NameNode的基于JMX的度量系统与JVM暂停监控

// 开启守护thread-(低优先级, 不阻塞thread运行, 随同thread消失而消失, 开启另一thread去守护thread)
  public class Daemon extends Thread {

  {
    setDaemon(true);                              // always a daemon
  }

// 核心循环任务-通过程序反应时间来测量是否暂停
  private class Monitor implements Runnable {
    @Override
    public void run() {
      // 时间统计工具
      Stopwatch sw = new Stopwatch();
      Map<String, GcTimes> gcTimesBeforeSleep = getGcTimes();
      while (shouldRun) {
        sw.reset().start();
        try {
          Thread.sleep(SLEEP_INTERVAL_MS);
        } catch (InterruptedException ie) {
          return;
        }
        long extraSleepTime = sw.elapsedMillis() - SLEEP_INTERVAL_MS;
        Map<String, GcTimes> gcTimesAfterSleep = getGcTimes();

        if (extraSleepTime > warnThresholdMs) {
          ++numGcWarnThresholdExceeded;
          LOG.warn(formatMessage(
              extraSleepTime, gcTimesAfterSleep, gcTimesBeforeSleep));
        } else if (extraSleepTime > infoThresholdMs) {
          ++numGcInfoThresholdExceeded;
          LOG.info(formatMessage(
              extraSleepTime, gcTimesAfterSleep, gcTimesBeforeSleep));
        }
        totalGcExtraSleepTime += extraSleepTime;
        gcTimesBeforeSleep = gcTimesAfterSleep;
      }
    }
  }