android开发中，常用定时器实现方式有以下几种：
1、Handler与sleep
2、Handler与postDelayed
3、Handler与timer
注：Handler的主要作用就是用来处理接收到的信息；用Handler消息传递机制是为了多个线程并发更新UI的同时，保证线程安全；
一、Handler 与 sleep
public class HandlerAndSleep { // 1、定义一个Handler类，用于处理接受到的Message. Handler handler = new Handler(Looper.getMainLooper()) { @Override public void handleMessage(@NonNull Message msg) { super.handleMessage(msg); // do something eg. Log.d("HandlerAndSleep", "HandlerAndSleep"); } }; // 2、新建一个实现Runnable接口的线程类 class MyThread implements Runnable { @Override public void run() { while (true) { try { Thread.sleep(10000); Message message = new Message(); message.what = 1; handler.sendMessage(message); } catch (Exception e) { e.printStackTrace(); } } } } public HandlerAndSleep() { //3、在需要启动线程的地方加入下面语句,启动线程后，线程每10s发送一次消息 new Thread(new MyThread()).start(); }}结果打印如下：
2022-05-30 11:30:42.320 17864-17864/com.yh.study D/HandlerAndSleep: HandlerAndSleep2022-05-30 11:30:52.324 17864-17864/com.yh.study D/HandlerAndSleep: HandlerAndSleep2022-05-30 11:31:02.326 17864-17864/com.yh.study D/HandlerAndSleep: HandlerAndSleep2022-05-30 11:31:12.335 17864-17864/com.yh.study D/HandlerAndSleep: HandlerAndSleep2022-05-30 11:31:22.338 17864-17864/com.yh.study D/HandlerAndSleep: HandlerAndSleep2022-05-30 11:31:32.339 17864-17864/com.yh.study D/HandlerAndSleep: HandlerAndSleep2022-05-30 11:31:42.347 17864-17864/com.yh.study D/HandlerAndSleep: HandlerAndSleep二、Handler 与 postDelayed
public class HandleAndPostDelayed { Handler handler = new Handler(){ @Override public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) { Log.d("HandleAndPostDelayed", "HandleAndPostDelayed"); return super.sendMessageAtTime(msg, uptimeMillis); } }; Runnable runnable = new Runnable() { @Override public void run() { // do something handler.postDelayed(this, 2000); } }; HandleAndPostDelayed() { // 开启定时器 handler.postDelayed(runnable, 2000); // 停止定时器// handler.removeCallbacks(runnable); }}结果打印如下：
2022-05-30 14:32:41.533 24351-24351/com.yh.study D/HandleAndPostDelayed: HandleAndPostDelayed2022-05-30 14:32:43.534 24351-24351/com.yh.study D/HandleAndPostDelayed: HandleAndPostDelayed2022-05-30 14:32:45.537 24351-24351/com.yh.study D/HandleAndPostDelayed: HandleAndPostDelayed2022-05-30 14:32:47.539 24351-24351/com.yh.study D/HandleAndPostDelayed: HandleAndPostDelayed2022-05-30 14:32:49.546 24351-24351/com.yh.study D/HandleAndPostDelayed: HandleAndPostDelayed2022-05-30 14:32:51.550 24351-24351/com.yh.study D/HandleAndPostDelayed: HandleAndPostDelayed三、Handler 与 timer
public class HandlerAndTimer { private final Timer timer = new Timer(); private TimerTask task; Handler handler = new Handler() { @Override public void handleMessage(@NonNull Message msg) { // do something Log.d("HandlerAndTimer", "HandlerAndTimer"); super.handleMessage(msg); } }; public HandlerAndTimer() { task = new TimerTask() { @Override public void run() { Message message = new Message(); message.what = 1; handler.sendMessage(message); } }; //启动定时器 参数对应为 TimerTask 延迟时间 间隔时间 timer.schedule(task, 2000, 2000); }}结果打印如下：
2022-05-30 14:43:40.922 25052-25052/com.yh.study D/HandlerAndTimer: HandlerAndTimer2022-05-30 14:43:42.924 25052-25052/com.yh.study D/HandlerAndTimer: HandlerAndTimer2022-05-30 14:43:44.925 25052-25052/com.yh.study D/HandlerAndTimer: HandlerAndTimer2022-05-30 14:43:46.926 25052-25052/com.yh.study D/HandlerAndTimer: HandlerAndTimer2022-05-30 14:43:48.927 25052-25052/com.yh.study D/HandlerAndTimer: HandlerAndTimer在Android上常用的定时器有两种，一种是Java.util.Timer，一种就是系统的AlarmService。
public class MainActivity extends AppCompatActivity { Handler handler = new Handler() { @Override public void handleMessage(@NonNull Message msg) { tCount++; timerTV.setText(tCount+""); super.handleMessage(msg); } }; Timer timer = new Timer(); int tCount = 0; int sCount = 0; TextView timerTV; TextView serviceTV; String ALARM_RECEIVER_ACTION = "com.yh.study.AlarmReceiver"; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); timerTV = findViewById(R.id.timer_tv); serviceTV = findViewById(R.id.service_tv); timer.schedule(new TimerTask() { @Override public void run() { handler.sendEmptyMessage(0); } }, 2*1000, 5*1000); AlarmReceiver alarmReceiver = new AlarmReceiver(new Handler() { @Override public void handleMessage(@NonNull Message msg) { sCount++; serviceTV.setText(sCount+""); super.handleMessage(msg); } }); IntentFilter intentFilter = new IntentFilter(); intentFilter.addAction(ALARM_RECEIVER_ACTION); registerReceiver(alarmReceiver, intentFilter); Intent intent = new Intent(ALARM_RECEIVER_ACTION); PendingIntent pendingIntent = PendingIntent.getBroadcast(MainActivity.this, 0, intent, 0);// AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE); alarmManager.setExactAndAllowWhileIdle(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime(), pendingIntent); }}public class AlarmReceiver extends BroadcastReceiver { Handler handler; AlarmReceiver(Handler handler) { this.handler = handler; } @Override public void onReceive(Context context, Intent intent) { Log.d("onReceive", "onReceive"); handler.sendEmptyMessage(0); Intent serviceIntent = new Intent(context, MyService.class); context.startService(serviceIntent); }}public class MyService extends IntentService { public MyService() { super("MyService"); } @Override protected void onHandleIntent(@Nullable Intent intent) { AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE); Intent alarmIntent = new Intent("com.yh.study.AlarmReceiver"); PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 0, alarmIntent, 0); alarmManager.setExactAndAllowWhileIdle(AlarmManager.RTC_WAKEUP, System.currentTimeMillis()+5*1000, pendingIntent); }}以下结论来源其他博主（小子在验证过程中，连接usb情况正常，拔掉usb后与结论不符，可能是小子哪里写的有问题）
在Timer中，当连接USB线进行调试时，会发现一切工作正常，每5秒更新一次界面，即使是按下电源键，仍然会5秒触发一次。 当拔掉USB线，按下电源键关闭屏幕后，过一段时间再打开，发现定时器明显没有继续计数，停留在了关闭电源键时的数字。
在AlarmService中，拔掉USB线，按下电源键，过一点时间再次打开屏幕，发现定时器一直在计数。
测试机型小米8,上timer，下AlarmService
