求助！！小程序获取用户openid，java后台服务器报错500

小程序获取openid，报java.lang.NullPointerException错误

看了下代码，参数都有传过去，实在找不出哪里有错

public static Map<String,Object> login(String code) throws Exception{

    if(code==null||code.equals("")) {
        throw new Exception("小程序登录凭证不能为空");
    }
    //返回结果
    Map<String,Object> ret=new HashMap<String,Object>();
    Map<String,String> params=new HashMap<String,String>();
    params.put("grant_type", grant_type);
    params.put("appid",appid);
    params.put("secret",app_secret);
    params.put("js_code", code);
    //调用获取access_token接口
    String result=httpPost(access_token_url,params);
    System.out.println(result);
    //根据请求结果判定，是否验证成功
    JSONObject obj=JSONObject.fromObject(result);
    if(obj!=null) {
        Object errCode=obj.get("errcode");
        if(errCode!=null) {
            throw new Exception("errCode:"+errCode);
        }else {
            Object session_key=obj.get("session_key");//会话密钥
            Object openid=obj.get("openid");//唯一标识
            ret.put("session_key", session_key);
            ret.put("openid", openid);
        }
    }   
    return ret;
}
public static String httpGet(String url) {
    DefaultHttpClient httpclient=new DefaultHttpClient();
    String body=null;
    HttpGet get=new HttpGet(url);
    body=invoke(httpclient,get);
    httpclient.getConnectionManager().shutdown();
    return body;
}
public static String httpPost(String url,Map<String,String> params) {
    System.out.println("httpPost:"+params+"url:"+url);
    DefaultHttpClient httpclient=new DefaultHttpClient();
    String body=null;
    HttpPost post=postForm(url,params);
    body=invoke(httpclient,post);
    httpclient.getConnectionManager();
    return body;
}
private static HttpPost postForm(String url,Map<String,String>params) {
    HttpPost httppost=new HttpPost();
    System.out.println(httppost);
    List<NameValuePair> nvps=new ArrayList<NameValuePair>();
    for(String key:params.keySet()) {
        if(key!=null) {
            nvps.add(new BasicNameValuePair(key,params.get(key)));
        }       
    }
    try {
        httppost.setEntity(new UrlEncodedFormEntity(nvps,HTTP.UTF_8));
    }catch(UnsupportedEncodingException e){
        e.printStackTrace();
    }
    return httppost;
}

private static String invoke(DefaultHttpClient httpclient,HttpUriRequest httppost) {
    HttpResponse response=sendRequest(httpclient,httppost);
    String body=parseResponse(response);
    return body;
}
private static HttpResponse sendRequest(DefaultHttpClient httpclient,HttpUriRequest httppost) {
    HttpResponse response=null;
    try {
        response=httpclient.execute(httppost);
        System.out.println(response);
    }catch(ClientProtocolException e) {
        e.printStackTrace();
    }catch(IOException e) {
        e.printStackTrace();
    }
    return response;
}
private static String parseResponse(HttpResponse response) {
    HttpEntity entity=response.getEntity();
    String charset=EntityUtils.getContentCharSet(entity);
    charset=StringUtils.isEmpty(charset)?"utf-8":charset;
    String body=null;
    try {
        body=EntityUtils.toString(entity,charset);
    }catch(ParseException e) {
        e.printStackTrace();
    }catch(IOException e) {
        e.printStackTrace();
    }
    return body;
}

```求好心人帮忙看看代码哪里有错T_T 感激不尽

sendRequest的sendRequest空了