Android Frameworks系列(一) startService启动

　　之前写过一篇从AIDL看Android跨进程通信 从Java层次去分析AIDL运行的原理，当时主要是为了学习Binder机制而写的，但是Binder机制比较复杂，打算过段时间单独的写一篇来分析。本篇文章分析startService的启动过程，也会涉及到使用Binder机制来进行跨进程通信，但是不会分析Binder机制的细节。不过还是强烈建议大家学习Binder机制，至少要了解Binder的基本架构，Android经常使用Binder来进行跨进程通信。
　　在Android中启动Service和Activity统一由ActivityManagerService（简称AMS）管理，AMS在系统启动时，就已经在ServiceManager中注册了，存活于system_server进程中。先来了解一下ActivityManagerService相关的类：
　　
　　从类图可以看到这是典型的Binder机制，如果使用过AIDL的话，对IBinder，IInterface应该很熟悉。IActivityManager继承自IInterface，是用来管理Activity和Service的接口。ActivityManagerProxy从名字可以看出其是代理，并且是AMS在普通应用进程中的代理，通过它可以使用AMS的功能。ActivityManagerNative则可以看做AMS和Binder之间的中介，这样AMS就可以通过它向客户端提供服务了。

　　一 ：从客户进程启动Service
　　我们还是使用从AIDL看Android跨进程通信 中的例子，从MainActivity来调用startService，一步一步分析：
　　1.1 ContextWrapper.java
　　

@Override
    public ComponentName startService(Intent service) {
        return mBase.startService(service);
    }

　　这里的mBase实际是ContextImpl的实例，所以接着看
　　
　　1.2 ContextImpl.java
　　
　　/frameworks/base/core/java/android/app/ContextImpl.java
　　

@Override
    public ComponentName startService(Intent service) {
        warnIfCallingFromSystemProcess();
        return startServiceCommon(service, mUser);
    }
...省略...
private ComponentName startServiceCommon(Intent service, UserHandle user) {
        try {
            validateServiceIntent(service);
            service.prepareToLeaveProcess(this);
            ComponentName cn = ActivityManagerNative.getDefault().startService(
                mMainThread.getApplicationThread(), service, service.resolveTypeIfNeeded(
                            getContentResolver()), getOpPackageName(), user.getIdentifier());
            if (cn != null) {
                if (cn.getPackageName().equals("!")) {
                    throw new SecurityException(
                            "Not allowed to start service " + service
                            + " without permission " + cn.getClassName());
                } else if (cn.getPackageName().equals("!!")) {
                    throw new SecurityException(
                            "Unable to start service " + service
                            + ": " + cn.getClassName());
                }
            }
            return cn;
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

　　在startServiceCommon（）方法中首先验证intent是否合法，然后重点看第11~13行。想要继续跟下去就得先搞清楚ActivityManagerNative.getDefault() 是个什么鬼。

　　1.3 ActivityManagerNative.java
　　/frameworks/base/core/java/android/app/ActivityManagerNative.java

/**
     * Cast a Binder object into an activity manager interface, generating
     * a proxy if needed.
     */
    static public IActivityManager asInterface(IBinder obj) {
        if (obj == null) {
            return null;
        }
        IActivityManager in =
            (IActivityManager)obj.queryLocalInterface(descriptor);
        if (in != null) {
            return in;
        }

        return new ActivityManagerProxy(obj);
    }

    /**
     * Retrieve the system's default/global activity manager.
     */
    static public IActivityManager getDefault() {
        return gDefault.get();
    }

...省略...

private static final Singleton gDefault = new Singleton() {
        protected IActivityManager create() {
            IBinder b = ServiceManager.getService("activity");
            if (false) {
                Log.v("ActivityManager", "default service binder = " + b);
            }
            IActivityManager am = asInterface(b);
            if (false) {
                Log.v("ActivityManager", "default service = " + am);
            }
            return am;
        }
    };

　　第22行，将会回调Singleton的create（）方法，对应28~38行代码，第29行通过ServiceManager.getService("activity") 可以获取一个IBinder，AMS在系统启动的时候已经在ServiceManager中注册过，这里我们只用知道通过这个IBinder对象（具体是BinderProxy对象）我们可以发起远程调用。通过第15行生成了一个ActivityManagerProxy并且将其作为单例保存起来，下次调用可以直接使用了。
　　![这里写图片描述]
　　
　　
　　上图也可以印证我们的分析。ActivityManagerNative.getDefault() 返回的是ActivityManagerProxy实例，那么1.2中将会调用ActivityManagerProxy.startService()方法。

　　1.4 ActivityManagerProxy.java
　　该类是在ActivityMangerNative中声明。
　　

 public ComponentName startService(IApplicationThread caller, Intent service,
            String resolvedType, String callingPackage, int userId) throws RemoteException
    {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IActivityManager.descriptor);
        data.writeStrongBinder(caller != null ? caller.asBinder() : null);
        service.writeToParcel(data, 0);
        data.writeString(resolvedType);
        data.writeString(callingPackage);
        data.writeInt(userId);
        mRemote.transact(START_SERVICE_TRANSACTION, data, reply, 0);
        reply.readException();
        ComponentName res = ComponentName.readFromParcel(reply);
        data.recycle();
        reply.recycle();
        return res;
    }

　　第12行mRemote是在ActivityManagerProxy的构造方法中赋值的，还记得刚刚new ActivityManagerProxy时传入了一个IBinder（BinderProxy）吗？通过它的transact()方法像远端AMS发起请求。

　　二 system_server进程中AMS响应

　　根据Binder机制，ActivityManagerNative的onTransact()方法将会回调，因为这个方法比较长，我只截取了响应startService动作的代码。
　　
　　2.1 ActivityManagerNative.java

 @Override
    public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
            throws RemoteException {
        switch (code) {
        ...省略...
        case START_SERVICE_TRANSACTION: {
            data.enforceInterface(IActivityManager.descriptor);
            IBinder b = data.readStrongBinder();
            IApplicationThread app = ApplicationThreadNative.asInterface(b);
            Intent service = Intent.CREATOR.createFromParcel(data);
            String resolvedType = data.readString();
            String callingPackage = data.readString();
            int userId = data.readInt();
            ComponentName cn = startService(app, service, resolvedType, callingPackage, userId);
            reply.writeNoException();
            ComponentName.writeToParcel(cn, reply);
            return true;
        }
        ...省略...
       }
     }

　　第9行，这里将要涉及到另外一个跨进程通信，主要是干什么的呢，我们知道MainActivity所在进程A通过IActivityManager接口可以调用AMS的服务，那么如果AMS想要访问进程A的服务该怎么办呢，通过这里的IApplicationThread接口可以做到。下面是相关类图：
　　
　　
　　
　　和之前的IActivityManager类似，使用IApplicationThread也是基于Binder架构的，所以它们的结构都很像，所以这里的ApplicationThreadProxy就是A进程在AMS的一个Binder代理，通过它AMS就可以调用A进程的服务。
　　第14行startService()是IActivityManager接口的方法，因为ActivityManagerNative是一个抽象类，并且AMS继承了ActivityManagerNative，所以这里startService（）的真正实现在ActivityManagerService中。

　　2.2 ActivityManagerService.java
　　/frameworks/base/services/core/java/com/android/server/am
　　

 @Override
    public ComponentName startService(IApplicationThread caller, Intent service,
            String resolvedType, String callingPackage, int userId)
            throws TransactionTooLargeException {
        enforceNotIsolatedCaller("startService");
        // Refuse possible leaked file descriptors
        if (service != null && service.hasFileDescriptors() == true) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
        }

        if (callingPackage == null) {
            throw new IllegalArgumentException("callingPackage cannot be null");
        }

        if (DEBUG_SERVICE) Slog.v(TAG_SERVICE,
                "startService: " + service + " type=" + resolvedType);
        synchronized(this) {
            final int callingPid = Binder.getCallingPid();
            final int callingUid = Binder.getCallingUid();
            final long origId = Binder.clearCallingIdentity();
            ComponentName res = mServices.startServiceLocked(caller, service,
                    resolvedType, callingPid, callingUid, callingPackage, userId);
            Binder.restoreCallingIdentity(origId);
            return res;
        }
    }

　　重点看第21，22行
　　mServices.startServiceLocked(caller, service,resolvedType, callingPid, callingUid, callingPackage, userId);
　　mServices是ActiveServices的实例，所以将要执行ActiveServices的startServiceLocked()方法，一般带有Locked后缀的都是需要保证线程安全的。

　　2.3 ActiveServices.java
　　/frameworks/base/services/core/java/com/android/server/am

ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
            int callingPid, int callingUid, String callingPackage, final int userId)
            throws TransactionTooLargeException {
        if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE, "startService: " + service
                + " type=" + resolvedType + " args=" + service.getExtras());

        final boolean callerFg;
        if (caller != null) {
            final ProcessRecord callerApp = mAm.getRecordForAppLocked(caller);
            if (callerApp == null) {
                throw new SecurityException(
                        "Unable to find app for caller " + caller
                        + " (pid=" + Binder.getCallingPid()
                        + ") when starting service " + service);
            }
            callerFg = callerApp.setSchedGroup != ProcessList.SCHED_GROUP_BACKGROUND;
        } else {
            callerFg = true;
        }


        ServiceLookupResult res =
            retrieveServiceLocked(service, resolvedType, callingPackage,
                    callingPid, callingUid, userId, true, callerFg, false);
        if (res == null) {
            return null;
        }
        if (res.record == null) {
            return new ComponentName("!", res.permission != null
                    ? res.permission : "private to package");
        }

        ServiceRecord r = res.record;

        if (!mAm.mUserController.exists(r.userId)) {
            Slog.w(TAG, "Trying to start service with non-existent user! " + r.userId);
            return null;
        }

        if (!r.startRequested) {
            final long token = Binder.clearCallingIdentity();
            try {
                // Before going further -- if this app is not allowed to run in the
                // background, then at this point we aren't going to let it period.
                final int allowed = mAm.checkAllowBackgroundLocked(
                        r.appInfo.uid, r.packageName, callingPid, true);
                if (allowed != ActivityManager.APP_START_MODE_NORMAL) {
                    Slog.w(TAG, "Background start not allowed: service "
                            + service + " to " + r.name.flattenToShortString()
                            + " from pid=" + callingPid + " uid=" + callingUid
                            + " pkg=" + callingPackage);
                    return null;
                }
            } finally {
                Binder.restoreCallingIdentity(token);
            }
        }

        NeededUriGrants neededGrants = mAm.checkGrantUriPermissionFromIntentLocked(
                callingUid, r.packageName, service, service.getFlags(), null, r.userId);

        // If permissions need a review before any of the app components can run,
        // we do not start the service and launch a review activity if the calling app
        // is in the foreground passing it a pending intent to start the service when
        // review is completed.
        if (Build.PERMISSIONS_REVIEW_REQUIRED) {
            if (!requestStartTargetPermissionsReviewIfNeededLocked(r, callingPackage,
                    callingUid, service, callerFg, userId)) {
                return null;
            }
        }

        if (unscheduleServiceRestartLocked(r, callingUid, false)) {
            if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "START SERVICE WHILE RESTART PENDING: " + r);
        }
        r.lastActivity = SystemClock.uptimeMillis();
        r.startRequested = true;
        r.delayedStop = false;
        r.pendingStarts.add(new ServiceRecord.StartItem(r, false, r.makeNextStartId(),
                service, neededGrants));

        final ServiceMap smap = getServiceMap(r.userId);
        boolean addToStarting = false;
        if (!callerFg && r.app == null
                && mAm.mUserController.hasStartedUserState(r.userId)) {
            ProcessRecord proc = mAm.getProcessRecordLocked(r.processName, r.appInfo.uid, false);
            if (proc == null || proc.curProcState > ActivityManager.PROCESS_STATE_RECEIVER) {
                // If this is not coming from a foreground caller, then we may want
                // to delay the start if there are already other background services
                // that are starting.  This is to avoid process start spam when lots
                // of applications are all handling things like connectivity broadcasts.
                // We only do this for cached processes, because otherwise an application
                // can have assumptions about calling startService() for a service to run
                // in its own process, and for that process to not be killed before the
                // service is started.  This is especially the case for receivers, which
                // may start a service in onReceive() to do some additional work and have
                // initialized some global state as part of that.
                if (DEBUG_DELAYED_SERVICE) Slog.v(TAG_SERVICE, "Potential start delay of "
                        + r + " in " + proc);
                if (r.delayed) {
                    // This service is already scheduled for a delayed start; just leave
                    // it still waiting.
                    if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE, "Continuing to delay: " + r);
                    return r.name;
                }
                if (smap.mStartingBackground.size() >= mMaxStartingBackground) {
                    // Something else is starting, delay!
                    Slog.i(TAG_SERVICE, "Delaying start of: " + r);
                    smap.mDelayedStartList.add(r);
                    r.delayed = true;
                    return r.name;
                }
                if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE, "Not delaying: " + r);
                addToStarting = true;
            } else if (proc.curProcState >= ActivityManager.PROCESS_STATE_SERVICE) {
                // We slightly loosen when we will enqueue this new service as a background
                // starting service we are waiting for, to also include processes that are
                // currently running other services or receivers.
                addToStarting = true;
                if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE,
                        "Not delaying, but counting as bg: " + r);
            } else if (DEBUG_DELAYED_STARTS) {
                StringBuilder sb = new StringBuilder(128);
                sb.append("Not potential delay (state=").append(proc.curProcState)
                        .append(' ').append(proc.adjType);
                String reason = proc.makeAdjReason();
                if (reason != null) {
                    sb.append(' ');
                    sb.append(reason);
                }
                sb.append("): ");
                sb.append(r.toString());
                Slog.v(TAG_SERVICE, sb.toString());
            }
        } else if (DEBUG_DELAYED_STARTS) {
            if (callerFg) {
                Slog.v(TAG_SERVICE, "Not potential delay (callerFg=" + callerFg + " uid="
                        + callingUid + " pid=" + callingPid + "): " + r);
            } else if (r.app != null) {
                Slog.v(TAG_SERVICE, "Not potential delay (cur app=" + r.app + "): " + r);
            } else {
                Slog.v(TAG_SERVICE,
                        "Not potential delay (user " + r.userId + " not started): " + r);
            }
        }

        return startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
    }

　　这个方法比较长，为了方便分析，先把介绍一下方法的参数：
　　　　caller:IApplicationThread类型，这里具体是ApplicationThreadProxy类型，通过它可以和应用进程进行通信。
　　　　service：Intent类型，主要包含了将要启动service的信息
　　　　resolvedType:String 类型，这里为null
　　　　callingPid：发起call的进程的pid
　　　　callingUid：发起call进程的uid
　　　　callingPackage：发起call进程的package信息
　　　　userId：这里为0
　　如图：

　　

　　第22、23行代码主要是检索服务信息，可以获取到ServiceRecord和相应的权限信息。84~145行主要是对非前台进程的调度。最终会调用第147行startServiceInnerLocked()方法。　　

ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
            boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
        ServiceState stracker = r.getTracker();
        if (stracker != null) {
            stracker.setStarted(true, mAm.mProcessStats.getMemFactorLocked(), r.lastActivity);
        }
        r.callStart = false;
        synchronized (r.stats.getBatteryStats()) {
            r.stats.startRunningLocked();
        }
        String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false, false);
        if (error != null) {
            return new ComponentName("!!", error);
        }

        if (r.startRequested && addToStarting) {
            boolean first = smap.mStartingBackground.size() == 0;
            smap.mStartingBackground.add(r);
            r.startingBgTimeout = SystemClock.uptimeMillis() + BG_START_TIMEOUT;
            if (DEBUG_DELAYED_SERVICE) {
                RuntimeException here = new RuntimeException("here");
                here.fillInStackTrace();
                Slog.v(TAG_SERVICE, "Starting background (first=" + first + "): " + r, here);
            } else if (DEBUG_DELAYED_STARTS) {
                Slog.v(TAG_SERVICE, "Starting background (first=" + first + "): " + r);
            }
            if (first) {
                smap.rescheduleDelayedStarts();
            }
        } else if (callerFg) {
            smap.ensureNotStartingBackground(r);
        }

        return r.name;
    }

　　第11行代码会调用bringUpServiceLocked()，从名字可以看出，这里应该会生成目标Service，并且会判断目标Service的进程是否存在，如果存在则直接去生成，如果不存在，则从Zygote fork一个进程，然后在生成相应的Service实例。
　　

private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
            boolean whileRestarting, boolean permissionsReviewRequired)
            throws TransactionTooLargeException {
        //Slog.i(TAG, "Bring up service:");
        //r.dump("  ");

        if (r.app != null && r.app.thread != null) {
            sendServiceArgsLocked(r, execInFg, false);
            return null;
        }

        if (!whileRestarting && r.restartDelay > 0) {
            // If waiting for a restart, then do nothing.
            return null;
        }

        if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Bringing up " + r + " " + r.intent);

        // We are now bringing the service up, so no longer in the
        // restarting state.
        if (mRestartingServices.remove(r)) {
            r.resetRestartCounter();
            clearRestartingIfNeededLocked(r);
        }

        // Make sure this service is no longer considered delayed, we are starting it now.
        if (r.delayed) {
            if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE, "REM FR DELAY LIST (bring up): " + r);
            getServiceMap(r.userId).mDelayedStartList.remove(r);
            r.delayed = false;
        }

        // Make sure that the user who owns this service is started.  If not,
        // we don't want to allow it to run.
        if (!mAm.mUserController.hasStartedUserState(r.userId)) {
            String msg = "Unable to launch app "
                    + r.appInfo.packageName + "/"
                    + r.appInfo.uid + " for service "
                    + r.intent.getIntent() + ": user " + r.userId + " is stopped";
            Slog.w(TAG, msg);
            bringDownServiceLocked(r);
            return msg;
        }

        // Service is now being launched, its package can't be stopped.
        try {
            AppGlobals.getPackageManager().setPackageStoppedState(
                    r.packageName, false, r.userId);
        } catch (RemoteException e) {
        } catch (IllegalArgumentException e) {
            Slog.w(TAG, "Failed trying to unstop package "
                    + r.packageName + ": " + e);
        }

        final boolean isolated = (r.serviceInfo.flags&ServiceInfo.FLAG_ISOLATED_PROCESS) != 0;
        final String procName = r.processName;
        ProcessRecord app;

        if (!isolated) {
            app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);
            if (DEBUG_MU) Slog.v(TAG_MU, "bringUpServiceLocked: appInfo.uid=" + r.appInfo.uid
                        + " app=" + app);
            if (app != null && app.thread != null) {
                try {
                    app.addPackage(r.appInfo.packageName, r.appInfo.versionCode, mAm.mProcessStats);
                    realStartServiceLocked(r, app, execInFg);
                    return null;
                } catch (TransactionTooLargeException e) {
                    throw e;
                } catch (RemoteException e) {
                    Slog.w(TAG, "Exception when starting service " + r.shortName, e);
                }

                // If a dead object exception was thrown -- fall through to
                // restart the application.
            }
        } else {
            // If this service runs in an isolated process, then each time
            // we call startProcessLocked() we will get a new isolated
            // process, starting another process if we are currently waiting
            // for a previous process to come up.  To deal with this, we store
            // in the service any current isolated process it is running in or
            // waiting to have come up.
            app = r.isolatedProc;
        }

        // Not running -- get it started, and enqueue this service record
        // to be executed when the app comes up.
        if (app == null && !permissionsReviewRequired) {
            if ((app=mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
                    "service", r.name, false, isolated, false)) == null) {
                String msg = "Unable to launch app "
                        + r.appInfo.packageName + "/"
                        + r.appInfo.uid + " for service "
                        + r.intent.getIntent() + ": process is bad";
                Slog.w(TAG, msg);
                bringDownServiceLocked(r);
                return msg;
            }
            if (isolated) {
                r.isolatedProc = app;
            }
        }

        if (!mPendingServices.contains(r)) {
            mPendingServices.add(r);
        }

        if (r.delayedStop) {
            // Oh and hey we've already been asked to stop!
            r.delayedStop = false;
            if (r.startRequested) {
                if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE,
                        "Applying delayed stop (in bring up): " + r);
                stopServiceLocked(r);
            }
        }

        return null;
    }

　　程序会运行到第60行会获取目标进程的信息，63行会判断进程是否存在，如果存在直接会运行第66行realStartServiceLocked（从方法名可以看出真正的启动Service应该就在这个方法中）方法，并返回。如果不存在则会执行到第89行的mAm.startProcessLocked方法，这里的mAm是AMS的实例，所以程序会回到AMS的startProcessLocked方法。通过分析我们验证了之前的判断。并且还可以预测，将来在目标进程生成之后，还是会调用realStartServiceLocked方法去生成Service。

　　三 AMS通知Zygote孵化目标进程
　　根据之前的分析，到这一步，AMS将会通知Zygote去孵化目标进程。这个过程也是一个跨进程的通信，但是并不是Binder去实现的，而是使用的Socket，Zygote孵化进程将来也会去写一篇文章去分析，这里只用知道通过其生成了一个目标进程。目标进程又会通过Binder向system_server进程发起attachApplication请求。那么此时又会回调到AMS的attachApplication方法。
　　
　　四 AMS通知目标进程创建并启动Service
　　4.1 ActivityManagerService.java
　　

 @Override
    public final void attachApplication(IApplicationThread thread) {
        synchronized (this) {
            int callingPid = Binder.getCallingPid();
            final long origId = Binder.clearCallingIdentity();
            attachApplicationLocked(thread, callingPid);
            Binder.restoreCallingIdentity(origId);
        }
    }

　　基于第三节的分析，在Zygote孵化目标进程成功后，目标进程会通过Binder请求调用attachApplication服务，所以用调用上边的代码。第6行代码会接着调用attachApplicationLocked（之前已经说过方法名带Locked一般表示线程安全）方法，由于这个方法比较长，由于篇幅限制，我只截取最重要的一部分
　　

private final boolean attachApplicationLocked(IApplicationThread thread,
            int pid) {
            ...省略...
             boolean badApp = false;
        boolean didSomething = false;

        // See if the top visible activity is waiting to run in this process...
        if (normalMode) {
            try {
                if (mStackSupervisor.attachApplicationLocked(app)) {
                    didSomething = true;
                }
            } catch (Exception e) {
                Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
                badApp = true;
            }
        }

        // Find any services that should be running in this process...
        if (!badApp) {
            try {
                didSomething |= mServices.attachApplicationLocked(app, processName);
            } catch (Exception e) {
                Slog.wtf(TAG, "Exception thrown starting services in " + app, e);
                badApp = true;
            }
        }
            ...省略...
            }

　　第22行 mServices.attachApplicationLocked(),mServices我们已经遇到过了，就是ActiveServices，那么又会执行其对应的方法。
　　
　　4.2 ActiveServices.java
　　

boolean attachApplicationLocked(ProcessRecord proc, String processName)
            throws RemoteException {
        boolean didSomething = false;
        // Collect any services that are waiting for this process to come up.
        if (mPendingServices.size() > 0) {
            ServiceRecord sr = null;
            try {
                for (int i=0; isize(); i++) {
                    sr = mPendingServices.get(i);
                    if (proc != sr.isolatedProc && (proc.uid != sr.appInfo.uid
                            || !processName.equals(sr.processName))) {
                        continue;
                    }

                    mPendingServices.remove(i);
                    i--;
                    proc.addPackage(sr.appInfo.packageName, sr.appInfo.versionCode,
                            mAm.mProcessStats);
                    realStartServiceLocked(sr, proc, sr.createdFromFg);
                    didSomething = true;
                    if (!isServiceNeeded(sr, false, false)) {
                        // We were waiting for this service to start, but it is actually no
                        // longer needed.  This could happen because bringDownServiceIfNeeded
                        // won't bring down a service that is pending...  so now the pending
                        // is done, so let's drop it.
                        bringDownServiceLocked(sr);
                    }
                }
            } catch (RemoteException e) {
                Slog.w(TAG, "Exception in new application when starting service "
                        + sr.shortName, e);
                throw e;
            }
        }
        // Also, if there are any services that are waiting to restart and
        // would run in this process, now is a good time to start them.  It would
        // be weird to bring up the process but arbitrarily not let the services
        // run at this point just because their restart time hasn't come up.
        if (mRestartingServices.size() > 0) {
            ServiceRecord sr;
            for (int i=0; isize(); i++) {
                sr = mRestartingServices.get(i);
                if (proc != sr.isolatedProc && (proc.uid != sr.appInfo.uid
                        || !processName.equals(sr.processName))) {
                    continue;
                }
                mAm.mHandler.removeCallbacks(sr.restarter);
                mAm.mHandler.post(sr.restarter);
            }
        }
        return didSomething;
    }

　　第19行realStartServiceLocked(sr, proc, sr.createdFromFg);还记得么，在第二节最后，我们分析了孵化目标进程后，最后还是会执行realStartServiceLocked，在这里得到了印证。
　　

 private final void realStartServiceLocked(ServiceRecord r,
            ProcessRecord app, boolean execInFg) throws RemoteException {
        if (app.thread == null) {
            throw new RemoteException();
        }
        if (DEBUG_MU)
            Slog.v(TAG_MU, "realStartServiceLocked, ServiceRecord.uid = " + r.appInfo.uid
                    + ", ProcessRecord.uid = " + app.uid);
        r.app = app;
        r.restartTime = r.lastActivity = SystemClock.uptimeMillis();

        final boolean newService = app.services.add(r);
        bumpServiceExecutingLocked(r, execInFg, "create");
        mAm.updateLruProcessLocked(app, false, null);
        mAm.updateOomAdjLocked();

        boolean created = false;
        try {
            if (LOG_SERVICE_START_STOP) {
                String nameTerm;
                int lastPeriod = r.shortName.lastIndexOf('.');
                nameTerm = lastPeriod >= 0 ? r.shortName.substring(lastPeriod) : r.shortName;
                EventLogTags.writeAmCreateService(
                        r.userId, System.identityHashCode(r), nameTerm, r.app.uid, r.app.pid);
            }
            synchronized (r.stats.getBatteryStats()) {
                r.stats.startLaunchedLocked();
            }
            mAm.notifyPackageUse(r.serviceInfo.packageName,
                                 PackageManager.NOTIFY_PACKAGE_USE_SERVICE);
            app.forceProcessStateUpTo(ActivityManager.PROCESS_STATE_SERVICE);
            app.thread.scheduleCreateService(r, r.serviceInfo,
                    mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
                    app.repProcState);
            r.postNotification();
            created = true;
        } catch (DeadObjectException e) {
            Slog.w(TAG, "Application dead when creating service " + r);
            mAm.appDiedLocked(app);
            throw e;
        } finally {
            if (!created) {
                // Keep the executeNesting count accurate.
                final boolean inDestroying = mDestroyingServices.contains(r);
                serviceDoneExecutingLocked(r, inDestroying, inDestroying);

                // Cleanup.
                if (newService) {
                    app.services.remove(r);
                    r.app = null;
                }

                // Retry.
                if (!inDestroying) {
                    scheduleServiceRestartLocked(r, false);
                }
            }
        }

        if (r.whitelistManager) {
            app.whitelistManager = true;
        }

        requestServiceBindingsLocked(r, execInFg);

        updateServiceClientActivitiesLocked(app, null, true);

        // If the service is in the started state, and there are no
        // pending arguments, then fake up one so its onStartCommand() will
        // be called.
        if (r.startRequested && r.callStart && r.pendingStarts.size() == 0) {
            r.pendingStarts.add(new ServiceRecord.StartItem(r, false, r.makeNextStartId(),
                    null, null));
        }

        sendServiceArgsLocked(r, execInFg, true);

        if (r.delayed) {
            if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE, "REM FR DELAY LIST (new proc): " + r);
            getServiceMap(r.userId).mDelayedStartList.remove(r);
            r.delayed = false;
        }

        if (r.delayedStop) {
            // Oh and hey we've already been asked to stop!
            r.delayedStop = false;
            if (r.startRequested) {
                if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE,
                        "Applying delayed stop (from start): " + r);
                stopServiceLocked(r);
            }
        }
    }

　　这里主要看32~34行 app.thread.scheduleCreateService（）其中app.thread是IApplicationThread，可见这里是system_server进程需要调用目标进程scheduleCreateService，那么在第二节分析，其实IApplicationThread的具体实现也就是ApplicationThreadProxy，通过它发起Binder通信。

　　4.3 ApplicationThreadProxy.java
　　

public final void scheduleCreateService(IBinder token, ServiceInfo info,
            CompatibilityInfo compatInfo, int processState) throws RemoteException {
        Parcel data = Parcel.obtain();
        data.writeInterfaceToken(IApplicationThread.descriptor);
        data.writeStrongBinder(token);
        info.writeToParcel(data, 0);
        compatInfo.writeToParcel(data, 0);
        data.writeInt(processState);
        try {
            mRemote.transact(SCHEDULE_CREATE_SERVICE_TRANSACTION, data, null,
                    IBinder.FLAG_ONEWAY);
        } catch (TransactionTooLargeException e) {
            Log.e("CREATE_SERVICE", "Binder failure starting service; service=" + info);
            throw e;
        }
        data.recycle();
    }

　　第10~11行，发起远程Binder通信，标识为SCHEDULE_CREATE_SERVICE_TRANSACTION，通过binder机制，最终将在ApplicationThreadNative的onTransact中响应。

　　4.4 ApplicationThreadNative.java
　　

 @Override
    public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
            throws RemoteException {
        switch (code) {
        ......
        case SCHEDULE_CREATE_SERVICE_TRANSACTION: {
            data.enforceInterface(IApplicationThread.descriptor);
            IBinder token = data.readStrongBinder();
            ServiceInfo info = ServiceInfo.CREATOR.createFromParcel(data);
            CompatibilityInfo compatInfo = CompatibilityInfo.CREATOR.createFromParcel(data);
            int processState = data.readInt();
            scheduleCreateService(token, info, compatInfo, processState);
            return true;
        }
        ......
        }
        return super.onTransact(code, data, reply, flags);
        }

　　第12行scheduleCreateService方法，真正的实现是在ApplicationThread，原因是ApplicationThreadNative是个抽象类，ApplicationThread是其实现类。

　　4.5 ActivityThread::ApplicationThread.java
　　

public final void scheduleCreateService(IBinder token,
                ServiceInfo info, CompatibilityInfo compatInfo, int processState) {
            updateProcessState(processState, false);
            CreateServiceData s = new CreateServiceData();
            s.token = token;
            s.info = info;
            s.compatInfo = compatInfo;

            sendMessage(H.CREATE_SERVICE, s);
        }

　　第9行，sendMessage()发送了一个tag为H.CREATE_SERVICE的消息，这里H类继承自Handler，是ActivityThread中的内部类，主要用于处理各种消息

　　4.6 ActivityThread::H.java

public void handleMessage(Message msg) {
    switch (msg.what) {
                case CREATE_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
                    handleCreateService((CreateServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                 }
     }

　　第5行，handleCreateService（）

private void handleCreateService(CreateServiceData data) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();

        LoadedApk packageInfo = getPackageInfoNoCheck(
                data.info.applicationInfo, data.compatInfo);
        Service service = null;
        try {
            java.lang.ClassLoader cl = packageInfo.getClassLoader();
            service = (Service) cl.loadClass(data.info.name).newInstance();
        } catch (Exception e) {
            if (!mInstrumentation.onException(service, e)) {
                throw new RuntimeException(
                    "Unable to instantiate service " + data.info.name
                    + ": " + e.toString(), e);
            }
        }

        try {
            if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);

            ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
            context.setOuterContext(service);

            Application app = packageInfo.makeApplication(false, mInstrumentation);
            service.attach(context, this, data.info.name, data.token, app,
                    ActivityManagerNative.getDefault());
            service.onCreate();
            mServices.put(data.token, service);
            try {
                ActivityManagerNative.getDefault().serviceDoneExecuting(
                        data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }
        } catch (Exception e) {
            if (!mInstrumentation.onException(service, e)) {
                throw new RuntimeException(
                    "Unable to create service " + data.info.name
                    + ": " + e.toString(), e);
            }
        }
    }

　　第6~7行获取apk的信息，包括路径，包名。
　　第10~11行通过ClassLoader将对应名字的Service加载到内存中来，这样就获取到了Service的对象。
　　第23~24行获取ContextImpl对象。
　　第26行获取Application对象。
　　第27~28行通过attach方法将contextImpl，Application与Service对象关联起来。
　　第29行调用onCreate方法，一般我们在使用Service时会重写该方法。这里Service的生命周期就开始了。
　　第32~33行通过IActivityManager通知AMS 我们需要的Service已经启动了。
　　上述整个启动过程的时序图：