public static String parsingCamelCaseToUnderline(String params){
    char[] charArray = params.toCharArray();
    params = "";
    for (int i = 0; i < charArray.length; i++) {
        char c = charArray[i];
        if( i == 0 ){
            params += Character.toLowerCase(c);
        }else{
            params += (Character.isUpperCase(c) ? "_" + Character.toLowerCase(c) : c);
        }
    }
    return params;
}

System.out.println(parsingCamelCaseToUnderline("createTime"));
System.out.println(parsingCamelCaseToUnderline("CreateTime"));
System.out.println(parsingCamelCaseToUnderline("UreateTime"));
console ......
create_time
create_time
ureate_time