guava RateLimiter 實現原理

RateLimiter 是guava 用於限流的工具類，是限制每秒能有多少個請求放行的意思

原理大體是這樣的：計算出多少us 內能夠放行多少請求

爲了提高效率，它是會計算下一次要執行的時間點，若是到了這個時間點才執行，不然會進行sleep 等待

public double acquire(int permits) {
    long microsToWait = reserve(permits);
    stopwatch.sleepMicrosUninterruptibly(microsToWait);
    return 1.0 * microsToWait / SECONDS.toMicros(1L);
  }
sleep 等待邏輯在stopwatch.sleepMicrosUninterruptibly(microsToWait);

 public static void sleepUninterruptibly(long sleepFor, TimeUnit unit) {
    boolean interrupted = false;
    try {
      long remainingNanos = unit.toNanos(sleepFor);
      long end = System.nanoTime() + remainingNanos;
      while (true) {
        try {
          // TimeUnit.sleep() treats negative timeouts just like zero.
          NANOSECONDS.sleep(remainingNanos);
          return;
        } catch (InterruptedException e) {
          interrupted = true;
          remainingNanos = end - System.nanoTime();
        }
      }
    } finally {
      if (interrupted) {
        Thread.currentThread().interrupt();
      }
    }