WatchDog简介：
当应用超过一定时间无响应的时候，系统为了不让应用长时处于不可操作的状态，会弹出一个“无响应”（ANR）的对话框，用户可以选择强制关闭，从而关掉这个进程。
ANR机制是针对应用的，对于系统进程来说，如果长时间“无响应”，Android系统设计了WatchDog机制来管控。如果超过了“无响应”的延时，那么系统WatchDog会触发自杀机制。
WatchDog注册与启动
在各种Service里面都有注册，例如AMS里：
Watchdog.getInstance().addMonitor(this);
Watchdog.getInstance().addThread(mHandler);
该Service还需要实现 implements Watchdog.Monitor 回调方法
@Override
public void monitor() {
ActivityManagerService.this.monitor();
}
启动在startOtherServices里
private void startOtherServices() {
//........
traceBeginAndSlog("StartWatchdog");
Watchdog.getInstance().start();
traceEnd();
//........
}
WatchDog运行机制
初始化，将前台、主线程、UI、IO、Display等线程加入检测队列
private Watchdog() {
mThread = new Thread(this::run, "watchdog");
// Initialize handler checkers for each common thread we want to check. Note
// that we are not currently checking the background thread, since it can
// potentially hold longer running operations with no guarantees about the timeliness
// of operations there.
// The shared foreground thread is the main checker. It is where we
// will also dispatch monitor checks and do other work.
mMonitorChecker = new HandlerChecker(FgThread.getHandler(),
"foreground thread", DEFAULT_TIMEOUT);
mHandlerCheckers.add(mMonitorChecker);
// Add checker for main thread. We only do a quick check since there
// can be UI running on the thread.
mHandlerCheckers.add(new HandlerChecker(new Handler(Looper.getMainLooper()),
"main thread", DEFAULT_TIMEOUT));
// Add checker for shared UI thread.
mHandlerCheckers.add(new HandlerChecker(UiThread.getHandler(),
"ui thread", DEFAULT_TIMEOUT));
// And also check IO thread.
mHandlerCheckers.add(new HandlerChecker(IoThread.getHandler(),
"i/o thread", DEFAULT_TIMEOUT));
// And the display thread.
mHandlerCheckers.add(new HandlerChecker(DisplayThread.getHandler(),
"display thread", DEFAULT_TIMEOUT));
// And the animation thread.
mHandlerCheckers.add(new HandlerChecker(AnimationThread.getHandler(),
"animation thread", DEFAULT_TIMEOUT));
// And the surface animation thread.
mHandlerCheckers.add(new HandlerChecker(SurfaceAnimationThread.getHandler(),
"surface animation thread", DEFAULT_TIMEOUT));
// Initialize monitor for Binder threads.
addMonitor(new BinderThreadMonitor());
mInterestingJavaPids.add(Process.myPid());
// See the notes on DEFAULT_TIMEOUT.
assert DB ||
DEFAULT_TIMEOUT > ZygoteConnectionConstants.WRAPPED_PID_TIMEOUT_MILLIS;
mTraceErrorLogger = new TraceErrorLogger();
}
WatchDog线程一直循环检测异常
private void run() {
boolean waitedHalf = false;
while (true) {
List<HandlerChecker> blockedCheckers = Collections.emptyList();
String subject = "";
boolean allowRestart = true;
int debuggerWasConnected = 0;
boolean doWaitedHalfDump = false;
final ArrayList<Integer> pids;
synchronized (mLock) {
long timeout = CHECK_INTERVAL;
// Make sure we (re)spin the checkers that have become idle within
// this wait-and-check interval
for (int i=0; i<mHandlerCheckers.size(); i++) {
HandlerChecker hc = mHandlerCheckers.get(i);
hc.scheduleCheckLocked();
}
if (debuggerWasConnected > 0) {
debuggerWasConnected--;
}
// NOTE: We use uptimeMillis() here because we do not want to increment the time we
// wait while asleep. If the device is asleep then the thing that we are waiting
// to timeout on is asleep as well and won't have a chance to run, causing a false
// positive on when to kill things.
long start = SystemClock.uptimeMillis();
while (timeout > 0) {
if (Debug.isDebuggerConnected()) {
debuggerWasConnected = 2;
}
try {
mLock.wait(timeout);
// Note: mHandlerCheckers and mMonitorChecker may have changed after waiting
} catch (InterruptedException e) {
Log.wtf(TAG, e);
}
if (Debug.isDebuggerConnected()) {
debuggerWasConnected = 2;
}
timeout = CHECK_INTERVAL - (SystemClock.uptimeMillis() - start);
}
final int waitState = evaluateCheckerCompletionLocked();
if (waitState == COMPLETED) {
// The monitors have returned; reset
waitedHalf = false;
continue;
} else if (waitState == WAITING) {
// still waiting but within their configured intervals; back off and recheck
continue;
} else if (waitState == WAITED_HALF) {
if (!waitedHalf) {
Slog.i(TAG, "WAITED_HALF");
waitedHalf = true;
// We've waited half, but we'd need to do the stack trace dump w/o the lock.
pids = new ArrayList<>(mInterestingJavaPids);
doWaitedHalfDump = true;
} else {
continue;
}
} else {
// something is overdue!
blockedCheckers = getBlockedCheckersLocked();
subject = describeCheckersLocked(blockedCheckers);
allowRestart = mAllowRestart;
pids = new ArrayList<>(mInterestingJavaPids);
}
} // END synchronized (mLock)
if (doWaitedHalfDump) {
// We've waited half the deadlock-detection interval. Pull a stack
// trace and wait another half.
ActivityManagerService.dumpStackTraces(pids, null, null,
getInterestingNativePids(), null, subject);
continue;
}
// If we got here, that means that the system is most likely hung.
// First collect stack traces from all threads of the system process.
// Then kill this process so that the system will restart.
EventLog.writeEvent(EventLogTags.WATCHDOG, subject);
final UUID errorId;
if (mTraceErrorLogger.isAddErrorIdEnabled()) {
errorId = mTraceErrorLogger.generateErrorId();
mTraceErrorLogger.addErrorIdToTrace("system_server", errorId);
} else {
errorId = null;
}
// Log the atom as early as possible since it is used as a mechanism to trigger
// Perfetto. Ideally, the Perfetto trace capture should happen as close to the
// point in time when the Watchdog happens as possible.
FrameworkStatsLog.write(FrameworkStatsLog.SYSTEM_SERVER_WATCHDOG_OCCURRED, subject);
long anrTime = SystemClock.uptimeMillis();
StringBuilder report = new StringBuilder();
report.append(MemoryPressureUtil.currentPsiState());
ProcessCpuTracker processCpuTracker = new ProcessCpuTracker(false);
StringWriter tracesFileException = new StringWriter();
final File stack = ActivityManagerService.dumpStackTraces(
pids, processCpuTracker, new SparseArray<>(), getInterestingNativePids(),
tracesFileException, subject);
// Give some extra time to make sure the stack traces get written.
// The system's been hanging for a minute, another second or two won't hurt much.
SystemClock.sleep(5000);
processCpuTracker.update();
report.append(processCpuTracker.printCurrentState(anrTime));
report.append(tracesFileException.getBuffer());
// Trigger the kernel to dump all blocked threads, and backtraces on all CPUs to the kernel log
doSysRq('w');
doSysRq('l');
// Try to add the error to the dropbox, but assuming that the ActivityManager
// itself may be deadlocked. (which has happened, causing this statement to
// deadlock and the watchdog as a whole to be ineffective)
Thread dropboxThread = new Thread("watchdogWriteToDropbox") {
public void run() {
// If a watched thread hangs before init() is called, we don't have a
// valid mActivity. So we can't log the error to dropbox.
if (mActivity != null) {
mActivity.addErrorToDropBox(
"watchdog", null, "system_server", null, null, null,
null, report.toString(), stack, null, null, null,
errorId);
}
}
};
dropboxThread.start();
try {
dropboxThread.join(2000); // wait up to 2 seconds for it to return.
} catch (InterruptedException ignored) {}
IActivityController controller;
synchronized (mLock) {
controller = mController;
}
if (controller != null) {
Slog.i(TAG, "Reporting stuck state to activity controller");
try {
Binder.setDumpDisabled("Service dumps disabled due to hung system process.");
// 1 = keep waiting, -1 = kill system
int res = controller.systemNotResponding(subject);
if (res >= 0) {
Slog.i(TAG, "Activity controller requested to coninue to wait");
waitedHalf = false;
continue;
}
} catch (RemoteException e) {
}
}
// Only kill the process if the debugger is not attached.
if (Debug.isDebuggerConnected()) {
debuggerWasConnected = 2;
}
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);
WatchdogDiagnostics.diagnoseCheckers(blockedCheckers);
Slog.w(TAG, "*** GOODBYE!");
if (!Build.IS_USER && isCrashLoopFound()
&& !WatchdogProperties.should_ignore_fatal_count().orElse(false)) {
breakCrashLoop();
}
Process.killProcess(Process.myPid());
System.exit(10);
}
waitedHalf = false;
}
}
run()方法内部执行逻辑有点复杂，把他们拆分成五个部分：
检测1：遍历mHandlerCheckers列表，执行scheduleCheckLocked()来开启检测；
检测2：开启定期检测，每一次检查的间隔时间由CHECK_INTERVAL常量设定，默认为30秒；
检测3：检查HanddlerChecker的完成状态：COMPLETED表示已经完成；WAITING和WAITED_HALF表示还在等待，但未超时；OVERDUE表示已经超时；
检测4：如果存在超时的HandlerChecker，获取阻塞的HandlerChecker，生成一些描述信息；
检测5：保存日志，打印调用栈，然后kill系统进程；
总结
Watchdog是个相对比较简单的监控线程，主要承担检测system_server进程的任务，如果system_server进程异常，就执行kill让系统重启。
