写这篇文章的目的,是看到群里有人在实现延迟的时候,用如下的第四种方法,个人感觉有点不妥,为了防止更多的人有这种想法,所以自己抽空深入分析,就分析的结果,写下此文,希望对部分人有启示作用。
答: 1.java.util.Timer类的:
- public void schedule(TimerTask task, long delay) {
- if (delay < 0)
- throw new IllegalArgumentException("Negative delay.");
- sched(task, System.currentTimeMillis()+delay, 0);
- }
2.android.os.Handler类:
- public final boolean postDelayed(Runnable r, long delayMillis)
- {
- return sendMessageDelayed(getPostMessage(r), delayMillis);
- }
3.android.app.AlarmManager类:
- @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);
- }
4.Thread.sleep()然后在一定时间之后再执行想执行的代码:
- new Thread(new Runnable() {
- Thead.sleep(4 * 1000);
- doTask();
- }).start()
答:
- 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) {
- }
- }
- }
是通过wait和延迟时间到达的时候,调用notify来唤起线程继续执行,这样来实现延迟的话,我们可以回开启一个新的线程,貌似为了个延迟没必要这样吧,定时,频繁执行的任务,再考虑这个吧。
- 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;
- }......
- }
- }
当我们向MessageQueue插入一条延迟的Message的时候,Looper在执行loop方法,底层会调用epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);其中的timeoutMillis参数指定了在没有事件发生的时候epoll_wait调用阻塞的毫秒数(milliseconds)。这样我们在之前的时间内这个时候阻塞了是会释放cpu的资源,等到延迟的时间到了时候,再监控到事件发生。在这里可能有人会有疑问,一直阻塞,那我接下来的消息应该怎么执行呢?我们可以看到当我们插入消息的时候的方法:
- 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;
- }
阻塞了有两种方式唤醒,一种是超时了,一种是被主动唤醒了,在上面我们可以看到当有消息进入的时候,我们会唤醒继续执行,所以我们的即时消息在延迟消息之后插入是没有关系的。然后在延迟时间到了的时候,我们也会被唤醒,执行对应的消息send,以达到延迟时间执行某个任务的目的。
优势:这种延迟在阻塞的时候,是会释放cpu的锁,不会过多地占用cpu的资源。
- IAlarmManager mService.set(mPackageName, type, triggerAtMillis, windowMillis, intervalMillis, flags,
- operation, recipientWrapper, listenerTag, workSource, alarmClock);
这里是通过aidl与AlarmManagerService的所在进程进行通信,具体的实现是在AlarmManagerService类里面:
- 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);
- }
- }
- }
虽然有人觉得用AlarmManager能够在应用关闭的情况下,定时器还能再唤起,经过自己的测试,当杀掉应用程序的进程,AlarmManager的receiver也是接收不到消息的,但是我相信在这里定时器肯定是发送了,但是作为接收方的应用程序进程被杀掉了,执行不了对应的代码。不过有人也觉得AlarmManager更耗电,是因为我们执行定时任务的情况会频繁唤起cpu,但是如果只是用来只是执行延迟任务的话,个人觉得和Handler.postDelayed()相比应该也不会耗电多的。
如上面我们看到的这样,如果是单纯的实现一个任务的延迟的话,我们可以用Handler.postDelayed()和AlarmManager.set()来实现,用(4)的方法Thread.sleep()的话,首先开启一个新的线程,然后会持有cpu的资源,用(1)的方法,Timer,会开启一个死循环的线程,这样在资源上面都有点浪费。
如果大家还有更好的延迟解决方案,可以拿出来大家探讨,如果文章有不对的地方,欢迎拍砖。
来源: https://juejin.im/post/5a28fdebf265da43133d0b76