Android Service的启动过程分析

时间:2021-05-21

Android Service的启动过程分析

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:

Service启动过程

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:

@Overridepublic 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:

@Overridepublic 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。

感谢阅读,希望能帮助到大家,谢谢大家对本站的支持!

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