Dubbo TPS限流算法

public class DefaultTPSLimiter implements TPSLimiter {

    private final ConcurrentMap<String, StatItem> stats = new ConcurrentHashMap<String, StatItem>();

    @Override
    public boolean isAllowable(URL url, Invocation invocation) {
        // 时间间隔内的上限值
        int rate = url.getMethodParameter(invocation.getMethodName(), TPS_LIMIT_RATE_KEY, -1);
        // 时间间隔
        long interval = url.getMethodParameter(invocation.getMethodName(), TPS_LIMIT_INTERVAL_KEY, DEFAULT_TPS_LIMIT_INTERVAL);
        String serviceKey = url.getServiceKey();
        if (rate > 0) {
            StatItem statItem = stats.get(serviceKey);
            if (statItem == null) {
                stats.putIfAbsent(serviceKey, new StatItem(serviceKey, rate, interval));
                statItem = stats.get(serviceKey);
            } else {
                // 如果配置有修改，则直接覆盖。
                // 这里不直接修改字段，而是重新生成一个不可变对象。(值得学习)
                if (statItem.getRate() != rate || statItem.getInterval() != interval) {
                    stats.put(serviceKey, new StatItem(serviceKey, rate, interval));
                    statItem = stats.get(serviceKey);
                }
            }
            return statItem.isAllowable();
        } else {
            // 上限值为负数则不限流
            StatItem statItem = stats.get(serviceKey);
            if (statItem != null) {
                // ? 不太理解，为啥删除前还有判断一下
                stats.remove(serviceKey);
            }
        }

        return true;
    }

}

class StatItem {

    private final String name;

    private final AtomicLong lastResetTime;

    private final long interval;

    private final AtomicInteger token;

    private final int rate;

    StatItem(String name, int rate, long interval) {
        this.name = name;
        this.rate = rate;
        this.interval = interval;
        this.lastResetTime = new AtomicLong(System.currentTimeMillis());
        this.token = new AtomicInteger(rate);
    }

    
    public boolean isAllowable() {
        long now = System.currentTimeMillis();
        // 重置时间间隔内的计数
        if (now > lastResetTime.get() + interval) {
            token.set(rate);
            lastResetTime.set(now);
        }

        return token.decrementAndGet() >= 0;
    }

    public long getInterval() {
        return interval;
    }


    public int getRate() {
        return rate;
    }


    long getLastResetTime() {
        return lastResetTime.get();
    }

    int getToken() {
        return token.get();
    }

    @Override
    public String toString() {
        return "StatItem " +
            "[name=" + name + ", " +
            "rate = " + rate + ", " +
            "interval = " + interval + ']';
    }

}