計數器實現限流 - Java實現

實現時需要注意，用於計數的變量，自增操作應該爲原子操作。

import java.util.concurrent.atomic.AtomicLong;

public class RateLimiter {

    private final static AtomicLong ZERO = new AtomicLong(0);
    private AtomicLong counter = ZERO;
    private static long timestamp = System.currentTimeMillis();
    private long permitsPerSecond;

    public RateLimiter(long permitsPerSecond) {
        this.permitsPerSecond = permitsPerSecond;
    }

    public boolean tryAcquire() {
        long now = System.currentTimeMillis();
        if (now - timestamp < 1000) {
            if (counter.get() < permitsPerSecond) {
                counter.incrementAndGet();
                return true;
            } else {
                return false;
            }
        } else {
            counter = ZERO;
            timestamp = now;
            return false;
        }
    }

}

用計數器來做限流，實現簡單，並且很容易的擴展到分佈式場景中，用redis保存計數值，並設置自動過期時間。缺點就是，在計數器置0前後的極短時間裏，可能會有 2*permitsPerSecond 的突發流量。