String line = null;
            while ((line = reader.readLine()) != null) {
                strBuf.append(line).append("\n");
            }
            res = strBuf.toString();
            reader.close();
            reader = null;
        } catch (Exception e) {
            System.out.println("发送POST请求出错。" + urlStr);
            e.printStackTrace();
        } finally {
            if (conn != null) {
                conn.disconnect();
                conn = null;
            }
        }
        return res;
    }

以上是网上可以搜到显示最多的代码：

实际使用中发现有部分接口无法调通，后来发现是在

 strBuf.append("\r\n").append("--").append(BOUNDARY).append("\r\n");

多了换行符，应改为：

 strBuf.append("--").append(BOUNDARY).append("\r\n");

完整代码如下：

 OutputStream out = new DataOutputStream(conn.getOutputStream());
            if (fileMap != null) {
                String filename = fileMap.get("filename");
                String upload = fileMap.get("upload");
                StringBuffer strBuf = new StringBuffer();
                strBuf.append("--").append(BOUNDARY).append("\r\n");
                strBuf.append("Content-Disposition: form-data;name=\"filename\"\r\n\r\n" + filename + "\r\n"); 
                strBuf.append("--").append(BOUNDARY).append("\r\n");
                strBuf.append("Content-Disposition: form-data;name=\"upload\";filename=\"" + upload + "\"\r\n"); 
                strBuf.append("\r\n");
               // strBuf.append("test:ai\n");
                out.write(strBuf.toString().getBytes());
                logger.debug(strBuf.toString());
                DataInputStream in = new DataInputStream(new FileInputStream(file));
                int bytes = 0;
                byte[] bufferOut = new byte[1024];
                while ((bytes = in.read(bufferOut)) != -1) {
                    out.write(bufferOut, 0, bytes);
                }
                in.close();
                
                byte[] endData = ("\r\n--" + BOUNDARY + "--\r\n").getBytes();
                out.write(endData);
                out.flush();
                out.close();
            }
            
            // 读取返回数据
            StringBuffer strBuf = new StringBuffer();
            BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
            String line = null;
            while ((line = reader.readLine()) != null) {
                strBuf.append(line).append("\n");
            }
            res = strBuf.toString();
            reader.close();
            reader = null;
            logger.info("上传完成:"+res);

这个要根据接收者的接收协议来确定使用哪一种方式。