Android看门狗（WatchDog）

一：概念

Android WatchDog（看门狗）是Android系统中用于监控系统关键服务的运行状态的机制，其核心目标是检测系统服务是否因死锁、阻塞或异常导致长时间无响应，并在必要时触发系统恢复（如重启）。

二：核心功能

2.1 服务状态监控

定期检查关键系统服务（如ActivityManager、WindowManager等）是否正常响应，防止服务阻塞导致系统卡死。

2.2 超时处理

若某个服务未在规定时间内更新“心跳”（monitor），WatchDog判定为超时，触发后续处理流程。

2.3 日志收集与调试

超时发生时，自动收集系统堆栈信息（包括所有线程的调用栈），帮助定位问题根源。

2.4 系统恢复

在严重超时情况下，可能强制重启系统进程（system_server）或整个设备，避免用户长时间面对无响应界面。

三：实现逻辑

3.1 启动

WatchDog像系统服务一样，是由SystemServer启动的，具体启动逻辑如下

frameworks/base/services/java/com/android/server/SystemServer.javaprivate void startBootstrapServices(@NonNull TimingsTraceAndSlog t) {...//Watchdog对象的创建及线程的启动t.traceBegin("StartWatchdog");final Watchdog watchdog = Watchdog.getInstance();watchdog.start();mDumper.addDumpable(watchdog);t.traceEnd();...//Watchdog初始化t.traceBegin("InitWatchdog");watchdog.init(mSystemContext, mActivityManagerService);t.traceEnd();...}

3.2 初始化

WatchDog运行在"watchdog"线程中
"watchdog.monitor"后台线程，负责处理通过Handler发送的任务
HandlerChecker一个与ServiceThread的Looper关联的Handler。这个Handler会将消息和任务发送到ServiceThread中执行，用于监控线程的响应性，确保线程正常运行
把"monitor thread"、"foreground thread"、"main thread"、"ui thread"、"i/o thread"、"display thread"、"animation thread"、"surface animation thread"等系统关键线程加入监控中
初始化Binder线程的监视器

frameworks/base/services/core/java/com/android/server/Watchdog.javaprivate Watchdog() {//新建一个名为"watchdog"的线程mThread = new Thread(this::run, "watchdog");...//启动一个名为"watchdog.monitor"的后台线程，负责处理通过Handler发送的任务ServiceThread t = new ServiceThread("watchdog.monitor",android.os.Process.THREAD_PRIORITY_DEFAULT, true /*allowIo*/);t.start();//创建一个与ServiceThread的Looper关联的Handler。这个Handler会将消息和任务发送到ServiceThread中执行，用于监控线程的响应性，确保线程正常运行mMonitorChecker = new HandlerChecker(new Handler(t.getLooper()), "monitor thread", mLock);//监控"monitor thread"、"foreground thread"、"main thread"、"ui thread"、"i/o thread"、"display thread"、"animation thread"、"surface animation thread"等系统关键线程mHandlerCheckers.add(withDefaultTimeout(mMonitorChecker));...//初始化Binder线程的监视器addMonitor(new BinderThreadMonitor());...}public void start() {//启动"watchdog"线程mThread.start();}public void init(Context context, ActivityManagerService activity) {mActivity = activity;//注册重启广播context.registerReceiver(new RebootRequestReceiver(),new IntentFilter(Intent.ACTION_REBOOT),android.Manifest.permission.REBOOT, null);...}

3.3 周期性检测

检查周期为15s，最大等待时间为60s
用不同的状态表示不同的等待时间：WAITING（等待了15s之内）/WAITED_UNTIL_PRE_WATCHDOG（等待了15-60s）/OVERDUE（等待超过60s）/COMPLETED（已完成）
等待时间在15s-30s，会收集堆栈信息但不立即重启；如果超过60s，会收集堆栈信息并立刻重启
检测手段：调用被监控线程的monitor函数，根据返回时间来判断超时时间

    private void run() {boolean waitedHalf = false;while (true) {List<HandlerChecker> blockedCheckers = Collections.emptyList();...boolean doWaitedPreDump = false;//watchdog超时时间（60s）final long watchdogTimeoutMillis = mWatchdogTimeoutMillis;//watchdog检查间隔（15s）final long checkIntervalMillis = watchdogTimeoutMillis / PRE_WATCHDOG_TIMEOUT_RATIO;...synchronized (mLock) {long sfHangTime;long timeout = checkIntervalMillis;...for (int i=0; i<mHandlerCheckers.size(); i++) {HandlerCheckerAndTimeout hc = mHandlerCheckers.get(i);//把mMonitorQueue中的monitor加入到mMonitors中，并清空mMonitorQueuehc.checker().scheduleCheckLocked(hc.customTimeoutMillis().orElse(watchdogTimeoutMillis * Build.HW_TIMEOUT_MULTIPLIER));}...long start = SystemClock.uptimeMillis();while (timeout > 0) {...try {//等待一个检查周期（15s）mLock.wait(timeout);// Note: mHandlerCheckers and mMonitorChecker may have changed after waiting} catch (InterruptedException e) {Log.wtf(TAG, e);}...timeout = checkIntervalMillis - (SystemClock.uptimeMillis() - start);}...if (sfHangTime > TIME_SF_WAIT * 2) {...} else {//针对每一个HandlerChecker的等待时间，返回不用的状态（WAITING-等待了15s之内/WAITED_UNTIL_PRE_WATCHDOG-等待了15-60s/OVERDUE-等待超过60s/COMPLETED-已完成）final int waitState = evaluateCheckerCompletionLocked();if (waitState == COMPLETED) {//monitor的thread已按时返回心跳，重置waitedHalf，继续下一次循环...waitedHalf = false;continue;} else if (waitState == WAITING) {//monitor的thread没有在15s内返回心跳，继续等待，不做处理，继续下一次循环continue;} else if (waitState == WAITED_UNTIL_PRE_WATCHDOG) {//monitor的thread没有在15s-60s内返回心跳if (!waitedHalf) {//monitor的thread没有在15s-30s内返回心跳Slog.i(TAG, "WAITED_UNTIL_PRE_WATCHDOG");waitedHalf = true;//获取阻塞的线程blockedCheckers = getCheckersWithStateLocked(WAITED_UNTIL_PRE_WATCHDOG);subject = describeCheckersLocked(blockedCheckers);pids = new ArrayList<>(mInterestingJavaPids);doWaitedPreDump = true;} else {//monitor的thread没有在30s-60s内返回心跳continue;}} else {//monitor的thread没有在60s内返回心跳// something is overdue!blockedCheckers = getCheckersWithStateLocked(OVERDUE);subject = describeCheckersLocked(blockedCheckers);allowRestart = mAllowRestart;pids = new ArrayList<>(mInterestingJavaPids);}}} // END synchronized (mLock)//打印堆栈到日志中logWatchog(doWaitedPreDump, subject, pids);if (doWaitedPreDump) {//monitor的thread没有在15s-30s内返回心跳，继续下一次循环continue;}...if (debuggerWasConnected >= 2) {Slog.w(TAG, "Debugger connected: Watchdog is *not* killing the system process");} else if (debuggerWasConnected > 0) {Slog.w(TAG, "Debugger was connected: Watchdog is *not* killing the system process");} else if (!allowRestart) {Slog.w(TAG, "Restart not allowed: Watchdog is *not* killing the system process");} else {Slog.w(TAG, "*** WATCHDOG KILLING SYSTEM PROCESS: " + subject);//诊断被阻塞的检查器并记录相关信息WatchdogDiagnostics.diagnoseCheckers(blockedCheckers);Slog.w(TAG, "*** GOODBYE!");...exceptionHang.WDTMatterJava(330);if (mSfHang) {...} else {//把WatchDog进程杀掉（WatchDog自杀）Process.killProcess(Process.myPid());}//终止当前运行的JVMSystem.exit(10);}waitedHalf = false;}}public static class HandlerChecker implements Runnable {public void scheduleCheckLocked(long handlerCheckerTimeoutMillis) {mWaitMaxMillis = handlerCheckerTimeoutMillis;if (mCompleted) {//把mMonitorQueue中的monitor加入到mMonitors中，并清空mMonitorQueuemMonitors.addAll(mMonitorQueue);mMonitorQueue.clear();}...}public void run() {final int size = mMonitors.size();for (int i = 0 ; i < size ; i++) {synchronized (mLock) {mCurrentMonitor = mMonitors.get(i);}//检查监控线程状态，如果被监控线程卡住，这里也会卡住mCurrentMonitor.monitor();}synchronized (mLock) {mCompleted = true;mCurrentMonitor = null;}}}

四：总结

4.1 如何将特定线程加入WatchDog监控

答：可参考AMS

4.1.1 实现Watchdog.Monitor接口

public class ActivityManagerService extends IActivityManager.Stub
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback, ActivityManagerGlobalLock {

public void monitor() {
synchronized (this) { }
}

}

2.1.2 初始化时把自身加入到WatchDog的监控线程中

public ActivityManagerService(Context systemContext, ActivityTaskManagerService atm) {
...
//AMS把自身加入到WatchDog的监控线程中
Watchdog.getInstance().addMonitor(this);
Watchdog.getInstance().addThread(mHandler);
...
}