java发送post请求，使用multipart/form-data的方式传递参数

/**

 * 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。