private final ConcurrentHashMap<CacheKey, CachedRange> cache = new ConcurrentHashMap<CacheKey, CachedRange>();
 
 CacheKey：业务场景
 CachedRange：当前批次可用的id区间范围

 public long nextId(String app, String key) {
       synchronized (this){
           CacheKey cacheKey = new CacheKey(app, key);
           CachedRange cachedRange = this.cache.get(cacheKey);
           if (cachedRange == null || cachedRange.range.getEnd() < cachedRange.pos) {
               IDRange range = this.service.getNextRange(app, key, this.size);
               cachedRange = new CachedRange(range, range.getStart()) ;
           }
           long pos = cachedRange.pos;
           cachedRange.pos += 1;
           this.cache.put(cacheKey, cachedRange);
           return pos;
       }
   }
   
size：表示一次获取id的区间长度
   

获取一个可用的IDRange, 结果为闭区间[a, b]

public IDRange getNextRange(String app, String key, int size) {

        synchronized (this) {
            IDRange result = new IDRange();
            IDRange range = this.get4Update(app, key, size * this.PRE_ALOCATE_BATCH_NUM);
            result.setApp(app);
            result.setKey(key);
            result.setStart(range.getStart());
            result.setEnd(range.getEnd());

            this.logger.info("return range: {}", result);
            return result;
        }
    }

// 数据库查询

 @Transactional(value="crtTransactionManager", propagation = Propagation.REQUIRED, rollbackFor = Exception.class)
    public IDRange get4Update(String app, String key, int size) {
        Map<String, String> params = new HashMap<String, String>();
        params.put("app", app);
        params.put("key", key);
        SqlSession sqlSession = this.commonSqlSessionTemplate.getSqlSessionFactory()
                .openSession();
        UniversalId universalId = sqlSession.selectOne("select4Update", params);
        if (universalId == null) {
            return null;
        }

        IDRange range = new IDRange();
        range.setApp(app);
        range.setKey(key);
        range.setStart(universalId.getValue() + 1);
        range.setEnd(universalId.getValue() + size - 1);

        universalId.setValue(universalId.getValue() + size);
        sqlSession.update("updateValue", universalId);
        return range;
    }