對jvm進行gc的時間、數量、jvm停頓時間的監控

在jdk中一個類可以獲得gc的信息：

    public static void main(String[] args) {
        List<GarbageCollectorMXBean> garbageCollectorMXBeans = ManagementFactory.getGarbageCollectorMXBeans();
        for (GarbageCollectorMXBean garbageCollectorMXBean : garbageCollectorMXBeans) {
            System.out.println(garbageCollectorMXBean.getName() + "#  time: " + garbageCollectorMXBean.getCollectionTime() + ", count: " + garbageCollectorMXBean.getCollectionCount());
        }
    }

在hadoop-common.jar中，有個JvmPauseMonitor$Monitor類，它能監控jvm暫停時間：

private class Monitor implements Runnable {
    @Override
    public void run() {
      StopWatch sw = new StopWatch();
      Map<String, GcTimes> gcTimesBeforeSleep = getGcTimes(); // 用ManagementFactory.getGarbageCollectorMXBeans()收集得到 name:GcTime的信息
      while (shouldRun) {
        sw.reset().start();
        try {
          Thread.sleep(SLEEP_INTERVAL_MS);
        } catch (InterruptedException ie) {
          return;
        }
        long extraSleepTime = sw.now(TimeUnit.MILLISECONDS) - SLEEP_INTERVAL_MS; // 關鍵在這里，減去自己sleep的時間，大約等於系統暫停的時間（因為還有run方法執行其他代碼的時間）
        Map<String, GcTimes> gcTimesAfterSleep = getGcTimes();

        if (extraSleepTime > warnThresholdMs) { // 達到warn的閾值則打印警告
          ++numGcWarnThresholdExceeded;
          LOG.warn(formatMessage(
              extraSleepTime, gcTimesAfterSleep, gcTimesBeforeSleep));
        } else if (extraSleepTime > infoThresholdMs) {
          ++numGcInfoThresholdExceeded;
          LOG.info(formatMessage(
              extraSleepTime, gcTimesAfterSleep, gcTimesBeforeSleep));
        }
        totalGcExtraSleepTime += extraSleepTime; // 記錄jvm暫停的時間（大約）
        gcTimesBeforeSleep = gcTimesAfterSleep;
      }
    }
  }