待签名参数按照字段名的ascii码从小到大排序

 public static String getSign(Map<String, String> params) {
           Map<String, String> sortMap = new TreeMap<String, String>();
           sortMap.putAll(params);
           // 以k1=v1&k2=v2...方式拼接参数
           StringBuilder builder = new StringBuilder();
           for (Map.Entry<String, String> s : sortMap.entrySet()) {
               String k = s.getKey();
               String v = s.getValue();
               if (StringUtils.isBlank(v)) {// 过滤空值
                   continue;
               }
               builder.append(k).append("=").append(v).append("&");
           }
           if (!sortMap.isEmpty()) {
               builder.deleteCharAt(builder.length() - 1);
           }
           return builder.toString();
       }

    public static void main(String[] args) {
          Map<String,String> paraMap = new HashMap<String,String>();
          paraMap.put("total_fee","200");
          paraMap.put("appid", "wxd678efh567hg6787");
          paraMap.put("body", "测试");
          paraMap.put("out_trade_no","20150806125346");
          System.out.println(getSign(paraMap));
    }