Android Service的启动过程.doc

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：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 ;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, mApatibilityInfoForPackageLocked(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; = info; patInfo = 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( .applicationInfo, patInfo); Service service = null; try java.lang.ClassLoader cl = packageInfo.getClassLoader(); service = (Service) cl.loadClass().newInstance(); catch (Exception e) if (!mInstrumentation.onException(service, e) throw new RuntimeException( Unable to instantiate service + + : + e.toString(), e); try if (localLOGV) Slog.v(TAG, Creating service + ); ContextImpl context = ContextImpl.createAppContext(this, packageInfo); context.setOuterContext(service); Application app = packageInfo.makeApplication(false, mInstrumentation); service.attach(context, this, , 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 + + : + 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().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 (Eception 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 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, ent); catch (TransactionTooLargeException e) if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, Transaction too large: intent= + ent); caughtException = e; catch (RemoteException e) / Remote process gone. well 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)m