Android Service啟動繫結流程詳解

2023-03-09 06:00:27

前言

本文基於Android 11，參考《Android進階解密》一書資料。瞭解Service的啟動和繫結流程，以及Service的Context建立過程。

由於基於分析流程，忽略很多細節分支。各位在看原始碼的時候，要儘可能忽略細節，分析整體流程之後，還有精力的話再去看細節。例如有些屬性是在後面賦值的，如果在前面追究，難哦。

另：閱讀這種流程需要很大的耐心和毅力。建議在心情愉悅想要學習的時候搭配原始碼一起食用。

一、Service 的啟動流程

1、ContextImpl.startService

啟動一個Service，通常在Activity呼叫startService來啟動。

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

2、ContextImpl.startServiceCommon

startServiceCommon檢查intent內容是否合法，然後做一些離開當前程序的準備操作。呼叫 ActivityManager.getService()獲得AMS的本地參照，並呼叫其startService函數。

也就是說通過Binder機制跨程序通訊呼叫了AMS的startService函數。

private ComponentName startServiceCommon(Intent service, boolean requireForeground,
        UserHandle user) {
    try {
        //檢查intent 的compant和package是否合法
        validateServiceIntent(service);
        ...
        ComponentName cn = ActivityManager.getService().startService(
                mMainThread.getApplicationThread(), service,
                service.resolveTypeIfNeeded(getContentResolver()), requireForeground,
                getOpPackageName(), getAttributionTag(), user.getIdentifier());
        ...
        return cn;
    } catch (RemoteException e) {
        throw e.rethrowFromSystemServer();
    }
}

通過 ActivityManager.getService()的實現。

    @UnsupportedAppUsage
    public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }
    @UnsupportedAppUsage
    private static final Singleton<IActivityManager> IActivityManagerSingleton =
            new Singleton<IActivityManager>() {
                @Override
                protected IActivityManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };

3、AMS.startService

AMS.startService函數獲取呼叫Pid和Uid,然後呼叫ActiveService的startServiceLocked函數。

@Override
public ComponentName startService(IApplicationThread caller, Intent service,
        String resolvedType, boolean requireForeground, String callingPackage,
        String callingFeatureId, int userId)
        throws TransactionTooLargeException {
    ...
    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、ActiveService.startServiceLock

ActiveService.startServiceLock函數，對一些合法性的檢查，例如前臺Service的許可權、限制性後臺Service進行延遲執行（standby）。並將要啟動的資訊封裝成ServiceRecord。然後呼叫了startServiceInnerLocked函數。

ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
        int callingPid, int callingUid, boolean fgRequired, String callingPackage,
        @Nullable String callingFeatureId, final int userId)
        throws TransactionTooLargeException {
    return startServiceLocked(caller, service, resolvedType, callingPid, callingUid, fgRequired,
            callingPackage, callingFeatureId, userId, false);
}
ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
        int callingPid, int callingUid, boolean fgRequired, String callingPackage,
        @Nullable String callingFeatureId, final int userId,
        boolean allowBackgroundActivityStarts) throws TransactionTooLargeException {
    final boolean callerFg;
    if (caller != null) {
        //獲取呼叫Service的應用程式程序描述
        final ProcessRecord callerApp = mAm.getRecordForAppLocked(caller);
        if (callerApp == null) {
          ...
        }
        callerFg = callerApp.setSchedGroup != ProcessList.SCHED_GROUP_BACKGROUND;
    } else {
        callerFg = true;
    }
    //檢索ServiceRecord,包括同應用和其他應用
    ServiceLookupResult res =
        retrieveServiceLocked(service, null, resolvedType, callingPackage,
                callingPid, callingUid, userId, true, callerFg, false, false);
    ...
    //要啟動的ServiceRecord
    ServiceRecord r = res.record;
    ...
    r.lastActivity = SystemClock.uptimeMillis();
    r.startRequested = true;
    r.delayedStop = false;
    r.fgRequired = fgRequired;
    r.pendingStarts.add(new ServiceRecord.StartItem(r, false, r.makeNextStartId(),
            service, neededGrants, callingUid));
    ComponentName cmp = startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
    ...
    return cmp;
}

5、ActiveServices.startServiceInnerLocker

呼叫了bringUpServiceLocked函數,會將ServiceRecord新增到ServiceMap型別的smap集合，進行快取。

ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
        boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
    r.callStart = false;
    ...
    String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false, false);
    ...
    return r.name;
}

6、 ActiveService.bringUpServiceLocked

分析一：首次啟動Service時，在執行bringUpServiceLocked函數，ServiceRecord是屬於新建立的,而非從AMS的快取mServices中檢索而來，所以此時的ServiceRecord的ProcessRecord型別app和IApplicationThread型別thread都是null。只有啟動過後的ServiceRecord才有值，才會執行sendServiceArgsLocked函數，重複呼叫Service的生命週期onStartCommand,而不呼叫onCreate函數。

  private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
            boolean whileRestarting, boolean permissionsReviewRequired)
            throws TransactionTooLargeException {
        //分析一：未啟動過的ServiceRecord兩者都是null，重複啟動會執行該函數，
        //會重複呼叫service的onStartCommand函數。
        if (r.app != null &amp;&amp; r.app.thread != null) {
            sendServiceArgsLocked(r, execInFg, false);
            return null;
        }
		...
        final boolean isolated = (r.serviceInfo.flags&amp;ServiceInfo.FLAG_ISOLATED_PROCESS) != 0;
        final String procName = r.processName;
        HostingRecord hostingRecord = new HostingRecord("service", r.instanceName);
        ProcessRecord app;
        if (!isolated) {
        	////通過AMS獲取service所在程序的ProcessRecord。ProcessList=&gt;MyProcessMap=》會快取已建立過程序的ProcessRecord
            app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);
            if (app != null &amp;&amp; app.thread != null) {
                try {
                    app.addPackage(r.appInfo.packageName, r.appInfo.longVersionCode, mAm.mProcessStats);
                    //啟動服務
                   realStartServiceLocked(r, app, execInFg);
                    return null;
                } catch (TransactionTooLargeException e) {
                    throw e;
                } catch (RemoteException e) {
                    Slog.w(TAG, "Exception when starting service " + r.shortInstanceName, e);
                }
            }
        }
        //如果service所在的程序未啟動，通過AMS啟動該程序，可以參考應用程序的啟動流程
          if (app == null &amp;&amp; !permissionsReviewRequired) {
            if ((app=mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
                    hostingRecord, ZYGOTE_POLICY_FLAG_EMPTY, false, isolated, false)) == null) {;
                bringDownServiceLocked(r);
                return msg;
            }
            if (isolated) {
                r.isolatedProc = app;
            }
        }
        //等待程序啟動完畢重啟啟動
        if (!mPendingServices.contains(r)) {
            mPendingServices.add(r);
        }
		...
        return null;
    }

7、ActiveService.realStartServiceLocked

   private final void realStartServiceLocked(ServiceRecord r,
            ProcessRecord app, boolean execInFg) throws RemoteException {
        //將ProcessRecord設定給ServiceRecord
        r.setProcess(app);
		//登記當ServiceRecord到ProcessRecordd的陣列mServices，表示Service已經啟動（實際未啟動）
        final boolean newService = app.startService(r);
        boolean created = false;
        try {
			...
            app.thread.scheduleCreateService(r, r.serviceInfo,
                    mAm.compatibilityInfoForPackage(r.serviceInfo.applicationInfo),
                    app.getReportedProcState());
          	...
        } catch (DeadObjectException e) {
            Slog.w(TAG, "Application dead when creating service " + r);
            mAm.appDiedLocked(app, "Died when creating service");
            throw e;
        } 
        //會呼叫Service的onStartCommand函數
         sendServiceArgsLocked(r, execInFg, true);
        ...
    }

通過ProcessRecord物件的IApplicationThread參照，通過Binder機制呼叫了應用程式的ApplicationThread的scheduleCreateService函數。

8、ApplicationThread.scheduleCreateService

將ServiceInfo等相關資訊封裝到CreateServiceData中，並行送給ActivityThread的H型別的mH物件。

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);
}

9、H.handleMesssage

呼叫了ActivityThread的handleCreateService函數。

case CREATE_SERVICE:
    handleCreateService((CreateServiceData)msg.obj);
    break;

10、ActivityThread.handleCreateService

    private void handleCreateService(CreateServiceData data) {
        ...
        //獲取當前應用的描述資訊LoadedApk
        LoadedApk packageInfo = getPackageInfoNoCheck(
                data.info.applicationInfo, data.compatInfo);
        Service service = null;
        try {
           //建立Service的上下問文
           ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
           //獲取當前應用Applcation物件
            Application app = packageInfo.makeApplication(false, mInstrumentation);
            //通過反射建立Service物件
            java.lang.ClassLoader cl = packageInfo.getClassLoader();
            service = packageInfo.getAppFactory()
                    .instantiateService(cl, data.info.name, data.intent);
           //初始化資源
            context.getResources().addLoaders(
                    app.getResources().getLoaders().toArray(new ResourcesLoader[0]));
		   //context 與service相互繫結
            context.setOuterContext(service);
            service.attach(context, this, data.info.name, data.token, app,
                    ActivityManager.getService());
            //呼叫Service的生命週期onCreate函數，意味Service建立完畢
            service.onCreate();
            //快取Service
            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);
            }
        }
    }

通過ContextImpl.createAppContext建立Service的上下文context，通過packageInfo.getAppFactory().instantiateService反射獲得當前Service物件service，將context與service相互繫結。然後呼叫service.onCreate。至此，Service建立完畢。

二、Service的繫結

1、 ContextImpl.bindService

public boolean bindService(Intent service, ServiceConnection conn, int flags) {
	//系統程序呼叫繫結服務或傳送廣播都會發出警告
    warnIfCallingFromSystemProcess();
    return bindServiceCommon(service, conn, flags, null, mMainThread.getHandler(), null,
            getUser());
}

2、ContextImpl.bindServiceCommon

在分析一，主要判斷入參Executor executor或UserHandle user哪個為null，總有一個為null，但最終都是呼叫了LoadedApk的getServiceDispatcherCommon函數來獲取ServiceDispathcer型別sd。影響只是回撥程式碼是在主執行緒執行，還是執行緒池。這裡傳入ActivityThread的H物件，意味著後續連線成功回撥onServiceConnected是在主執行緒。

分析二：通過Binder機制呼叫AMS的bindIsolatedService函數。

private boolean bindServiceCommon(Intent service, ServiceConnection conn, int flags,
        String instanceName, Handler handler, Executor executor, UserHandle user) {
    // Keep this in sync with DevicePolicyManager.bindDeviceAdminServiceAsUser.
    IServiceConnection sd;
    if (conn == null) {
        throw new IllegalArgumentException("connection is null");
    }
    if (handler != null && executor != null) {
        throw new IllegalArgumentException("Handler and Executor both supplied");
    }
    if (mPackageInfo != null) {
        if (executor != null) {//分析一：無論哪個分支，都是獲得ServiceConnect的本地參照sd,兩者最終都是
        //呼叫LoadedApk的getServiceDispatcherCommon
            sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(), executor, flags);
        } else {
            //正常使用走這個分支
            sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(), handler, flags);
        }
    } else {
        throw new RuntimeException("Not supported in system context");
    }
    //檢查compant and package is null ?
    validateServiceIntent(service);
    try {
        IBinder token = getActivityToken();
        if (token == null && (flags&BIND_AUTO_CREATE) == 0 && mPackageInfo != null
                && mPackageInfo.getApplicationInfo().targetSdkVersion
                < android.os.Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
            flags |= BIND_WAIVE_PRIORITY;
        }
        service.prepareToLeaveProcess(this);
        //分析二：呼叫AMS.bindIsolatedService
        int res = ActivityManager.getService().bindIsolatedService(
            mMainThread.getApplicationThread(), getActivityToken(), service,
            service.resolveTypeIfNeeded(getContentResolver()),
            sd, flags, instanceName, getOpPackageName(), user.getIdentifier());
        if (res < 0) {
            throw new SecurityException(
                    "Not allowed to bind to service " + service);
        }
        return res != 0;
    } catch (RemoteException e) {
        throw e.rethrowFromSystemServer();
    }
}

IServiceConnection連線的建立會先從快取中獲取，避免每次都要新建。分析一：通過executor或handler建立ServiceDispatcher型別的sd,含有靜態內部類InnerConnection的參照mIServiceConnection。繼承自IServiceConnection.Stub，也就是InnerConnection是實現者，遠端呼叫代理在其他程序，例如SystemServer程序中的ActiveService。

private IServiceConnection getServiceDispatcherCommon(ServiceConnection c,
        Context context, Handler handler, Executor executor, int flags) {
    synchronized (mServices) {
        LoadedApk.ServiceDispatcher sd = null;
        //從快取獲取
        ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher> map = mServices.get(context);
        if (map != null) {
            sd = map.get(c);
        }
        if (sd == null) {
        	//分析一：通過executor或handler建立ServiceDispatcher
            if (executor != null) {
                sd = new ServiceDispatcher(c, context, executor, flags);
            } else {
                sd = new ServiceDispatcher(c, context, handler, flags);
            }
            if (DEBUG) Slog.d(TAG, "Creating new dispatcher " + sd + " for conn " + c);
            if (map == null) {
                map = new ArrayMap<>();
                mServices.put(context, map);
            }
            map.put(c, sd);
        } else {
            sd.validate(context, handler, executor);
        }
        return sd.getIServiceConnection();
    }
}

3、AMS.bindIsolatedService

AMS經過兩次過載函數bindIsolatedService呼叫，簡單檢查相關合法性。然後呼叫ActiveService型別的mService的bindServiceLocked函數。

4、ActiveService.bindServiceLocked

int bindServiceLocked(IApplicationThread caller, IBinder token, Intent service,
        String resolvedType, final IServiceConnection connection, int flags,
        String instanceName, String callingPackage, final int userId)
        throws TransactionTooLargeException {
    //發起繫結service的app程序描述
    final ProcessRecord callerApp = mAm.getRecordForAppLocked(caller);
    ...
    ServiceLookupResult res =
        retrieveServiceLocked(service, instanceName, resolvedType, callingPackage,
                Binder.getCallingPid(), Binder.getCallingUid(), userId, true,
                callerFg, isBindExternal, allowInstant);
    ...
    ServiceRecord s = res.record;
    ...
        //描述Service和應用程式程序之間的關聯，內部維護Service、程序、IntentFilter以及所有繫結資訊。
        AppBindRecord b = s.retrieveAppBindingLocked(service, callerApp);
        //描述應用程式與service建立的一次通訊（繫結）
        ConnectionRecord c = new ConnectionRecord(b, activity,
                connection, flags, clientLabel, clientIntent,
                callerApp.uid, callerApp.processName, callingPackage);
        IBinder binder = connection.asBinder();
        s.addConnection(binder, c);
        b.connections.add(c);
        if (activity != null) {
            activity.addConnection(c);
        }
        b.client.connections.add(c);
        c.startAssociationIfNeeded();
        ...
        //啟動Service，可以參考Service的啟動
        if ((flags&Context.BIND_AUTO_CREATE) != 0) {
            s.lastActivity = SystemClock.uptimeMillis();
            if (bringUpServiceLocked(s, service.getFlags(), callerFg, false,
                    permissionsReviewRequired) != null) {
                return 0;
            }
        }
        ...
        //表示Service已啟動，且已返回binder,可以通過binder存取介面
        if (s.app != null && b.intent.received) {
            // Service is already running, so we can immediately
            // publish the connection.
            try {
                //建立連線
                c.conn.connected(s.name, b.intent.binder, false);
            } catch (Exception e) {
                Slog.w(TAG, "Failure sending service " + s.shortInstanceName
                        + " to connection " + c.conn.asBinder()
                        + " (in " + c.binding.client.processName + ")", e);
            }
            //第一個繫結該Service的程序，且要重綁
            if (b.intent.apps.size() == 1 && b.intent.doRebind) {
                requestServiceBindingLocked(s, b.intent, callerFg, true);
            }
        } else if (!b.intent.requested) {//首次繫結，執行此次
            requestServiceBindingLocked(s, b.intent, callerFg, false);
        }
        ...
}

AppBindRecord 描述應用程式程序和Service的關聯，包括誰繫結了Service的ProcessRecord，繫結資訊IntentBindRecord，當前服務ServiceRecord，當前應用程序的所有連線記錄connections。

5、requestServiceBindingLocked

呼叫了ApplicationThread的scheduleBindService函數。

private final boolean requestServiceBindingLocked(ServiceRecord r, IntentBindRecord i,
        boolean execInFg, boolean rebind) throws TransactionTooLargeException {
           ...
            r.app.thread.scheduleBindService(r, i.intent.getIntent(), rebind, r.app.getReportedProcState());
           ...
}

6、ApplicationThread.scheduleBindService

將資料封裝 BindServiceData，傳送個ActivityThread的H型別的mH處理。

public final void scheduleBindService(IBinder token, Intent intent,
        boolean rebind, int processState) {
    updateProcessState(processState, false);
    BindServiceData s = new BindServiceData();
    s.token = token;
    s.intent = intent;
    s.rebind = rebind;
    sendMessage(H.BIND_SERVICE, s);
}

7 、 H.handleMessage

case BIND_SERVICE:
    handleBindService((BindServiceData)msg.obj);

8、ActivityThread.handleBindService

handleBindService函數有兩個分支，即是否重新系結。

如果當前程序第一個與Service繫結，且呼叫過了onUbBinder方法，那麼這裡的data.rebind將為true,直接執行Service的onRebind函數即可。另外一種就是沒有繫結過，那麼需要執行Service的onBind函數。然後還要執行AMS的publishService函數。

private void handleBindService(BindServiceData data) {
    Service s = mServices.get(data.token);
    if (s != null) {
           ...
            try {
                if (!data.rebind) {
                    IBinder binder = s.onBind(data.intent);
                    ActivityManager.getService().publishService(
                            data.token, data.intent, binder);
                } else {
                    s.onRebind(data.intent);
                    ActivityManager.getService().serviceDoneExecuting(
                            data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
                }
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        ...
    }
}

9、AMS.publishService

public void publishService(IBinder token, Intent intent, IBinder service) {
    // Refuse possible leaked file descriptors
    if (intent != null &amp;&amp; intent.hasFileDescriptors() == true) {
        throw new IllegalArgumentException("File descriptors passed in Intent");
    }
    synchronized(this) {
        if (!(token instanceof ServiceRecord)) {
            throw new IllegalArgumentException("Invalid service token");
        }
        mServices.publishServiceLocked((ServiceRecord)token, intent, service);
    }
}

10、ActiveService.publishServiceLocked

分析一：可見在第4步bindServiceLocked函數，IntentBindRecord物件的屬性binder、requested、received都是false。

在ServiceRecord的所有連線記錄connections中，通過intent查詢對應之前已經儲存的ConnectionRecord,並呼叫其IServiceConnection的connected函數。

在第2步的時候呼叫bindServiceCommon函數時，會建立ServiceDispatcher時，內部持有InnerConnection範例，這裡的IServiceConnection代理參照指向該InnerConnection範例,這裡會呼叫其connected函數。

    void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
        final long origId = Binder.clearCallingIdentity();
        try {
            if (r != null) {
                Intent.FilterComparison filter
                        = new Intent.FilterComparison(intent);
                IntentBindRecord b = r.bindings.get(filter);
                if (b != null &amp;&amp; !b.received) {//分析1
                    b.binder = service;
                    b.requested = true;
                    b.received = true;
                    ArrayMap&lt;IBinder, ArrayList&lt;ConnectionRecord&gt;&gt; connections = r.getConnections();
                    for (int conni = connections.size() - 1; conni &gt;= 0; conni--) {
                        ArrayList&lt;ConnectionRecord&gt; clist = connections.valueAt(conni);
                        for (int i=0; i&lt;clist.size(); i++) {
                            ConnectionRecord c = clist.get(i);
                            if (!filter.equals(c.binding.intent.intent)) {
							 ...
                                continue;
                            }
                            ...
                            try {
                                c.conn.connected(r.name, service, false);
                            } catch (Exception e) {
                                Slog.w(TAG, "Failure sending service " + r.shortInstanceName
                                      + " to connection " + c.conn.asBinder()
                                      + " (in " + c.binding.client.processName + ")", e);
                            }
                        }
                    }
                }
                serviceDoneExecutingLocked(r, mDestroyingServices.contains(r), false);
            }
        } finally {
            Binder.restoreCallingIdentity(origId);
        }
    }

11、InnerConnection.connected

private static class InnerConnection extends IServiceConnection.Stub {
    @UnsupportedAppUsage
    final WeakReference<LoadedApk.ServiceDispatcher> mDispatcher;
    InnerConnection(LoadedApk.ServiceDispatcher sd) {
        mDispatcher = new WeakReference<LoadedApk.ServiceDispatcher>(sd);
    }
    public void connected(ComponentName name, IBinder service, boolean dead)
            throws RemoteException {
        LoadedApk.ServiceDispatcher sd = mDispatcher.get();
        if (sd != null) {
            sd.connected(name, service, dead);
        }
    }
}

12、ServiceDispatcher.connected

這裡呼叫了 mActivityThread.post(new RunConnection(name, service, 0, dead))，執行RunConnection的run函數。這裡的話run函數執行程式碼又回到了應用程序的主執行緒。

public void connected(ComponentName name, IBinder service, boolean dead) {
    if (mActivityExecutor != null) {
        mActivityExecutor.execute(new RunConnection(name, service, 0, dead));
    } else if (mActivityThread != null) {
        mActivityThread.post(new RunConnection(name, service, 0, dead));
    } else {
        doConnected(name, service, dead);
    }
}

13、RunConnection.run

RunConnection是ServiceDispatcher的內部類，這裡執行SD的doConnected函數。

public void run() {
    if (mCommand == 0) {
        doConnected(mName, mService, mDead);
    } else if (mCommand == 1) {
        doDeath(mName, mService);
    }
}

14、ServiceDispatcher.doConnected

這裡呼叫了ServiceConnection物件的onServiceConnected函數，也就是我們發起繫結，呼叫context.bindService的引數。

public void doConnected(ComponentName name, IBinder service, boolean dead) {
        ...
        mConnection.onServiceConnected(name, service);
        ...
}

到此,Service的繫結流程分析完畢。

三、Service的Context

在第一節Service的啟動流程最後函數呼叫了ActivityThread的handleCreateService函數。

    private void handleCreateService(CreateServiceData data) {
        unscheduleGcIdler();
		//應用的描述資訊
        LoadedApk packageInfo = getPackageInfoNoCheck(
                data.info.applicationInfo, data.compatInfo);
        Service service = null;
        try {
			//分析一
            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);
            context.getResources().addLoaders(
                    app.getResources().getLoaders().toArray(new ResourcesLoader[0]));
			//分析二
            context.setOuterContext(service);
            //分析三
            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);
            }
        }
    }

分析一：通過ContextImpl的靜態函數createAppContext返回了一個ContextImpl型別的context。createAppContext又呼叫了過載函數createAppContext。直接新建了ContextImpl範例context，建構函式傳遞了ActivityThread型別的mainThread和LoadedApk型別的packageInfo。並給context設定了資源環境和是否Syetem屬性。

static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo) {
    return createAppContext(mainThread, packageInfo, null);
}
static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo,
        String opPackageName) {
    if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
    ContextImpl context = new ContextImpl(null, mainThread, packageInfo, null, null, null, null,
            0, null, opPackageName);
    context.setResources(packageInfo.getResources());
    context.mIsSystemOrSystemUiContext = isSystemOrSystemUI(context);
    return context;
}

ContextImpl類有一個Context型別的mOuterContext屬性，在建構函式時指向了自己。

回到handleCreateService函數的分析二，在建立好Service物件service之後，將service作為引數傳遞給了context.setOuterContext函數。Service本身繼承自ContextWrapper,ContextWrapper又是Context的子類。這時候的setOuterContext函數將service設定給了context的mOuterContext屬性。意味著當前上下文context持有當前新建的service參照。

在分析三，呼叫了service.attach函數，context並作為第一個引數被傳入。attach函數又呼叫了attachBaseContext函數。

public final void attach(
        Context context,
        ActivityThread thread, String className, IBinder token,
        Application application, Object activityManager) {
    attachBaseContext(context);
    mThread = thread; 
    mClassName = className;
    mToken = token;
    mApplication = application;
    mActivityManager = (IActivityManager)activityManager;
    mStartCompatibility = getApplicationInfo().targetSdkVersion
            &lt; Build.VERSION_CODES.ECLAIR;
    setContentCaptureOptions(application.getContentCaptureOptions());
}

attachBaseContext呼叫了父類別ContextWrapper的attachBaseContext函數

@Override
protected void attachBaseContext(Context newBase) {
    super.attachBaseContext(newBase);
    if (newBase != null) {
        newBase.setContentCaptureOptions(getContentCaptureOptions());
    }
}

ContextWrapper將一路傳遞過來的上下文base設定給你了mBase屬性。

protected void attachBaseContext(Context base) {
    if (mBase != null) {
        throw new IllegalStateException("Base context already set");
    }
    mBase = base;
}

也就是說，我們在啟動Service時，會同時建立Service的上下文context，並將其儲存到Service的父類別ContextWrapper的mBases屬性中，同時context也會有當前Service參照，儲存在mOuterContext變數中。

總結

Service的啟動和繫結從AMS轉移到ActiveService
Service的啟動，會先判斷程序是否建立，提前啟動程序，再啟動自己。
Service重複啟動，會重複呼叫onStratCommand及後續生命週期函數。
Service的繫結，會先走一趟Service的啟動流程，再繫結。
應用程序與SytemServer程序（AMS、ActiveService）的互動式通過Binder機制進行，通過AIDL各持有雙方介面。應用程序通過H物件，將現成重新切回主執行緒（所有應用誇程序通訊應如此）。
Service在應用和AMS兩邊都會做快取，以便快速在找到使用。應用程式儲存在ArrayMap<IBinder, Service>型別的mServices;ActiveService則是ArraySet<ServiceRecord>型別的mServices。

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