浅谈浅谈Android ANR在线监控原理在线监控原理
Android中的中的Watchdog
在Android中，Watchdog是用来监测关键服务是否发生了死锁，如果发生了死锁就kill进程，重启SystemServer
Android的Watchdog是在SystemServer中进行初始化的，所以Watchdog是运行在SystemServer进程中
Watchdog是运行一个单独的线程中的，每次wait 30s之后就会发起一个监测行为，如果系统休眠了，那Watchdog的wait行为
也会休眠，此时需要等待系统唤醒之后才会重新恢复监测
想要被Watchdog监测的对象需要实现Watchdog.Monitor接口的monitor()方法，然后调用addMonitor()方法
其实framework里面的Watchdog实现除了能监控线程死锁以外还能够监控线程卡顿，addMonitor()方法是监控线程死锁的，而
addThread()方法是监控线程卡顿的
Watchdog线程死锁监控实现线程死锁监控实现
Watchdog监控线程死锁需要被监控的对象实现Watchdog.Monitor接口的monitor()方法，然后再调用addMonitor()方法，例如
ActivityManagerService：
public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
public ActivityManagerService(Context systemContext) {
Watchdog.getInstance().addMonitor(this);
}
public void monitor() {
synchronized (this) { }
}
// ...
}
如上是从ActivityManagerService提取出来关于Watchdog监控ActivityManagerService这个对象锁的相关代码，而监控的实现
如下，Watchdog是一个线程对象，start这个线程之后就会每次wait 30s后检查一次，如此不断的循环检查：
public void addMonitor(Monitor monitor) {
synchronized (this) {
if (isAlive()) {
throw new RuntimeException("Monitors can't be added once the Watchdog is running");
}
mMonitorChecker.addMonitor(monitor);
}
}
@Override
public void run() {
boolean waitedHalf = false;
while (true) {
final ArrayList<HandlerChecker> 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<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) {
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