java防止同时多个相同请求并发问题

1、单机版本的解决方案

    select  max(id)+1   from  bill

boolean i = false;
try{
    Object maxId = billService.findMaxId();
    //锁住本对象
    synchronized(maxId){
        StringBuffer br = new StringBuffer();
        br.append(bill.getUserId());
        br.append(RedisConstant.INCOME_KEY);
        String key = br.toString();
        QueryWrapper userQueryWrapper = new QueryWrapper();
        userQueryWrapper.eq("id", bill.getUserId());