public class JavaRequest {
     private static final String BASE_URL = "http://localhost:8080/dsdemo/";
     public static String userToken = null;
     public static String problemName = null;
     public static String sendPost(String sufUrl, JSONObject params) {
        DataOutputStream out = null;
        BufferedReader in = null;
        String result = "";
        try {
            URL realUrl = new URL(BASE_URL+sufUrl);
            // 打开和URL之间的连接
            HttpURLConnection connection = (HttpURLConnection) realUrl.openConnection();
            connection.setDoOutput(true);  
            connection.setDoInput(true);  
            connection.setUseCaches(false);  
            connection.setInstanceFollowRedirects(true);  
            connection.setRequestMethod("POST"); // 设置请求方式  
            connection.setRequestProperty("Connection", "Keep-Alive");
            connection.connect();
            out = new DataOutputStream (connection.getOutputStream());
            // 发送请求参数，防止中文乱码
            out.write(jsonToUrlParams(params).getBytes("UTF-8"));
            // flush输出流的缓冲
            out.flush();
            // 定义BufferedReader输入流来读取URL的响应
            in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String line;
            while ((line = in.readLine()) != null) {
                result += line;
            }
        } catch (Exception e) {
            System.out.println("发送 POST 请求出现异常！");
            e.printStackTrace();
        }
        //使用finally块来关闭输出流、输入流
        finally{
            try{
                if(out!=null){
                    out.close();
                }
                if(in!=null){
                    in.close();
                }
            }
            catch(IOException ex){
                ex.printStackTrace();
            }
        }
        System.out.println(result);
        return result;
    }    
     
    private static String jsonToUrlParams(JSONObject params){
        StringBuilder sb = new StringBuilder();
        sb.append("userToken=").append(userToken);
        for(Iterator<?> it = params.keys(); it.hasNext(); ){
            String key = (String) it.next();
            sb.append("&").append(key).append("=").append(params.get(key));
        }
        return sb.toString();
    }
}

 <script type="text/javascript">
      var sendData = {
          username: "hjz",
          password: "123"
      };
    (function (){
         $.ajax({  
             type:'post',      
             url:'http://localhost:8080/dsdemo/userRegister',  
             data: sendData,  
             dataType:'json',  
             success:function(data){  
                 alert(JSON.stringify(data));
             } ,
             error: function(data){
                 alert(JSON.stringify(data));
             }
       });  
    })(); 
  </script>