看门狗最初的意义是因为早期嵌入式设备上的程序经常跑飞（比如说电磁干扰等），所以专门设置了一个硬件看门狗，每隔一段时间，看门狗就去检查某个参数是不是被设置了，如果发现该参数被设置了，则判断为系统出错，然后强制重启。
Watchdog是Android用于对SystemServer的参数设置进行监听的看门狗。那它看的是哪几个门呢，主要是几个重要的service的门。
- ActivityManagerService
- PowerManagerService
- WindowManagerService
一旦发现service出了问题，就会杀掉system_server,而这也会使zygote随其一起自杀，最后导致重启java世界。
那system_server是如何使用Watchdog来为自己服务的呢？
system_server和Watchdog的交互流程可以总结为以下三个步骤：
- Watchdog.getInstance().init()
- Watchdog.getInstance().start().
- Watchdog.getInstance().addMonitor()
这三个步骤都非常简单。先看第一步
创建和初始化Watchdog
getInstance用于创建Watchdog
public static Watchdog getInstance() {
if (sWatchdog == null) {
sWatchdog = new Watchdog();
}
return sWatchdog;
}
private Watchdog() {
super("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));
// Initialize monitor for Binder threads.
addMonitor(new BinderThreadMonitor());
}
接着看看Init函数做了些什么
public void init(Context context, ActivityManagerService activity) {
mResolver = context.getContentResolver();
mActivity = activity;
context.registerReceiver(new RebootRequestReceiver(),
new IntentFilter(Intent.ACTION_REBOOT),
android.Manifest.permission.REBOOT, null);
}
2.让Watchdog看门狗跑起来
SystemServer调用了Watchdog的start函数，这将导致Watchdog的run在另外一个线程中被执行。
public void run() {
boolean waitedHalf = false;
while (true) {
final ArrayList blockedCheckers;
final String subject;
final boolean allowRestart;
int debuggerWasConnected = 0;
synchronized (this) {
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 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 {
wait(timeout);
} 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) {
// We've waited half the deadlock-detection interval. Pull a stack
// trace and wait another half.
ArrayList pids = new ArrayList();
pids.add(Process.myPid());
ActivityManagerService.dumpStackTraces(true, pids, null, null,
NATIVE_STACKS_OF_INTEREST);
waitedHalf = true;
}
continue;
}
// something is overdue!
blockedCheckers = getBlockedCheckersLocked();
subject = describeCheckersLocked(blockedCheckers);
allowRestart = mAllowRestart;
}
// 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);
ArrayList pids = new ArrayList();
pids.add(Process.myPid());
if (mPhonePid > 0) pids.add(mPhonePid);
// Pass !waitedHalf so that just in case we somehow wind up here without having
// dumped the halfway stacks, we properly re-initialize the trace file.
final File stack = ActivityManagerService.dumpStackTraces(
!waitedHalf, pids, null, null, NATIVE_STACKS_OF_INTEREST);
// 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(2000);
// Pull our own kernel thread stacks as well if we're configured for that
if (RECORD_KERNEL_THREADS) {
dumpKernelStackTraces();
}
// 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() {
mActivity.addErrorToDropBox(
"watchdog", null, "system_server", null, null,
subject, null, stack, null);
}
};
dropboxThread.start();
try {
dropboxThread.join(2000); // wait up to 2 seconds for it to return.
} catch (InterruptedException ignored) {}
IActivityController controller;
synchronized (this) {
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);
for (int i=0; i 隔一段时间给另外一个线程发送一条monitor消息，那个线程将检查各个service的健康情况。而看门狗会等待检查结果，如果最后没有返回结果，那么它会杀掉systemServer.
3.列队检查
要想支持看门狗的检查，就需要让这些Service实现Monitor接口，
public interface Monitor {
void monitor();
}
例如WindowManagerServer
public class WindowManagerService extends IWindowManager.Stub
implements ==Watchdog.Monitor,== WindowManagerPolicy.WindowManagerFuncs
然后Watchdog就会调用它们的monitor函数进行检查了。
那么Service的健康是如何判定的呢。我们以WindowManagerService为例，先看看它是怎么把自己交给看门狗检查的，代码如下：
// Add ourself to the Watchdog monitors.
//在构造函数中把自己加入了Watchdog的检查列队中
Watchdog.getInstance().addMonitor(this);
而Watchdog调用各个monitor函数到底又检查了什么呢？再看看它实现的monitor函数吧。
WindowManagerServer-->
@Override
public void monitor() {
//原来monitor检查的就是这些service是不是又发生死锁了
synchronized (mWindowMap) { }
}
原来，watchdog最怕系统服务死锁了，对于这种情况也只能采取杀系统的方式了。
说明：这种情况我只碰过一次，原因是一个函数占着锁，但长时间没有返回。没有返回的原因是这个函数需要和硬件交互，而硬件又没有及时返回。