Android：ANR的触发原理

一、概述

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。一般地，这时往往会弹出一个提示框，告知用户当前xxx未响应，用户可选择继续等待或者Force Close。

那么哪些场景会造成ANR呢？

• 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

二、Service

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

2.1 埋炸弹

文章startService流程分析详细介绍Service启动流程. 其中在Service进程attach到system_server进程的过程中会调用realStartServiceLocked()方法来埋下炸弹.

2.1.1 AS.realStartServiceLocked

[-> ActiveServices.java]

private final void realStartServiceLocked(ServiceRecord r, ProcessRecord app, boolean execInFg) throws RemoteException {
    ...
    //发送delay消息(SERVICE_TIMEOUT_MSG)，【见小节2.1.2】
    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 {
        ...
    }
}

2.1.2 AS.bumpServiceExecutingLocked

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【见2.3.1】
    mAm.mHandler.sendMessageAtTime(msg,
        proc.execServicesFg ? (now+SERVICE_TIMEOUT) : (now+ SERVICE_BACKGROUND_TIMEOUT));
}

该方法的主要工作发送delay消息(SERVICE_TIMEOUT_MSG). 炸弹已埋下, 我们并不希望炸弹被引爆, 那么就需要在炸弹爆炸之前拆除炸弹.

2.2 拆炸弹

在system_server进程AS.realStartServiceLocked()调用的过程会埋下一颗炸弹, 超时没有启动完成则会爆炸. 那么什么时候会拆除这颗炸弹的引线呢? 经过Binder等层层调用进入目标进程的主线程handleCreateService()的过程.

2.2.1 AT.handleCreateService

[-> ActivityThread.java]

    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) {
            ...
        }
    }

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.

2.2.2 AS.serviceDoneExecutingLocked

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);
        ...
    }
    ...
}

该方法的主要工作是当service启动完成，则移除服务超时消息SERVICE_TIMEOUT_MSG。

2.3 引爆炸弹

前面介绍了埋炸弹和拆炸弹的过程, 如果在炸弹倒计时结束之前成功拆卸炸弹,那么就没有爆炸的机会, 但是世事难料. 总有些极端情况下无法即时拆除炸弹,导致炸弹爆炸, 其结果就是App发生ANR. 接下来,带大家来看看炸弹爆炸的现场:

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,

2.3.1 MainHandler.handleMessage

[-> ActivityManagerService.java ::MainHandler]

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;
            ...
        }
        ...
    }
}

2.3.2 AS.serviceTimeout

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]”;

三 BroadcastReceiver

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

3.1 埋炸弹

文章Android Broadcast广播机制分析详细介绍广播启动流程，通过调用 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.

3.1.1 processNextBroadcast

[-> BroadcastQueue.java]

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【见小节3.3.2】
                    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【见小节3.2.2】
                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【见小节3.1.2】
            setBroadcastTimeoutLocked(timeoutTime);
        }
        ...
    }
}

对于广播超时处理时机：

1. 首先在part3的过程中setBroadcastTimeoutLocked(timeoutTime) 设置超时广播消息；
2. 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;

3.1.2 setBroadcastTimeoutLocked

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。

3.2 拆炸弹

broadcast跟service超时机制大抵相同，但有一个非常隐蔽的技能点，那就是通过静态注册的广播超时会受SharedPreferences(简称SP)的影响。

3.2.1 sendFinished

关于广播是否考虑SP的情况取决于如下代码：

public final void finish() {
    if (mType == TYPE_COMPONENT) {
        final IActivityManager mgr = ActivityManager.getService();
        if (QueuedWork.hasPendingWork()) {
            //当SP有未同步到磁盘的工作，则需等待其完成，才告知系统已完成该广播
            QueuedWork.queue(new Runnable() {
                public void run() {
                    sendFinished(mgr);
                }
            }, false);
        } else {
            sendFinished(mgr);
        }
    } else if (mOrderedHint && mType != TYPE_UNREGISTERED) {
        final IActivityManager mgr = ActivityManager.getService();
        sendFinished(mgr);
    }
}

可见，只有XML静态注册的广播超时检测过程会考虑是否有SP尚未完成，动态广播并不受其影响。

3.2.2 cancelBroadcastTimeoutLocked

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

移除广播超时消息BROADCAST_TIMEOUT_MSG

3.3 引爆炸弹

3.3.1 BroadcastHandler.handleMessage

[-> BroadcastQueue.java ::BroadcastHandler]

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;
            ...
        }
        ...
    }
}

3.3.2 broadcastTimeoutLocked

[-> BroadcastRecord.java]

//fromMsg = true
final void broadcastTimeoutLocked(boolean fromMsg) {
    if (fromMsg) {
        mPendingBroadcastTimeoutMessage = false;
    }

    if (mOrderedBroadcasts.size() == 0) {
        return;
    }

    long now = SystemClock.uptimeMillis();
    BroadcastRecord r = mOrderedBroadcasts.get(0);
    if (fromMsg) {
        if (mService.mDidDexOpt) {
            mService.mDidDexOpt = false;
            long timeoutTime = SystemClock.uptimeMillis() + mTimeoutPeriod;
            setBroadcastTimeoutLocked(timeoutTime);
            return;
        }

        if (!mService.mProcessesReady) {
            return; //当系统还没有准备就绪时，广播处理流程中不存在广播超时
        }

        long timeoutTime = r.receiverTime + mTimeoutPeriod;
        if (timeoutTime > now) {
            //如果当前正在执行的receiver没有超时，则重新设置广播超时
            setBroadcastTimeoutLocked(timeoutTime);
            return;
        }
    }

    BroadcastRecord br = mOrderedBroadcasts.get(0);
    if (br.state == BroadcastRecord.WAITING_SERVICES) {
        //广播已经处理完成，但需要等待已启动service执行完成。当等待足够时间，则处理下一条广播。
        br.curComponent = null;
        br.state = BroadcastRecord.IDLE;
        processNextBroadcast(false);
        return;
    }

    r.receiverTime = now;
    //当前BroadcastRecord的anr次数执行加1操作
    r.anrCount++;

    if (r.nextReceiver <= 0) {
        return;
    }
    ...

    Object curReceiver = r.receivers.get(r.nextReceiver-1);
    //查询App进程
    if (curReceiver instanceof BroadcastFilter) {
        BroadcastFilter bf = (BroadcastFilter)curReceiver;
        if (bf.receiverList.pid != 0
                && bf.receiverList.pid != ActivityManagerService.MY_PID) {
            synchronized (mService.mPidsSelfLocked) {
                app = mService.mPidsSelfLocked.get(
                        bf.receiverList.pid);
            }
        }
    } else {
        app = r.curApp;
    }

    if (app != null) {
        anrMessage = "Broadcast of " + r.intent.toString();
    }

    if (mPendingBroadcast == r) {
        mPendingBroadcast = null;
    }

    //继续移动到下一个广播接收者
    finishReceiverLocked(r, r.resultCode, r.resultData,
            r.resultExtras, r.resultAbort, false);
    scheduleBroadcastsLocked();

    if (anrMessage != null) {
        // [见小节3.3.3]
        mHandler.post(new AppNotResponding(app, anrMessage));
    }
}

1. mOrderedBroadcasts已处理完成，则不会anr;
2. 正在执行dexopt，则不会anr;
3. 系统还没有进入ready状态(mProcessesReady=false)，则不会anr;
4. 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;

3.3.3 AppNotResponding

[-> BroadcastQueue.java]

private final class AppNotResponding implements Runnable {
    ...
    public void run() {
        // 进入ANR处理流程
        mService.appNotResponding(mApp, null, null, false, mAnnotation);
    }
}

四 ContentProvider

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.

4.1 埋炸弹

文章理解ContentProvider原理详细介绍了Provider启动流程. 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.

4.1.1 AMS.attachApplicationLocked

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(...);
    ...
}

10s之后引爆该炸弹

4.2 拆炸弹

当provider成功publish之后,便会拆除该炸弹.

4.2.1 AMS.publishContentProviders

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();
               }
               ...
           }
       }
   }    
}

4.3 引爆炸弹

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,

4.3.1 MainHandler.handleMessage

[-> ActivityManagerService.java ::MainHandler]

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) {
                    //【见小节4.3.2】
                    processContentProviderPublishTimedOutLocked(app);
                }
            } break;
            ...
        }
        ...
    }
}

4.3.2 AMS.processContentProviderPublishTimedOutLocked

private final void processContentProviderPublishTimedOutLocked(ProcessRecord app) {
    //[见4.3.3]
    cleanupAppInLaunchingProvidersLocked(app, true); 
    //[见小节4.3.4]
    removeProcessLocked(app, false, true, "timeout publishing content providers");
}

4.3.3 AMS.cleanupAppInLaunchingProvidersLocked

boolean cleanupAppInLaunchingProvidersLocked(ProcessRecord app, boolean alwaysBad) {
    boolean restart = false;
    for (int i = mLaunchingProviders.size() - 1; i >= 0; i--) {
        ContentProviderRecord cpr = mLaunchingProviders.get(i);
        if (cpr.launchingApp == app) {
            if (!alwaysBad && !app.bad && cpr.hasConnectionOrHandle()) {
                restart = true;
            } else {
                //移除死亡的provider
                removeDyingProviderLocked(app, cpr, true);
            }
        }
    }
    return restart;
}

removeDyingProviderLocked()的功能跟进程的存活息息相关：详见ContentProvider引用计数 []小节4.5]

• 对于stable类型的provider(即conn.stableCount > 0),则会杀掉所有跟该provider建立stable连接的非persistent进程.
• 对于unstable类的provider(即conn.unstableCount > 0),并不会导致client进程被级联所杀.

4.3.4 AMS.removeProcessLocked

private final boolean removeProcessLocked(ProcessRecord app, boolean callerWillRestart, boolean allowRestart, String reason) {
    final String name = app.processName;
    final int uid = app.uid;

    //移除mProcessNames中的相应对象
    removeProcessNameLocked(name, uid);
    if (mHeavyWeightProcess == app) {
        mHandler.sendMessage(mHandler.obtainMessage(CANCEL_HEAVY_NOTIFICATION_MSG,
                mHeavyWeightProcess.userId, 0));
        mHeavyWeightProcess = null;
    }
    boolean needRestart = false;
    if (app.pid > 0 && app.pid != MY_PID) {
        int pid = app.pid;
        synchronized (mPidsSelfLocked) {
            mPidsSelfLocked.remove(pid);
            mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
        }

        ...
        boolean willRestart = false;
        if (app.persistent && !app.isolated) {
            if (!callerWillRestart) {
                willRestart = true;
            } else {
                needRestart = true;
            }
        }
        app.kill(reason, true); //杀进程
        handleAppDiedLocked(app, willRestart, allowRestart);
        if (willRestart) {
            removeLruProcessLocked(app);
            addAppLocked(app.info, false, null /* ABI override */);
        }
    } else {
        mRemovedProcesses.add(app);
    }
    return needRestart;
}

五、总结

当出现ANR时，都是调用到AMS.appNotResponding()方法，详细过程见文章理解Android ANR的信息收集过程. 当然这里介绍的provider例外.

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：

• 超过一定时间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(), 这个就不介绍了，很少使用，由用户自定义超时时间.

最后

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