Android Service的启动过程

刚开始学习Service的时候以为它是一个线程的封装,也可以执行耗时操作。其实不然,Service是运行在主线程的。直接执行耗时操作是会阻塞主线程的。长时间就直接ANR了。

我们知道Service可以执行一些后台任务,是后台任务不是耗时的任务,后台和耗时是有区别的喔。
这样就很容易想到音乐播放器,天气预报这些应用是要用到Service的。当然如果要在Service中执行耗时操作的话,开个线程就可以了。

关于Service的运行状态有两种,启动状态和绑定状态,两种状态可以一起。
启动一个Service只需调用Context的startService方法,传进一个Intent即可。看起来好像很简单的说,那是因为 Android为了方便开发者,做了很大程度的封装。那么你真的有去学习过Service是怎么启动的吗?Service的onCreate方法回调前都 做了哪些准备工作?

先上一张图大致了解下,灰色背景框起来的是同一个类中的方法,如下图:
Android Service的启动过程0

那接下来就从源码的角度来分析Service的启动过程。

当然是从Context的startService方法开始,Context的实现类是ContextImpl,那么我们就看到ContextImpl的startService方法即可,如下:

@Override
public ComponentName startService(Intent service) {
    warnIfCallingFromSystemProcess();
    return startServiceCommon(service, mUser);
}

会转到startServiceCommon方法,那跟进startServiceCommon方法方法瞧瞧。

private ComponentName startServiceCommon(Intent service, UserHandle user) {
    try {
        validateServiceIntent(service);
        service.prepareToLeaveProcess();
        ComponentName cn = ActivityManagerNative.getDefault().startService(
            mMainThread.getApplicationThread(), service, service.resolveTypeIfNeeded(
                        getContentResolver()), getOpPackageName(), user.getIdentifier());

    //代码省略

        return cn;
    } catch (RemoteException e) {
        throw new RuntimeException("Failure from system", e);
    }
}

可以看到调用了ActivityManagerNative.getDefault()的startService方法来启动 Service,ActivityManagerNative.getDefault()是ActivityManagerService,简称AMS。

那么现在启动Service的过程就转移到了ActivityManagerService,我们关注ActivityManagerService的startService方法即可,如下:

@Override
public ComponentName startService(IApplicationThread caller, Intent service,
        String resolvedType, String callingPackage, int userId)
        throws TransactionTooLargeException {

     //代码省略

    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;
    }
}

在上述的代码中,调用了ActiveServices的startServiceLocked方法,那么现在Service的启动过程从AMS转移到了ActiveServices了。

继续跟进ActiveServices的startServiceLocked方法,如下:

ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
        int callingPid, int callingUid, String callingPackage, int userId)
        throws TransactionTooLargeException {

    //代码省略

    ServiceLookupResult res =
        retrieveServiceLocked(service, resolvedType, callingPackage,
                callingPid, callingUid, userId, true, callerFg);

    //代码省略


    ServiceRecord r = res.record;

    //代码省略

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

在startServiceLocked方法中又会调用startServiceInnerLocked方法,

我们瞧瞧startServiceInnerLocked方法,

ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
        boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
    ProcessStats.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);

    //代码省略

    return r.name;
}

startServiceInnerLocked方法内部调用了bringUpServiceLocked方法,此时启动过程已经快要离开ActiveServices了。继续看到bringUpServiceLocked方法。如下:

private final String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
        boolean whileRestarting) throws TransactionTooLargeException {

        //代码省略

        if (app != null && app.thread != null) {
            try {
                app.addPackage(r.appInfo.packageName, r.appInfo.versionCode, mAm.mProcessStats);
                realStartServiceLocked(r, app, execInFg);
                return null;
            } 

       //代码省略

       return null;
}

省略了大部分if判断,相信眼尖的你一定发现了核心的方法,那就是
realStartServiceLocked,没错,看名字就像是真正启动Service。那么事不宜迟跟进去探探吧。如下:

private final void realStartServiceLocked(ServiceRecord r,
        ProcessRecord app, boolean execInFg) throws RemoteException {

    //代码省略

    boolean created = false;
    try {

       //代码省略
        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;
    } 

    //代码省略

    sendServiceArgsLocked(r, execInFg, true);

    //代码省略

}

找到了。app.thread调用了scheduleCreateService来启动Service,而app.thread是一个ApplicationThread,也是ActivityThread的内部类。此时已经到了主线程。
那么我们探探ApplicationThread的scheduleCreateService方法。如下:

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);
}

对待启动的Service组件信息进行包装,然后发送了一个消息。我们关注这个CREATE_SERVICE消息即可。

public void handleMessage(Message msg) {

        //代码省略

        case CREATE_SERVICE:
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceCreate");
            handleCreateService((CreateServiceData)msg.obj);
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            break;

        //代码省略

}

在handleMessage方法中接收到这个消息,然后调用了handleCreateService方法,跟进handleCreateService探探究竟:

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) {
            // nothing to do.
        }
    } catch (Exception e) {
        if (!mInstrumentation.onException(service, e)) {
            throw new RuntimeException(
                "Unable to create service " + data.info.name
                + ": " + e.toString(), e);
        }
    }
}

终于击破,这个方法很核心的。一点点分析

首先获取到一个LoadedApk对象,在通过这个LoadedApk对象获取到一个类加载器,通过这个类加载器来创建Service。如下:

java.lang.ClassLoader cl = packageInfo.getClassLoader();
service = (Service) cl.loadClass(data.info.name).newInstance();

接着调用ContextImpl的createAppContext方法创建了一个ContextImpl对象。

之后再调用LoadedApk的makeApplication方法来创建Application,这个创建过程如下:

public Application makeApplication(boolean forceDefaultAppClass,
        Instrumentation instrumentation) {
    if (mApplication != null) {
        return mApplication;
    }

    Application app = null;

    String appClass = mApplicationInfo.className;
    if (forceDefaultAppClass || (appClass == null)) {
        appClass = "android.app.Application";
    }

    try {
        java.lang.ClassLoader cl = getClassLoader();
        if (!mPackageName.equals("android")) {
            initializeJavaContextClassLoader();
        }
        ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
        app = mActivityThread.mInstrumentation.newApplication(
                cl, appClass, appContext);
        appContext.setOuterContext(app);
    } catch (Exception e) {
        if (!mActivityThread.mInstrumentation.onException(app, e)) {
            throw new RuntimeException(
                "Unable to instantiate application " + appClass
                + ": " + e.toString(), e);
        }
    }
    mActivityThread.mAllApplications.add(app);
    mApplication = app;

    if (instrumentation != null) {
        try {
            instrumentation.callApplicationOnCreate(app);
        } catch (Exception e) {
            if (!instrumentation.onException(app, e)) {
                throw new RuntimeException(
                    "Unable to create application " + app.getClass().getName()
                    + ": " + e.toString(), e);
            }
        }
    }

    // Rewrite the R 'constants' for all library apks.
    SparseArray<String> packageIdentifiers = getAssets(mActivityThread)
            .getAssignedPackageIdentifiers();
    final int N = packageIdentifiers.size();
    for (int i = 0; i < N; i++) {
        final int id = packageIdentifiers.keyAt(i);
        if (id == 0x01 || id == 0x7f) {
            continue;
        }

        rewriteRValues(getClassLoader(), packageIdentifiers.valueAt(i), id);
    }

    return app;
}

当然Application是只有一个的,从上述代码中也可以看出。

在回来继续看handleCreateService方法,之后service调用了attach方法关联了ContextImpl和Application等

最后service回调了onCreate方法,

service.onCreate();
mServices.put(data.token, service);

并将这个service添加进了一个了列表进行管理。

至此service启动了起来,以上就是service的启动过程。

你可能还想要知道onStartCommand方法是怎么被回调的?可能细心的你发现了在ActiveServices的realStartServiceLocked方法中,那里还有一个sendServiceArgsLocked方法。是的,那个就是入口。

那么我们跟进sendServiceArgsLocked方法看看onStartCommand方法是怎么回调的。

private final void sendServiceArgsLocked(ServiceRecord r, boolean execInFg,
        boolean oomAdjusted) throws TransactionTooLargeException {
    final int N = r.pendingStarts.size();

        //代码省略

        try {

        //代码省略

            r.app.thread.scheduleServiceArgs(r, si.taskRemoved, si.id, flags, si.intent);
        } catch (TransactionTooLargeException e) {
            if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Transaction too large: intent="
                    + si.intent);
            caughtException = e;
        } catch (RemoteException e) {
            // Remote process gone...  we'll let the normal cleanup take care of this.
            if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Crashed while sending args: " + r);
            caughtException = e;
        } 

        //代码省略
}

可以看到onStartCommand方法回调过程和onCreate方法的是很相似的,都会转到app.thread。那么现在就跟进ApplicationThread的scheduleServiceArgs。
你也可能猜到了应该又是封装一些Service的信息,然后发送一个消息, handleMessage接收。是的,源码如下:

public final void scheduleServiceArgs(IBinder token, boolean taskRemoved, int startId,
    int flags ,Intent args) {
    ServiceArgsData s = new ServiceArgsData();
    s.token = token;
    s.taskRemoved = taskRemoved;
    s.startId = startId;
    s.flags = flags;
    s.args = args;

    sendMessage(H.SERVICE_ARGS, s);
}
public void handleMessage(Message msg) {

        //代码省略

        case SERVICE_ARGS:
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStart");
            handleServiceArgs((ServiceArgsData)msg.obj);
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            break;

        //代码省略
}

咦,真的是这样。谜底应该就在handleServiceArgs方法了,那么赶紧瞧瞧,源码如下:

private void handleServiceArgs(ServiceArgsData data) {
    Service s = mServices.get(data.token);
    if (s != null) {
        try {
            if (data.args != null) {
                data.args.setExtrasClassLoader(s.getClassLoader());
                data.args.prepareToEnterProcess();
            }
            int res;
            if (!data.taskRemoved) {
                res = s.onStartCommand(data.args, data.flags, data.startId);
            } else {
                s.onTaskRemoved(data.args);
                res = Service.START_TASK_REMOVED_COMPLETE;
            }

            QueuedWork.waitToFinish();

            try {
                ActivityManagerNative.getDefault().serviceDoneExecuting(
                        data.token, SERVICE_DONE_EXECUTING_START, data.startId, res);
            } catch (RemoteException e) {
                // nothing to do.
            }
            ensureJitEnabled();
        } catch (Exception e) {
            if (!mInstrumentation.onException(s, e)) {
                throw new RuntimeException(
                        "Unable to start service " + s
                        + " with " + data.args + ": " + e.toString(), e);
            }
        }
    }
}

可以看到回调了onStartCommand方法。

以上就是Service的启动过程的源码分析。

从中,我理解了Service的启动过程的同时,阅读源码的能力也提高了,分析源码的时候我没能力把每一个变量,每一个方法都搞懂,我关注的都是一 些关键的字眼,比如这篇文章就是start呀,service呀。会有那种感觉,就是这里没错了。当然如果陷入胡同了也要兜出来。

这样的分析也能够摸清整体的过程,对于细节,等我有扎实的功底了在去研究吧。

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