guava RateLimiter 实现原理

RateLimiter 是guava 用于限流的工具类，是限制每秒能有多少个请求放行的意思

原理大体是这样的：计算出多少us 内能够放行多少请求

为了提高效率，它是会计算下一次要执行的时间点，若是到了这个时间点才执行，不然会进行sleep 等待

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