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通过上一篇文章我们掌握了Android四种的基本使用，本篇从源码层面总结一下startService的执行过程。

本文依然按着是什么？有什么？怎么用？啥原理？的步骤来分析。

1、是什么

上一篇总结了“Service是Android系统中的四大组件之一，它是一种没有可视化界面，运行于后台的一种服务程序。属于计算型组件，用来在后台执行持续性的计算任务，重要性仅次于Activity活动”。

2、有什么

Service和Activity一样也有自己的生命周期，也需要在AndroidManifest.xml中注册。

2.1 在AndroidManifest.xml中注册

 <service android:name="com.example.testdemo.service.ServiceJia" />

2.2 Service生命周期

Service的生命周期有很多，本文只谈startService和stopService涉及到的。

onCreate

它只在Service刚被创建的时刻被调用，Service在运行中，这个方法将不会被调用。也就是只有经历过onDestroy生命周期以后再次startService(intent) 才会执行。

onStartCommand

OnStartCommand方法是最重要的方法，因为它在我们需要启动Service的时候被调用。形参时当初startService时传递进来的Intent，这样就可以给Service传值。在这个方法中，可以做服务启动后要执行的任务，但是切记这里也是在主线程运行，耗时的操作必须创建一个子线程来执行，否则可能引发ANR导致程序闪退。

关于onStartCommand的返回值介绍：

START_STICKY：此时Service被杀死以后将会被重新创建。但是不会重新传入原来的Intent对象，而是传入intent为null。

START_NOT_STICKY：此时Service被杀死以后不会被重新创建。

START_REDELIVER_INTENT：功能与START_STICKY类似在Service被杀死以后将会被重新创建。厉害的一点时，该返回值时Intent会重新传递给Service。

onDestroy

onDestory是在Service即将被销毁时执行的生命名周期，Service和Activity生命周期不一样，Service没有onStop生命周期。

日志打印：

调用startService后的生命周期：
2024-07-01 10:20:59.756 20505-20505/com.example.testdemo3 E/com.example.testdemo3.service.ServiceJia: onCreate:
2024-07-01 10:20:59.757 20505-20505/com.example.testdemo3 E/com.example.testdemo3.service.ServiceJia: onStartCommand:

调用stopService后的生命周期：
2024-07-01 10:21:06.861 20505-20505/com.example.testdemo3 E/com.example.testdemo3.service.ServiceJia: onDestroy:

1. 怎么用

关于使用方法上一篇已经总结，这里不在赘述。

4、啥原理

Service的启动方法是调用

Intent serviceIntent = new Intent(ServiceActivity.this, ServiceJia.class);
startService(serviceIntent);

然后我们顺着startService方法开始解析源码，SDK版本API 30：

4.1 从ContexWrapper的startService开始：

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

4.2 ContextImpl类startService

mBase的类型是Context，但实际代码逻辑是在它的实现类ContextImpl类。

 @Overridepublic ComponentName startService(Intent service) {warnIfCallingFromSystemProcess();return startServiceCommon(service, false, mUser);
}
//private ComponentName startServiceCommon(Intent service, boolean requireForeground,UserHandle user) {try {validateServiceIntent(service);service.prepareToLeaveProcess(this);ComponentName cn = ActivityManager.getService().startService(mMainThread.getApplicationThread(), service,service.resolveTypeIfNeeded(getContentResolver()), requireForeground,getOpPackageName(), getAttributionTag(), 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());} else if (cn.getPackageName().equals("?")) {throw new IllegalStateException("Not allowed to start service " + service + ": " + cn.getClassName());}}return cn;} catch (RemoteException e) {throw e.rethrowFromSystemServer();}}

在ContextImpl类的startService中调用了startServiceCommon方法，而其中的关键代码是ActivityManager.getService().startService方法调用。

4.3 来到ActivityManager类 

  /*** @hide*/@UnsupportedAppUsagepublic static IActivityManager getService() {return IActivityManagerSingleton.get();}@UnsupportedAppUsageprivate static final Singleton<IActivityManager> IActivityManagerSingleton =new Singleton<IActivityManager>() {@Overrideprotected IActivityManager create() {final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);final IActivityManager am = IActivityManager.Stub.asInterface(b);return am;}};
具体实现是在ActivityManagerService.java@Overridepublic ComponentName startService(IApplicationThread caller, Intent service,String resolvedType, boolean requireForeground, String callingPackage,String callingFeatureId, int userId)throws TransactionTooLargeException {enforceNotIsolatedCaller("startService");// Refuse possible leaked file descriptorsif (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 + " fg=" + requireForeground);synchronized(this) {final int callingPid = Binder.getCallingPid();final int callingUid = Binder.getCallingUid();final long origId = Binder.clearCallingIdentity();ComponentName res;try {res = mServices.startServiceLocked(caller, service,resolvedType, callingPid, callingUid,requireForeground, callingPackage, callingFeatureId, userId);} finally {Binder.restoreCallingIdentity(origId);}return res;}
}

4.4 进入ActiveServices类，用来辅助ActivityServiceManager管理Service的启动和停止等。

mServices = new ActiveServices(this);

上面先调用ActiveServices的startServiceLocked方法；

res = mServices.startServiceLocked(caller, service,resolvedType, callingPid, callingUid,requireForeground, callingPackage, callingFeatureId, userId);

然后startServiceLocked又调用本类的startServiceInnerLocked方法：

ComponentName cmp = startServiceInnerLocked(smap, service, r, callerFg, addToStarting); 

然后startServiceInnerLocked又调用本类的bringUpServiceLocked方法：

String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false, false);

然后在调用bringUpServiceLocked方法realStartServiceLocked：

realStartServiceLocked(r, app, execInFg); 

4.5调用ApplicationThread的scheduleCreateService方法

app.thread.scheduleCreateService(r, r.serviceInfo,mAm.compatibilityInfoForPackage(r.serviceInfo.applicationInfo),app.getReportedProcState());

之后调用了sendServiceArgsLocked(r, execInFg, true);他就是onStartCommand生命周期，此处伏笔，下面4.8中详聊。

4.6 进入ApplicationThread类：

        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);}

这里sendMessage方法不是Handler的哈，是封装以后的，继续看：

 void sendMessage(int what, Object obj) {sendMessage(what, obj, 0, 0, false);
}private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {if (DEBUG_MESSAGES) {Slog.v(TAG,"SCHEDULE " + what + " " + mH.codeToString(what) + ": " + arg1 + " / " + obj);}Message msg = Message.obtain();msg.what = what;msg.obj = obj;msg.arg1 = arg1;msg.arg2 = arg2;if (async) {msg.setAsynchronous(true);}mH.sendMessage(msg);}

4.7 这里发送了消息，然后去找对应what = H.CREATE_SERVICE的处理：

在handleMessage方法中处理：case CREATE_SERVICE:if (Trace.isTagEnabled(Trace.TRACE_TAG_ACTIVITY_MANAGER)) {Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,("serviceCreate: " + String.valueOf(msg.obj)));}handleCreateService((CreateServiceData)msg.obj);Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);break;

关键时刻来咯：

@UnsupportedAppUsageprivate 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 {if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);ContextImpl context = ContextImpl.createAppContext(this, packageInfo);Application app = packageInfo.makeApplication(false, mInstrumentation);java.lang.ClassLoader cl = packageInfo.getClassLoader();service = packageInfo.getAppFactory().instantiateService(cl, data.info.name, data.intent);// Service resources must be initialized with the same loaders as the application// context.context.getResources().addLoaders(app.getResources().getLoaders().toArray(new ResourcesLoader[0]));context.setOuterContext(service);service.attach(context, this, data.info.name, data.token, app,ActivityManager.getService());service.onCreate();mServices.put(data.token, service);try {ActivityManager.getService().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);}}}

这里先创建了一个ContextImpl实例，然后创建了Application（这里是个工厂模式等于空了才创建）；创建Service实例并与ContextImpl关联；然后调用onCreate的生命周期方法。最后有个mServices.put，后面其他场景会从里面取来使用。

4.8再看onStartCommand生命周期

先生是否记得上面4.4小节买过伏笔sendServiceArgsLocked(r, execInFg, true);

sendServiceArgsLocked的r.app.thread.scheduleServiceArgs(r, slice);这行代码很熟悉了，和调用onCreate时候是一样的模式。

进入ApplicationThread类：

public final void scheduleServiceArgs(IBinder token, ParceledListSlice args) {List<ServiceStartArgs> list = args.getList();for (int i = 0; i < list.size(); i++) {ServiceStartArgs ssa = list.get(i);ServiceArgsData s = new ServiceArgsData();s.token = token;s.taskRemoved = ssa.taskRemoved;s.startId = ssa.startId;s.flags = ssa.flags;s.args = ssa.args;sendMessage(H.SERVICE_ARGS, s);}}

二次封装的发消息：

sendMessage(H.SERVICE_ARGS, s);和onCreate一样。

handleMessage处理消息：

case SERVICE_ARGS:if (Trace.isTagEnabled(Trace.TRACE_TAG_ACTIVITY_MANAGER)) {Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,("serviceStart: " + String.valueOf(msg.obj)));}handleServiceArgs((ServiceArgsData)msg.obj);Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);break;

从mServices取出onCreate是存的service，然后调用onStartCommand生命周期。

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 {ActivityManager.getService().serviceDoneExecuting(data.token, SERVICE_DONE_EXECUTING_START, data.startId, res);} catch (RemoteException e) {throw e.rethrowFromSystemServer();}} catch (Exception e) {if (!mInstrumentation.onException(s, e)) {throw new RuntimeException("Unable to start service " + s+ " with " + data.args + ": " + e.toString(), e);}}}}

至此StartService的启动该流程分析完毕！

才疏学浅，如有错误，欢迎指正，多谢。