@Override
    public void close() throws Exception {
        reaper.initiateShutdown();
        // Improve shutdown time by waking up the reaper thread
        // blocked on poll by sending a no-op.
        timer.add(new TimerTask(0) {
            @Override
            public void run() {}
        });
        reaper.awaitShutdown();
        timer.close();
    }
SystemTimerReaper wraps a {@link Timer} and starts a reaper thread
to expire the tasks in the {@link Timer}.
server-common/src/main/java/org/apache/kafka/server/util/timer/SystemTimerReaper.java