Android的服务（Service）（二）Service的自动重启问题

继续上篇的分析，接下来是第二个问题”Service的自动重启问题“

（一）、Service的生命周期

（二）、Service的自动重启问题

    这里要说服务的自动重启问题，这个问题其实很简单，只有两个关键的方法。代码如下：
    这个方法在ActivityThread的一系列针对服务的handle方法中都有调用到ActivityManagerSerice的serviceDoneExecuting()方法，但是跟重启有关的只有handleServiceArgs()，因为只有在这里才有一个叫res的参数会起作用。

    private void handleServiceArgs(ServiceArgsData data) {
        Service s = mServices.get(data.token);
        if (s != null) {
            try {
                if (data.args != null) {
                    data.args.setExtrasClassLoader(s.getClassLoader());
                }
                int res;
                if (!data.taskRemoved) {
                    //就是回调了用户服务的onStartCommand生命周期，这个做应用的都知道了，

                    //这里可以通过设置其返回值来控制自己的服务是否允许被重新启动，顺理成章的这个值就是res
                    res = s.onStartCommand(data.args, data.flags, data.startId);
                } else {
                    s.onTaskRemoved(data.args);
                    res = Service.START_TASK_REMOVED_COMPLETE;
                }
                ...............
                try {
                    //看看系统用这个值都干了一些什么导致有这个特性
                    ActivityManagerNative.getDefault().serviceDoneExecuting(
                            data.token, 1, data.startId, res);
                } catch (RemoteException e) {
                    // nothing to do.
                }
                ensureJitEnabled();
            }
            ..................
        }
    }

    下面就是这个特性的关键代码，里面的注释已经写的很全了，关键其作用的就是stopIfKilled这个标志。

    void serviceDoneExecutingLocked(ServiceRecord r, int type, int startId, int res) {
        boolean inDestroying = mDestroyingServices.contains(r);
        if (r != null) {
            if (type == 1) {
                // This is a call from a service start...  take care of
                // book-keeping.
                r.callStart = true;
                switch (res) {
                    case Service.START_STICKY_COMPATIBILITY:
                    case Service.START_STICKY: {
                        // We are done with the associated start arguments.
                        r.findDeliveredStart(startId, true);
                        // Don't stop if killed.
                        r.stopIfKilled = false;
                        break;
                    }
                    case Service.START_NOT_STICKY: {
                        // We are done with the associated start arguments.
                        r.findDeliveredStart(startId, true);
                        if (r.getLastStartId() == startId) {
                            // There is no more work, and this service
                            // doesn't want to hang around if killed.
                            r.stopIfKilled = true;
                        }
                        break;
                    }
                    case Service.START_REDELIVER_INTENT: {
                        // We'll keep this item until they explicitly
                        // call stop for it, but keep track of the fact
                        // that it was delivered.
                        ServiceRecord.StartItem si = r.findDeliveredStart(startId, false);
                        if (si != null) {
                            si.deliveryCount = 0;
                            si.doneExecutingCount++;
                            // Don't stop if killed.
                            r.stopIfKilled = true;
                        }
                        break;
                    }
                    case Service.START_TASK_REMOVED_COMPLETE: {
                        // Special processing for onTaskRemoved().  Don't
                        // impact normal onStartCommand() processing.
                        r.findDeliveredStart(startId, true);
                        break;
                    }
                    default:
                        throw new IllegalArgumentException(
                                "Unknown service start result: " + res);
                }
                if (res == Service.START_STICKY_COMPATIBILITY) {
                    r.callStart = false;
                }
            }
            final long origId = Binder.clearCallingIdentity();
            serviceDoneExecutingLocked(r, inDestroying, inDestroying);
            Binder.restoreCallingIdentity(origId);
        } else {
            Slog.w(TAG, "Done executing unknown service from pid "
                    + Binder.getCallingPid());
        }
    }

    那么这个标志位又是在哪些情况下使得服务可以重启的呢？这种场景入口很多啊，比如系统清理进程等，总之就是APP Died的情况下，入口方法不列举了，最后都会执行到这来：

    final void killServicesLocked(ProcessRecord app, boolean allowRestart) {
        // Report disconnected services.
        if (false) {
            // XXX we are letting the client link to the service for
            // death notifications.
            if (app.services.size() > 0) {
                Iterator<ServiceRecord> it = app.services.iterator();
                while (it.hasNext()) {
                    ServiceRecord r = it.next();
                    for (int conni=r.connections.size()-1; conni>=0; conni--) {
                        ArrayList<ConnectionRecord> cl = r.connections.valueAt(conni);
                        for (int i=0; i<cl.size(); i++) {
                            ConnectionRecord c = cl.get(i);
                            if (c.binding.client != app) {
                                try {
                                    //c.conn.connected(r.className, null);
                                } catch (Exception e) {
                                    // todo: this should be asynchronous!
                                    Slog.w(TAG, "Exception thrown disconnected servce "
                                          + r.shortName
                                          + " from app " + app.processName, e);
                                }
                            }
                        }
                    }
                }
            }
        }

        // First clear app state from services.
        for (int i=app.services.size()-1; i>=0; i--) {
            ServiceRecord sr = app.services.valueAt(i);
            synchronized (sr.stats.getBatteryStats()) {
                sr.stats.stopLaunchedLocked();
            }
            if (sr.app != null) {
                sr.app.services.remove(sr);
            }
            sr.app = null;
            sr.isolatedProc = null;
            sr.executeNesting = 0;
            sr.forceClearTracker();
            if (mDestroyingServices.remove(sr)) {
                if (DEBUG_SERVICE) Slog.v(TAG, "killServices remove destroying " + sr);
            }

            final int numClients = sr.bindings.size();
            for (int bindingi=numClients-1; bindingi>=0; bindingi--) {
                IntentBindRecord b = sr.bindings.valueAt(bindingi);
                if (DEBUG_SERVICE) Slog.v(TAG, "Killing binding " + b
                        + ": shouldUnbind=" + b.hasBound);
                b.binder = null;
                b.requested = b.received = b.hasBound = false;
            }
        }

        // Clean up any connections this application has to other services.
        for (int i=app.connections.size()-1; i>=0; i--) {
            ConnectionRecord r = app.connections.valueAt(i);
            removeConnectionLocked(r, app, null);
        }
        app.connections.clear();

        ServiceMap smap = getServiceMap(app.userId);

        // Now do remaining service cleanup.
        for (int i=app.services.size()-1; i>=0; i--) {
            ServiceRecord sr = app.services.valueAt(i);
            // Sanity check: if the service listed for the app is not one
            // we actually are maintaining, drop it.
            if (smap.mServicesByName.get(sr.name) != sr) {
                ServiceRecord cur = smap.mServicesByName.get(sr.name);
                Slog.wtf(TAG, "Service " + sr + " in process " + app
                        + " not same as in map: " + cur);
                app.services.removeAt(i);
                continue;
            }

            // Any services running in the application may need to be placed
            // back in the pending list.
            // 这里还是分很多种情况的
            // 允许重启时，如果当前服务所在进程crash超过两次，并且不是persistent的进程就结束不会重启了
            if (allowRestart && sr.crashCount >= 2 && (sr.serviceInfo.applicationInfo.flags
                    &ApplicationInfo.FLAG_PERSISTENT) == 0) {
                Slog.w(TAG, "Service crashed " + sr.crashCount
                        + " times, stopping: " + sr);
                EventLog.writeEvent(EventLogTags.AM_SERVICE_CRASHED_TOO_MUCH,
                        sr.userId, sr.crashCount, sr.shortName, app.pid);
                bringDownServiceLocked(sr);
            } else if (!allowRestart) {
                // 不允许重启直接挂掉
                bringDownServiceLocked(sr);
            } else {
                //
                boolean canceled = scheduleServiceRestartLocked(sr, true);

                // Should the service remain running?  Note that in the
                // extreme case of so many attempts to deliver a command
                // that it failed we also will stop it here.
                if (sr.startRequested && (sr.stopIfKilled || canceled)) {
                    if (sr.pendingStarts.size() == 0) {
                        sr.startRequested = false;
                        if (sr.tracker != null) {
                            sr.tracker.setStarted(false, mAm.mProcessStats.getMemFactorLocked(),
                                    SystemClock.uptimeMillis());
                        }
                        if (!sr.hasAutoCreateConnections()) {
                            // Whoops, no reason to restart!
                            bringDownServiceLocked(sr);
                        }
                    }
                }
            }
        }

        if (!allowRestart) {
            app.services.clear();

            // Make sure there are no more restarting services for this process.
            for (int i=mRestartingServices.size()-1; i>=0; i--) {
                ServiceRecord r = mRestartingServices.get(i);
                if (r.processName.equals(app.processName) &&
                        r.serviceInfo.applicationInfo.uid == app.info.uid) {
                    mRestartingServices.remove(i);
                    clearRestartingIfNeededLocked(r);
                }
            }
            for (int i=mPendingServices.size()-1; i>=0; i--) {
                ServiceRecord r = mPendingServices.get(i);
                if (r.processName.equals(app.processName) &&
                        r.serviceInfo.applicationInfo.uid == app.info.uid) {
                    mPendingServices.remove(i);
                }
            }
        }

        // Make sure we have no more records on the stopping list.
        int i = mDestroyingServices.size();
        while (i > 0) {
            i--;
            ServiceRecord sr = mDestroyingServices.get(i);
            if (sr.app == app) {
                sr.forceClearTracker();
                mDestroyingServices.remove(i);
                if (DEBUG_SERVICE) Slog.v(TAG, "killServices remove destroying " + sr);
            }
        }

        app.executingServices.clear();
    }   

    private final boolean scheduleServiceRestartLocked(ServiceRecord r,
            boolean allowCancel) {
        boolean canceled = false;

        ServiceMap smap = getServiceMap(r.userId);
        if (smap.mServicesByName.get(r.name) != r) {
            ServiceRecord cur = smap.mServicesByName.get(r.name);
            Slog.wtf(TAG, "Attempting to schedule restart of " + r
                    + " when found in map: " + cur);
            return false;
        }

        final long now = SystemClock.uptimeMillis();

        if ((r.serviceInfo.applicationInfo.flags
                &ApplicationInfo.FLAG_PERSISTENT) == 0) {
            long minDuration = SERVICE_RESTART_DURATION;
            long resetTime = SERVICE_RESET_RUN_DURATION;

            // Any delivered but not yet finished starts should be put back
            // on the pending list.
            final int N = r.deliveredStarts.size();
            if (N > 0) {
                for (int i=N-1; i>=0; i--) {
                    ServiceRecord.StartItem si = r.deliveredStarts.get(i);
                    si.removeUriPermissionsLocked();
                    //注意了，这里的canceled如果为true还是需要结束服务的
                    //还要关注一下delivery的上限和doneExecuting的上限
                    if (si.intent == null) {
                        // We'll generate this again if needed.
                    } else if (!allowCancel || (si.deliveryCount < ServiceRecord.MAX_DELIVERY_COUNT
                            && si.doneExecutingCount < ServiceRecord.MAX_DONE_EXECUTING_COUNT)) {
                        //重新在pendingStart中添加si，所以会在下次执行时重新带入intent进去
                        r.pendingStarts.add(0, si);
                        long dur = SystemClock.uptimeMillis() - si.deliveredTime;
                        dur *= 2;
                        if (minDuration < dur) minDuration = dur;
                        if (resetTime < dur) resetTime = dur;
                    } else {
                        Slog.w(TAG, "Canceling start item " + si.intent + " in service "
                                + r.name);
                        canceled = true;
                    }
                }
                r.deliveredStarts.clear();
            }

            r.totalRestartCount++;
            if (r.restartDelay == 0) {
                r.restartCount++;
                r.restartDelay = minDuration;
            } else {
                // If it has been a "reasonably long time" since the service
                // was started, then reset our restart duration back to
                // the beginning, so we don't infinitely increase the duration
                // on a service that just occasionally gets killed (which is
                // a normal case, due to process being killed to reclaim memory).
                if (now > (r.restartTime+resetTime)) {
                    r.restartCount = 1;
                    r.restartDelay = minDuration;
                } else {
                    r.restartDelay *= SERVICE_RESTART_DURATION_FACTOR;
                    if (r.restartDelay < minDuration) {
                        r.restartDelay = minDuration;
                    }
                }
            }

            r.nextRestartTime = now + r.restartDelay;

            // Make sure that we don't end up restarting a bunch of services
            // all at the same time.
            boolean repeat;
            do {
                repeat = false;
                for (int i=mRestartingServices.size()-1; i>=0; i--) {
                    ServiceRecord r2 = mRestartingServices.get(i);
                    if (r2 != r && r.nextRestartTime
                            >= (r2.nextRestartTime-SERVICE_MIN_RESTART_TIME_BETWEEN)
                            && r.nextRestartTime
                            < (r2.nextRestartTime+SERVICE_MIN_RESTART_TIME_BETWEEN)) {
                        r.nextRestartTime = r2.nextRestartTime + SERVICE_MIN_RESTART_TIME_BETWEEN;
                        r.restartDelay = r.nextRestartTime - now;
                        repeat = true;
                        break;
                    }
                }
            } while (repeat);

        } else {
            // Persistent processes are immediately restarted, so there is no
            // reason to hold of on restarting their services.
            r.totalRestartCount++;
            r.restartCount = 0;
            r.restartDelay = 0;
            r.nextRestartTime = now;
        }

        if (!mRestartingServices.contains(r)) {
            r.createdFromFg = false;
            mRestartingServices.add(r);
            r.makeRestarting(mAm.mProcessStats.getMemFactorLocked(), now);
        }

        r.cancelNotification();

        mAm.mHandler.removeCallbacks(r.restarter);
        // 最关键的操作在这里，忘ActivityManagerService的handler里面post一个重启的Runnable
        // 这个东西前面启动过程创建ServiceRecord时有的，很简单就是一个ServiceRestarter，它里面保存了这个ServiceRecord本身
        // 重启的时候根据这个record就可以直接启动服务了
        mAm.mHandler.postAtTime(r.restarter, r.nextRestartTime);
        r.nextRestartTime = SystemClock.uptimeMillis() + r.restartDelay;
        Slog.w(TAG, "Scheduling restart of crashed service "
                + r.shortName + " in " + r.restartDelay + "ms");
        EventLog.writeEvent(EventLogTags.AM_SCHEDULE_SERVICE_RESTART,
                r.userId, r.shortName, r.restartDelay);

        return canceled;
    }

    private class ServiceRestarter implements Runnable {
        private ServiceRecord mService;

        void setService(ServiceRecord service) {
            mService = service;
        }

        public void run() {
            synchronized(mAm) {
                //后面的事情就顺利成章了。
                performServiceRestartLocked(mService);
            }
        }
    }

    整个这个过程中，有好几个参数控制着是否需要重启，也定了很多参数的上限等等，这里单独列出来解释一下。
    ServiceRecord.crashCount、ServiceRecord.StartItem.deliveryCount、ServiceRecord.StartItem.doneExecutingCount
    crashCount顾名思义啊，就是crash的次数，这个在handleAppCrashLocked()中自增的，很明显每crash一次就会自增，没什么好说的
    deliveryCount也很好理解，他是属于StartItem的，所以表示的是启动信息，是执行onStartCommand方法的次数，也就是外部startService的次数
    doneExecutingCount跟deliveryCount还很有关联，类似的也是说的这个服务执行的次数，那么它们有什么区别呢？
    还有两个标志位Service.START_FLAG_RETRY、Service.START_FLAG_REDELIVERY要一起看。这个在ActivesService.sendServiceArgsLocked()中可以看到。意思就是说这个服务是直接重启还是重新发送发送请求。
    它们还是互斥的，这点在serviceDoneExecutingLocked()方法的START_REDELIVER_INTENT分支处理中可以得到结论，总的来说就是说onStartCommand返回START_STICKY是允许重启，而START_REDELIVER_INTENT会重新将上次的intent请求发送出去，服务中会重新接收到这个。

最后将在下篇论第三个问题

（三）、Service与其客户端的绑定如何实现，即跨进程调用问题。