Java Servletでリクエストパラメータの存在確認 ref: http://qiita.com/coa00@github/items/1e33fdaa5fbb2d337878

file0.java

String [] keys = {"u","email"};
String ret = checkKeys(request,keys);

file1.java

    /**
     * キー一覧を渡して存在しないキーがあればListにいれてかえす。
     * @param req
     * @param keys
     * @return
     */
    public static List<String> getNotExistKeys(HttpServletRequest req,String keys[]){


        List<String> notExistKeys = new ArrayList<String>();
        boolean notExit;
        for(int i=0;i<keys.length;i++){
            notExit = true;

                        Enumeration<String> parameterNames = req.getParameterNames();
            while (parameterNames.hasMoreElements()) {

                String paramName = parameterNames.nextElement();
                if (keys[i].equals(paramName)){
                    String paramValues = req.getParameter(paramName);
                    if (!paramValues.isEmpty()) notExit = false;
                }
            }
            if (notExit == true) notExistKeys.add(keys[i]);
        }

        return notExistKeys;
    }

file2.java

    /**
     * checkKeys
     * @param req
     * @param keys
     * @return
     */
    public static String checkKeys(HttpServletRequest req,String keys[]){
        List<String> emptyParams = getNotExistKeys(req,keys);
        String callback = req.getParameter("callback");

        if (emptyParams.size()>0){
            return emptyParams.toString()+"が存在しません。";
        }else{
            return "OK";
        }

    }