private String camel4underline(String param){
        Pattern p=Pattern.compile("[A-Z]");
        if(param==null ||param.equals("")){
            return "";
        }
        StringBuilder builder=new StringBuilder(param);
        Matcher mc=p.matcher(param);
        int i=0;
        while(mc.find()){
            builder.replace(mc.start()+i, mc.end()+i, "_"+mc.group().toLowerCase());
            i++;
        }
        if('_' == builder.charAt(0)){
            builder.deleteCharAt(0);
        }
        return builder.toString();
    }

结果就是 userName 变成 user_name