ANR触发原理

ANR(Application Not responding)，是指应用程序未响应，Android系统对于一些事件需要在一定的时间范围内完成，如果超过预定时间能未能得到有效响应或者响应时间过长，都会造成ANR。

造成ANR的场景：

Service Timeout:比如前台服务在20s内未执行完成；
BroadcastQueue Timeout：比如前台广播在10s内未执行完成
ContentProvider Timeout：内容提供者,在publish过超时10s;
InputDispatching Timeout: 输入事件分发超时5s，包括按键和触摸事件。

触发ANR的过程可分为三个步骤: 埋炸弹, 拆炸弹, 引爆炸弹

<1> Service Timeout是位于”ActivityManager”线程中的AMS.MainHandler收到SERVICE_TIMEOUT_MSG消息时触发。

对于Service有两类:

对于前台服务，则超时为SERVICE_TIMEOUT = 20s；
对于后台服务，则超时为SERVICE_BACKGROUND_TIMEOUT = 200s

由变量ProcessRecord.execServicesFg来决定是否前台启动.

埋炸弹阶段：在Service进程attach到system_server进程的过程中会调用realStartServiceLocked()方法 (准确说是scheduleServiceTimeoutLocked方法) 中来埋下炸弹.

private final void realStartServiceLocked(ServiceRecord r, ProcessRecord app, boolean execInFg) throws RemoteException {
    ...
    //发送delay消息(SERVICE_TIMEOUT_MSG)，
    bumpServiceExecutingLocked(r, execInFg, "create");
    try {
        ...
        //最终执行服务的onCreate()方法
        app.thread.scheduleCreateService(r, r.serviceInfo,
                mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
                app.repProcState);
    } catch (DeadObjectException e) {
        mAm.appDiedLocked(app);
        throw e;
    } finally {
        ...
    }
}

private final void bumpServiceExecutingLocked(ServiceRecord r, boolean fg, String why) {
    ... 
    scheduleServiceTimeoutLocked(r.app);
}

void scheduleServiceTimeoutLocked(ProcessRecord proc) {
    if (proc.executingServices.size() == 0 || proc.thread == null) {
        return;
    }
    long now = SystemClock.uptimeMillis();
    Message msg = mAm.mHandler.obtainMessage(
            ActivityManagerService.SERVICE_TIMEOUT_MSG);
    msg.obj = proc;
    
    //当超时后仍没有remove该SERVICE_TIMEOUT_MSG消息，则执行service Timeout流程
    mAm.mHandler.sendMessageAtTime(msg,
        proc.execServicesFg ? (now+SERVICE_TIMEOUT) : (now+ SERVICE_BACKGROUND_TIMEOUT));
}

拆炸弹阶段：经过Binder等层层调用进入目标进程的主线程ActivityThread.handleCreateService()的过程. 在这个过程会创建目标服务对象,以及回调onCreate()方法, 紧接再次经过多次调用回到system_server来执行serviceDoneExecuting.

最终在serviceDoneExecutingLocked中移除服务超时消息SERVICE_TIMEOUT_MSG。

 private void handleCreateService(CreateServiceData data) {
        ...
        java.lang.ClassLoader cl = packageInfo.getClassLoader();
        Service service = (Service) cl.loadClass(data.info.name).newInstance();
        ...

        try {
            //创建ContextImpl对象
            ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
            context.setOuterContext(service);
            //创建Application对象
            Application app = packageInfo.makeApplication(false, mInstrumentation);
            service.attach(context, this, data.info.name, data.token, app,
                    ActivityManagerNative.getDefault());
            //调用服务onCreate()方法 
            service.onCreate();
            
            //拆除炸弹引线[见小节2.2.2]
            ActivityManagerNative.getDefault().serviceDoneExecuting(
                    data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
        } catch (Exception e) {
            ...
        }
    }

private void serviceDoneExecutingLocked(ServiceRecord r, boolean inDestroying, boolean finishing) {
    ...
    if (r.executeNesting <= 0) {
        if (r.app != null) {
            r.app.execServicesFg = false;
            r.app.executingServices.remove(r);
            if (r.app.executingServices.size() == 0) {
                //当前服务所在进程中没有正在执行的service
                mAm.mHandler.removeMessages(ActivityManagerService.SERVICE_TIMEOUT_MSG, r.app);
        ...
    }
    ...

引爆炸弹阶段：在system_server进程中有一个Handler线程, 名叫”ActivityManager”.当倒计时结束便会向该Handler线程发送一条信息SERVICE_TIMEOUT_MSG,

final class MainHandler extends Handler {
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case SERVICE_TIMEOUT_MSG: {
                ...
                //【见小节2.3.2】
                mServices.serviceTimeout((ProcessRecord)msg.obj);
            } break;
            ...
        }
        ...
    }
}

void serviceTimeout(ProcessRecord proc) {
    String anrMessage = null;

    synchronized(mAm) {
        if (proc.executingServices.size() == 0 || proc.thread == null) {
            return;
        }
        final long now = SystemClock.uptimeMillis();
        final long maxTime =  now -
                (proc.execServicesFg ? SERVICE_TIMEOUT : SERVICE_BACKGROUND_TIMEOUT);
        ServiceRecord timeout = null;
        long nextTime = 0;
        for (int i=proc.executingServices.size()-1; i>=0; i--) {
            ServiceRecord sr = proc.executingServices.valueAt(i);
            if (sr.executingStart < maxTime) {
                timeout = sr;
                break;
            }
            if (sr.executingStart > nextTime) {
                nextTime = sr.executingStart;
            }
        }
        if (timeout != null && mAm.mLruProcesses.contains(proc)) {
            Slog.w(TAG, "Timeout executing service: " + timeout);
            StringWriter sw = new StringWriter();
            PrintWriter pw = new FastPrintWriter(sw, false, 1024);
            pw.println(timeout);
            timeout.dump(pw, " ");
            pw.close();
            mLastAnrDump = sw.toString();
            mAm.mHandler.removeCallbacks(mLastAnrDumpClearer);
            mAm.mHandler.postDelayed(mLastAnrDumpClearer, LAST_ANR_LIFETIME_DURATION_MSECS);
            anrMessage = "executing service " + timeout.shortName;
        }
    }

    if (anrMessage != null) {
        //当存在timeout的service，则执行appNotResponding
        mAm.appNotResponding(proc, null, null, false, anrMessage);
    }
}

其中anrMessage的内容为”executing service [发送超时serviceRecord信息]”;

<2> BroadcastReceiver Timeout是位于”ActivityManager”线程中的BroadcastQueue.BroadcastHandler收到BROADCAST_TIMEOUT_MSG消息时触发。

对于广播队列有两个: foreground队列和background队列:

对于前台广播，则超时为BROADCAST_FG_TIMEOUT = 10s；
对于后台广播，则超时为BROADCAST_BG_TIMEOUT = 60s ;

埋炸弹阶段:

通过调用 BroadcastQueue.processNextBroadcast() 来处理广播.其流程为先处理并行广播,再处理当前有序广播,最后获取并处理下条有序广播.

final void processNextBroadcast(boolean fromMsg) {
    synchronized(mService) {
        ...
        //part 2: 处理当前有序广播
        do {
            r = mOrderedBroadcasts.get(0);
            //获取所有该广播所有的接收者
            int numReceivers = (r.receivers != null) ? r.receivers.size() : 0;
            if (mService.mProcessesReady && r.dispatchTime > 0) {
                long now = SystemClock.uptimeMillis();
                if ((numReceivers > 0) &&
                        (now > r.dispatchTime + (2*mTimeoutPeriod*numReceivers))) {
                    //当广播处理时间超时，则强制结束这条广播
                    broadcastTimeoutLocked(false);
                    ...
                }
            }
            if (r.receivers == null || r.nextReceiver >= numReceivers
                    || r.resultAbort || forceReceive) {
                if (r.resultTo != null) {
                    //处理广播消息消息
                    performReceiveLocked(r.callerApp, r.resultTo,
                        new Intent(r.intent), r.resultCode,
                        r.resultData, r.resultExtras, false, false, r.userId);
                    r.resultTo = null;
                }
                //拆炸弹
                cancelBroadcastTimeoutLocked();
            }
        } while (r == null);
        ...

        //part 3: 获取下条有序广播
        r.receiverTime = SystemClock.uptimeMillis();
        if (!mPendingBroadcastTimeoutMessage) {
            long timeoutTime = r.receiverTime + mTimeoutPeriod;
            //埋炸弹
            setBroadcastTimeoutLocked(timeoutTime);
        }
        ...
    }
}

对于广播超时处理时机：

首先在part3的过程中setBroadcastTimeoutLocked(timeoutTime) 设置超时广播消息；
然后在part2根据广播处理情况来处理：
- 当广播接收者等待时间过长，则调用broadcastTimeoutLocked(false);
- 当执行完广播,则调用cancelBroadcastTimeoutLocked;

final void setBroadcastTimeoutLocked(long timeoutTime) {
    if (! mPendingBroadcastTimeoutMessage) {
        Message msg = mHandler.obtainMessage(BROADCAST_TIMEOUT_MSG, this);
        mHandler.sendMessageAtTime(msg, timeoutTime);
        mPendingBroadcastTimeoutMessage = true;
    }
}

设置定时广播BROADCAST_TIMEOUT_MSG，即当前往后推mTimeoutPeriod时间广播还没处理完毕，则进入广播超时流程。

拆炸弹阶段：

在processNextBroadcast()过程, 执行完performReceiveLocked,便会拆除炸弹.

final void cancelBroadcastTimeoutLocked() {
    if (mPendingBroadcastTimeoutMessage) {
        mHandler.removeMessages(BROADCAST_TIMEOUT_MSG, this);
        mPendingBroadcastTimeoutMessage = false;
    }
}

引爆炸弹阶段：

BroadcastHandler.handleMessage

private final class BroadcastHandler extends Handler {
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case BROADCAST_TIMEOUT_MSG: {
                synchronized (mService) {
                    //【见小节3.3.2】
                    broadcastTimeoutLocked(true);
                }
            } break;
            ...
        }
        ...
    }
}

不会引爆的四种情况

mOrderedBroadcasts已处理完成，则不会anr;
正在执行dexopt，则不会anr;
系统还没有进入ready状态(mProcessesReady=false)，则不会anr;
如果当前正在执行的receiver没有超时，则重新设置广播超时，不会anr;

<3> ContentProvider Timeout是位于”ActivityManager”线程中的AMS.MainHandler收到CONTENT_PROVIDER_PUBLISH_TIMEOUT_MSG消息时触发。

ContentProvider 超时为CONTENT_PROVIDER_PUBLISH_TIMEOUT = 10s. 这个跟前面的Service和BroadcastQueue完全不同, 由Provider进程启动过程相关.

埋炸弹阶段：

埋炸弹的过程其实是在进程创建的过程,进程创建后会调用attachApplicationLocked()进入system_server进程. 10s之后引爆该炸弹

private final boolean attachApplicationLocked(IApplicationThread thread, int pid) {
    ProcessRecord app;
    if (pid != MY_PID && pid >= 0) {
        synchronized (mPidsSelfLocked) {
            app = mPidsSelfLocked.get(pid); // 根据pid获取ProcessRecord
        }
    } 
    ...
    
    //系统处于ready状态或者该app为FLAG_PERSISTENT进程则为true
    boolean normalMode = mProcessesReady || isAllowedWhileBooting(app.info);
    List<ProviderInfo> providers = normalMode ? generateApplicationProvidersLocked(app) : null;

    //app进程存在正在启动中的provider,则超时10s后发送CONTENT_PROVIDER_PUBLISH_TIMEOUT_MSG消息
    if (providers != null && checkAppInLaunchingProvidersLocked(app)) {
        Message msg = mHandler.obtainMessage(CONTENT_PROVIDER_PUBLISH_TIMEOUT_MSG);
        msg.obj = app;
        mHandler.sendMessageDelayed(msg, CONTENT_PROVIDER_PUBLISH_TIMEOUT);
    }
    
    thread.bindApplication(...);
    ...
}

拆炸弹阶段：

当provider成功publish之后,便会拆除该炸弹

public final void publishContentProviders(IApplicationThread caller, List<ContentProviderHolder> providers) {
   ...
   
   synchronized (this) {
       final ProcessRecord r = getRecordForAppLocked(caller);
       
       final int N = providers.size();
       for (int i = 0; i < N; i++) {
           ContentProviderHolder src = providers.get(i);
           ...
           ContentProviderRecord dst = r.pubProviders.get(src.info.name);
           if (dst != null) {
               ComponentName comp = new ComponentName(dst.info.packageName, dst.info.name);
               
               mProviderMap.putProviderByClass(comp, dst); //将该provider添加到mProviderMap
               String names[] = dst.info.authority.split(";");
               for (int j = 0; j < names.length; j++) {
                   mProviderMap.putProviderByName(names[j], dst);
               }

               int launchingCount = mLaunchingProviders.size();
               int j;
               boolean wasInLaunchingProviders = false;
               for (j = 0; j < launchingCount; j++) {
                   if (mLaunchingProviders.get(j) == dst) {
                       //将该provider移除mLaunchingProviders队列
                       mLaunchingProviders.remove(j);
                       wasInLaunchingProviders = true;
                       j--;
                       launchingCount--;
                   }
               }
               //成功pubish则移除该消息
               if (wasInLaunchingProviders) {
                   mHandler.removeMessages(CONTENT_PROVIDER_PUBLISH_TIMEOUT_MSG, r);
               }
               synchronized (dst) {
                   dst.provider = src.provider;
                   dst.proc = r;
                   //唤醒客户端的wait等待方法
                   dst.notifyAll();
               }
               ...
           }
       }
   }    
}

引爆炸弹阶段：

在system_server进程中有一个Handler线程, 名叫”ActivityManager”.当倒计时结束便会向该Handler线程发送一条信息CONTENT_PROVIDER_PUBLISH_TIMEOUT_MSG,

final class MainHandler extends Handler {
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case CONTENT_PROVIDER_PUBLISH_TIMEOUT_MSG: {
                ...
                ProcessRecord app = (ProcessRecord)msg.obj;
                synchronized (ActivityManagerService.this) {
                
                    processContentProviderPublishTimedOutLocked(app);
                }
            } break;
            ...
        }
        ...
    }
}

总结：

当出现ANR时，都是会调用到AMS.appNotResponding()方法

Timeout时长

对于前台服务，则超时为SERVICE_TIMEOUT = 20s；
对于后台服务，则超时为SERVICE_BACKGROUND_TIMEOUT = 200s
对于前台广播，则超时为BROADCAST_FG_TIMEOUT = 10s；
对于后台广播，则超时为BROADCAST_BG_TIMEOUT = 60s;
ContentProvider超时为CONTENT_PROVIDER_PUBLISH_TIMEOUT = 10s;

超时检测

Service超时检测机制：

超过timeout时长没有执行完相应操作来触发移除延时消息，则会触发anr;

BroadcastReceiver超时检测机制：

有序广播的总执行时间超过 2* receiver个数 * timeout时长，则会触发anr;
有序广播的某一个receiver执行过程超过 timeout时长，则会触发anr;

另外:

对于Service, Broadcast, Input发生ANR之后,最终都会调用AMS.appNotResponding;
对于provider,在其进程启动时publish过程可能会出现ANR, 则会直接杀进程以及清理相应信息,而不会弹出ANR的对话框. appNotRespondingViaProvider()过程会走appNotResponding(),