public void schedule(TimerTask task, long delay) {
 if (delay < 0)
  throw new IllegalArgumentException("Negative delay.");
 sched(task, System.currentTimeMillis()+delay, 0);
 }

public final boolean postDelayed(Runnable r, long delayMillis)
 {
 return sendMessageDelayed(getPostMessage(r), delayMillis);
 }

 @SystemApi
 @RequiresPermission(android.Manifest.permission.UPDATE_DEVICE_STATS)
 public void set(@AlarmType int type, long triggerAtMillis, long windowMillis,
  long intervalMillis, OnAlarmListener listener, Handler targetHandler,
  WorkSource workSource) {
 setImpl(type, triggerAtMillis, windowMillis, intervalMillis, 0, null, listener, null,
  targetHandler, workSource, null);
 }

new Thread(new Runnable(){
 Thead.sleep(4*1000);
 doTask();
}).start()

private void mainLoop() {
 while (true) {
  try {
  TimerTask task;
  boolean taskFired;
  synchronized(queue) {
   // Wait for queue to become non-empty
   while (queue.isEmpty() && newTasksMayBeScheduled)
   queue.wait();
   if (queue.isEmpty())
   break; // Queue is empty and will forever remain; die

   // Queue nonempty; look at first evt and do the right thing
   long currentTime, executionTime;
   task = queue.getMin();
   synchronized(task.lock) {
   if (task.state == TimerTask.CANCELLED) {
    queue.removeMin();
    continue; // No action required, poll queue again
   }
   currentTime = System.currentTimeMillis();
   executionTime = task.nextExecutionTime;
   if (taskFired = (executionTime<=currentTime)) {
    if (task.period == 0) { // Non-repeating, remove
    queue.removeMin();
    task.state = TimerTask.EXECUTED;
    } else { // Repeating task, reschedule
    queue.rescheduleMin(
     task.period<0 ? currentTime - task.period
      : executionTime + task.period);
    }
   }
   }
   if (!taskFired) // Task hasn't yet fired; wait
   queue.wait(executionTime - currentTime);
  }
  if (taskFired) // Task fired; run it, holding no locks
   task.run();
  } catch(InterruptedException e) {
  }
 }
 }

Message next() {
  ......
  int nextPollTimeoutMillis = 0;
  for (;;) {
   if (nextPollTimeoutMillis != 0) {
    Binder.flushPendingCommands();
   }
   nativePollOnce(ptr, nextPollTimeoutMillis);
   synchronized (this) {
    // Try to retrieve the next message. Return if found.
    final long now = SystemClock.uptimeMillis();
    Message prevMsg = null;
    Message msg = mMessages;
    if (msg != null && msg.target == null) {
     // Stalled by a barrier. Find the next asynchronous message in the queue.
     do {
      prevMsg = msg;
      msg = msg.next;
     } while (msg != null && !msg.isAsynchronous());
    }
    if (msg != null) {
     if (now < msg.when) {
      // Next message is not ready. Set a timeout to wake up when it is ready.
      nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
     } else {
      // Got a message.
      mBlocked = false;
      if (prevMsg != null) {
       prevMsg.next = msg.next;
      } else {
       mMessages = msg.next;
      }
      msg.next = null;
      if (DEBUG) Log.v(TAG, "Returning message: " + msg);
      msg.markInUse();
      return msg;
     }
    } else {
     // No more messages.
     nextPollTimeoutMillis = -1;
    }
  ......
  }
 }

boolean enqueueMessage(Message msg, long when) {
  if (msg.target == null) {
   throw new IllegalArgumentException("Message must have a target.");
  }
  if (msg.isInUse()) {
   throw new IllegalStateException(msg + " This message is already in use.");
  }
  synchronized (this) {
   if (mQuitting) {
    IllegalStateException e = new IllegalStateException(
      msg.target + " sending message to a Handler on a dead thread");
    Log.w(TAG, e.getMessage(), e);
    msg.recycle();
    return false;
   }
   msg.markInUse();
   msg.when = when;
   Message p = mMessages;
   boolean needWake;
   if (p == null || when == 0 || when < p.when) {
    msg.next = p;
    mMessages = msg;
    needWake = mBlocked;
   } else {
    needWake = mBlocked && p.target == null && msg.isAsynchronous();
    Message prev;
    for (;;) {
     prev = p;
     p = p.next;
     if (p == null || when < p.when) {
      break;
     }
     if (needWake && p.isAsynchronous()) {
      needWake = false;
     }
    }
    msg.next = p; // invariant: p == prev.next
    prev.next = msg;
   }
   mQuitting is false.
   if (needWake) {
    nativeWake(mPtr);
   }
  }
  return true;
 }

IAlarmManager mService.set(mPackageName, type, triggerAtMillis, windowMillis, intervalMillis, flags,
     operation, recipientWrapper, listenerTag, workSource, alarmClock);

 private final IBinder mService = new IAlarmManager.Stub() {
  @Override
  public void set(String callingPackage,
    int type, long triggerAtTime, long windowLength, long interval, int flags,
    PendingIntent operation, IAlarmListener directReceiver, String listenerTag,
    WorkSource workSource, AlarmManager.AlarmClockInfo alarmClock) {
   final int callingUid = Binder.getCallingUid();
   if (interval != 0) {
    if (directReceiver != null) {
     throw new IllegalArgumentException("Repeating alarms cannot use AlarmReceivers");
    }
   }
   if (workSource != null) {
    getContext().enforcePermission(
      android.Manifest.permission.UPDATE_DEVICE_STATS,
      Binder.getCallingPid(), callingUid, "AlarmManager.set");
   }
   // No incoming callers can request either WAKE_FROM_IDLE or
   // ALLOW_WHILE_IDLE_UNRESTRICTED -- we will apply those later as appropriate.
   flags &= ~(AlarmManager.FLAG_WAKE_FROM_IDLE
     | AlarmManager.FLAG_ALLOW_WHILE_IDLE_UNRESTRICTED);
   // Only the system can use FLAG_IDLE_UNTIL -- this is used to tell the alarm
   // manager when to come out of idle mode, which is only for DeviceIdleController.
   if (callingUid != Process.SYSTEM_UID) {
    flags &= ~AlarmManager.FLAG_IDLE_UNTIL;
   }

   if (windowLength == AlarmManager.WINDOW_EXACT) {
    flags |= AlarmManager.FLAG_STANDALONE;
   }
   if (alarmClock != null) {
    flags |= AlarmManager.FLAG_WAKE_FROM_IDLE | AlarmManager.FLAG_STANDALONE;
   } else if (workSource == null && (callingUid < Process.FIRST_APPLICATION_UID
     || Arrays.binarySearch(mDeviceIdleUserWhitelist,
       UserHandle.getAppId(callingUid)) >= 0)) {
    flags |= AlarmManager.FLAG_ALLOW_WHILE_IDLE_UNRESTRICTED;
    flags &= ~AlarmManager.FLAG_ALLOW_WHILE_IDLE;
   }
   setImpl(type, triggerAtTime, windowLength, interval, operation, directReceiver,
     listenerTag, flags, workSource, alarmClock, callingUid, callingPackage);
  }
 }
}