WatchDog 工作原理（上）

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来源：伯乐在线专栏作者 - gityuan

链接：http://android.jobbole.com/84881/

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一、概述

Android系统中，有硬件WatchDog用于定时检测关键硬件是否正常工作，类似地，在framework层有一个软件WatchDog用于定期检测关键系统服务是否发生死锁事件。WatchDog功能主要是分析系统核心服务和重要线程是否处于Blocked状态。

• 监视reboot广播；

• 监视mMonitors关键系统服务是否死锁。

二、启动流程

2.1 startOtherServices

[-> SystemServer.java]

private void startOtherServices () {

...

//创建watchdog【见小节2.2】

final Watchdog watchdog = Watchdog . getInstance ();

//注册reboot广播【见小节2.3】

watchdog . init ( context , mActivityManagerService );

...

mSystemServiceManager . startBootPhase ( SystemService . PHASE_LOCK_SETTINGS_READY );

...

mActivityManagerService . systemReady ( new Runnable () {

@Override

public void run () {

mSystemServiceManager . startBootPhase (

SystemService . PHASE_ACTIVITY_MANAGER_READY );

...

// watchdog启动【见小节3.1】

Watchdog . getInstance (). start ();

mSystemServiceManager . startBootPhase (

SystemService . PHASE_THIRD_PARTY_APPS_CAN_START );

}

}

}

2.2 getInstance

[-> Watchdog.java]

public static Watchdog getInstance () {

if ( sWatchdog == null ) {

//单例模式，创建实例对象【见小节2.2.1 】

sWatchdog = new Watchdog ();

}

return sWatchdog ;

}

2.2.1 创建Watchdog

[-> Watchdog.java]

public class Watchdog extends Thread {

...

private Watchdog () {

super ( "watchdog" );

//【见小节2.2.2 】

mMonitorChecker = new HandlerChecker ( FgThread . getHandler (),

"foreground thread" , DEFAULT_TIMEOUT );

mHandlerCheckers . add ( mMonitorChecker );

mHandlerCheckers . add ( new HandlerChecker ( new Handler ( Looper . getMainLooper ()),

"main thread" , DEFAULT_TIMEOUT ));

mHandlerCheckers . add ( new HandlerChecker ( UiThread . getHandler (),

"ui thread" , DEFAULT_TIMEOUT ));

mHandlerCheckers . add ( new HandlerChecker ( IoThread . getHandler (),

"i/o thread" , DEFAULT_TIMEOUT ));

mHandlerCheckers . add ( new HandlerChecker ( DisplayThread . getHandler (),

"display thread" , DEFAULT_TIMEOUT ));

//【见小节2.2.3】

addMonitor ( new BinderThreadMonitor ());

}

}

Watchdog继承于Thread，创建的线程名为”watchdog”。mHandlerCheckers是记录着所有的HandlerChecker对象的列表。

Watchdog监控的线程有：

线程名	对应handler	含义
main thread	new Handler(MainLooper)	当前主线程
android.fg	FgThread.getHandler	前台线程
android.ui	UiThread.getHandler	UI线程
android.io	IoThread.getHandler	I/O线程
android.display	DisplayThread.getHandler	Display线程

DEFAULT_TIMEOUT默认为60s，调试时才为10s方便找出潜在的ANR问题。

2.2.2 HandlerChecker

[-> Watchdog.java]

public final class HandlerChecker implements Runnable {

...

HandlerChecker ( Handler handler , String name , long waitMaxMillis ) {

mHandler = handler ;

mName = name ;

mWaitMax = waitMaxMillis ;

mCompleted = true ;

}

}

mMonitors记录所有Watchdog目前正在监控的服务。

2.2.3 监控Binder线程

在小节【2.2.1】创建Watchdog时，通过addMonitor(new BinderThreadMonitor())来监控Binder线程， 这里拆分两步骤：

• addMonitor

• new BinderThreadMonitor

2.2.3.1 addMonitor

public class Watchdog extends Thread {

public void addMonitor ( Monitor monitor ) {

synchronized ( this ) {

if ( isAlive ()) {

throw new RuntimeException ( "Monitors can't be added once the Watchdog is running" );

}

//此处mMonitorChecker数据类型为HandlerChecker

mMonitorChecker . addMonitor ( monitor );

}

}

public final class HandlerChecker implements Runnable {

private final ArrayList mMonitors = new ArrayList ();

public void addMonitor ( Monitor monitor ) {

//将上面的BinderThreadMonitor添加到mMonitors队列

mMonitors . add ( monitor );

}

...

}

}

将monitor添加到HandlerChecker的成员变量mMonitors列表中。

2.2.3.2 BinderThreadMonitor

private static final class BinderThreadMonitor implements Watchdog . Monitor {

public void monitor () {

Binder . blockUntilThreadAvailable ();

}

}

blockUntilThreadAvailable最终调用的是IPCThreadState，等待有空闲的binder线程

void IPCThreadState :: blockUntilThreadAvailable ()

{

pthread_mutex_lock ( & mProcess -> mThreadCountLock );

while ( mProcess -> mExecutingThreadsCount >= mProcess -> mMaxThreads ) {

//等待正在执行的binder线程小于进程最大binder线程上限(16个)

pthread_cond_wait ( & mProcess -> mThreadCountDecrement , & mProcess -> mThreadCountLock );

}

pthread_mutex_unlock ( & mProcess -> mThreadCountLock );

}

可见addMonitor(new BinderThreadMonitor())是将Binder线程添加到android.fg线程的handler(mMonitorChecker)来检查是否工作正常。

2.2.4 Monitor

public class Watchdog extends Thread {

public interface Monitor {

void monitor ();

}

}

能够被Watchdog监控的系统服务都实现了Watchdog.Monitor接口。 实现该接口类：

• ActivityManagerService

• PowerManagerService

• WindowManagerService

• InputManagerService

• NetworkManagementService

• MountService

• NativeDaemonConnector

• BinderThreadMonitor

• MediaProjectionManagerService

• MediaRouterService

• MediaSessionService

2.3 init

[-> Watchdog.java]

public void init ( Context context , ActivityManagerService activity ) {

mResolver = context . getContentResolver ();

mActivity = activity ;

//注册reboot广播接收者【见小节2.3.1】

context . registerReceiver ( new RebootRequestReceiver (),

new IntentFilter ( Intent . ACTION_REBOOT ),

android . Manifest . permission . REBOOT , null );

}

2.3.1 RebootRequestReceiver

[-> Watchdog.java]

final class RebootRequestReceiver extends BroadcastReceiver {

@Override

public void onReceive ( Context c , Intent intent ) {

if ( intent . getIntExtra ( "nowait" , 0 ) != 0 ) {

//【见小节2.3.2】

rebootSystem ( "Received ACTION_REBOOT broadcast" );

return ;

}

Slog . w ( TAG , "Unsupported ACTION_REBOOT broadcast: " + intent );

}

}

2.3.2 rebootSystem

[-> Watchdog.java]

void rebootSystem ( String reason ) {

Slog . i ( TAG , "Rebooting system because: " + reason );

IPowerManager pms = ( IPowerManager ) ServiceManager . getService ( Context . POWER_SERVICE );

try {

//通过PowerManager执行reboot操作

pms . reboot ( false , reason , false );

} catch