服務 - fwrite

服務概述#

服務特點
服務也是 AMS 所管理的對象
服務可以單獨啟動，而不用跟隨 Activity 一起啟動 （但同樣要啟動該服務所屬的進程，也就是 ActivityThread）
AMS 會通知 ActivityThread 來啟動服務
一般 App 啟動服務有兩種方式
通過 Context#startService 來啟動指定服務
通過 Context#bindService 綁定服務

綁定服務#

public class TestService extends AppCompatActivity {
    // 匿名類
    private ServiceConnection serviceConnection = new ServiceConnection() {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Intent intent = new Intent(this, MyService.class);
        bindService(intent, serviceConnection, BIND_AUTO_CREATE);
    }
    
    @Override
    public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
            // 向下轉型（當然你也可以用 Binder 提供的方法）
            MyService.MyBindClass bindClass = (MyService.MyBindClass) iBinder;	
        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {
        }
    };
}

ContextImpl - bindService 調用 AMS 註冊 Receiver#

Context 的實作類是 ContextImpl類 （原因可以看看 Activity#attach 函數，這裡就不細講），所以在使用 bindService 的時候，實作類就是 ContextImpl
取得 IServiceConnection 對象：透過 LoadApk#getServiceDispatcher 取得 IServiceConnection 對象，IServiceConnection 是 APP 端 IBinder Server 對象，將來讓 AMS 做回調 （下個小節說明）
取得 Binder Proxy 訪問 AMS：ActivityManager#getService 取得 Binder 代理類，透過代理類呼叫 AMS#bindIsolatedService 方法

// ContextImpl.java

    final @NonNull LoadedApk mPackageInfo;

    @Override
    public boolean bindService(Intent service, ServiceConnection conn, int flags) {
        ...

        return bindServiceCommon(service, conn, flags, null, mMainThread.getHandler(), null,
                getUser());
    }


    private boolean bindServiceCommon(Intent service, ServiceConnection conn, int flags,
            String instanceName, Handler handler, Executor executor, UserHandle user) {

        IServiceConnection sd;

        // 判空
        if (conn == null) {
            throw new IllegalArgumentException("connection is null");
        }
        if (handler != null && executor != null) {
            throw new IllegalArgumentException("Handler and Executor both supplied");
        }

        // mPackageInfo 類型 LoadedApk
        if (mPackageInfo != null) {

            // @ 1. 主要是要透過 LoadedApk#getServiceDispatcher，包裝出 IServiceConnection 
            if (executor != null) {
                sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(), executor, flags);
            } else {
                sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(), handler, flags);
            }
        } else {
            throw new RuntimeException("Not supported in system context");
        }

        validateServiceIntent(service);
        try {

            ... 省略部分

                // @ 2. 分析 AMS#bindIsolatedService 方法
            int res = ActivityManager.getService().bindIsolatedService(
                mMainThread.getApplicationThread(), getActivityToken(), service,
                service.resolveTypeIfNeeded(getContentResolver()),
                sd, flags, instanceName, getOpPackageName(), user.getIdentifier());

            // 綁定結果
            if (res < 0) {
                throw new SecurityException(
                        "Not allowed to bind to service " + service);
            }
            return res != 0;
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

IServiceConnection 創建 Binder 回調 - 服務連線#

分析 LoadedApk#getServiceDispatcherCommon 方法：可以發現它主要在創建 IServiceConnection 接口，該接口之後會讓 AMS 做回調
檢查 Cache：首先檢查本地緩存是否有該服務的 Binder 回調（Context 作為 Key），如果有的話直接返回，否則創建 ServiceDispatcher 對象
如果沒有就創建 LoadedApk.ServiceDispatcher 對象
透過 getIServiceConnection 取得 Binder Server 回調接口

// LoadedApk.java

public final class LoadedApk {

    // ServiceDispatcher 的緩存
    private final ArrayMap<Context, ArrayMap<ServiceConnection, 
        LoadedApk.ServiceDispatcher>> mServices = new ArrayMap<>();

    public final IServiceConnection getServiceDispatcher(ServiceConnection c,
            Context context, Handler handler, int flags) {
        // @ 追蹤 getServiceDispatcherCommon
        return getServiceDispatcherCommon(c, context, handler, null, flags);
    }


    private IServiceConnection getServiceDispatcherCommon(ServiceConnection c,
            Context context, Handler handler, Executor executor, int flags) {

        synchronized (mServices) {
            LoadedApk.ServiceDispatcher sd = null;
            ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher> map = 
                mServices.get(context);

            // 1. 判斷緩存
            if (map != null) {
                ...

                sd = map.get(c);    // 有緩存
            }

            if (sd == null) {
                // @ 2. 沒緩存，直接創建 (根據回傳的接口進行創建)
                if (executor != null) {
                    sd = new ServiceDispatcher(c, context, executor, flags);
                } else {
                    sd = new ServiceDispatcher(c, context, handler, flags);
                }

                ...

                if (map == null) {
                    map = new ArrayMap<>();
                    mServices.put(context, map);
                }
                map.put(c, sd);
            } else {
                sd.validate(context, handler, executor);
            }

            // 3. 分析 LoadedApk.ServiceDispatcher#getIServiceConnection
            return sd.getIServiceConnection();
        }

    }
}

ServiceDispatcher 類：是 LoadedApk 的靜態內部類，真正給 AMS 回調的 Binder 是 InnerConnection 類，當 AMS 回調時會呼叫 connected 方法
創建 IServiceConnection.Stub 給 AMS 回調
AMS 回調實，會創建 RunConnection 對象，讓 Handler 或 Executor 呼叫
App 端 IServiceConnection 服務實作，類關係圖

// LoadedApk.java

public final class LoadedApk {

    static final class ServiceDispatcher {

        private final ServiceDispatcher.InnerConnection mIServiceConnection;

        private static class InnerConnection extends IServiceConnection.Stub {

            // 弱引用
            final WeakReference<LoadedApk.ServiceDispatcher> mDispatcher;

            InnerConnection(LoadedApk.ServiceDispatcher sd) {
                mDispatcher = new WeakReference<LoadedApk.ServiceDispatcher>(sd);
            }

            // AMS 在呼叫時會使用 connected 方法
            public void connected(ComponentName name, IBinder service, boolean dead)
                    throws RemoteException {
                LoadedApk.ServiceDispatcher sd = mDispatcher.get();
                if (sd != null) {
                    // @ 分析 connected
                    sd.connected(name, service, dead);
                }
            }
        }

        // 建構式
        ServiceDispatcher(ServiceConnection conn,
                    Context context, Handler activityThread, int flags) {

            // 創建 InnerConnection 對象
            mIServiceConnection = new InnerConnection(this);

            ... 省略部分參數

        }

        IServiceConnection getIServiceConnection() {
            // 實現類實作 IServiceConnection.Stub 
            return mIServiceConnection;
        }

        public void connected(ComponentName name, IBinder service, boolean dead) {
            // 如果有設定 Executor 就透過 Executor 通知
            if (mActivityExecutor != null) {
                // 透過 Executor
                mActivityExecutor.execute(new RunConnection(name, service, 0, dead));
            } else if (mActivityThread != null) {
                // @ 透過 Handler (ActivityThread#H 類)
                mActivityThread.post(new RunConnection(name, service, 0, dead));
            } else {
                doConnected(name, service, dead);
            }
        }

    }
}

AMS 透過 Binder 回調 - ServiceConnection#

RunConnection 類：是一個 Runnable，以目前狀況來說是使用 Handler，這個 Runnable 會傳到 ActivityThread#H 類（也是個 Handler）讓它回調

// LoadedApk#ServiceDispatcher#RunConnection.java

    private final class RunConnection implements Runnable {

        // 構建
        RunConnection(ComponentName name, IBinder service, int command, boolean dead) {
            mName = name;
            mService = service;
            mCommand = command;
            mDead = dead;
        }

        public void run() {
            // 目前傳入的 mCommand = 0
            if (mCommand == 0) {
                // @ 分析 doConnected 方法
                doConnected(mName, mService, mDead);
            } else if (mCommand == 1) {
                doDeath(mName, mService);
            }
        }

        final ComponentName mName;
        final IBinder mService;
        final int mCommand;
        final boolean mDead;
    }

ServiceDispatcher#doConnected 方法：對 App 傳入的 ServiceConnection 接口進行回調，並有多個狀況如下

是否同名 Server cache	目標 IBinder 狀態	說明	處理行為
Y	比對相同	已連線	直接返回，不多作處理
Y	比對不相同	已連線，但取得不同 IBinder 代理	1. 先斷線 `onServiceDisconnected`，2. 再次連線 `onServiceConnected`
N	IBinder 不為空	第一次連線	連線呼叫 `onServiceConnected`

// LoadedApk#ServiceDispatcher.java

    private final ServiceConnection mConnection;

    public void doConnected(ComponentName name, IBinder service, boolean dead) {
        ServiceDispatcher.ConnectionInfo old;
        ServiceDispatcher.ConnectionInfo info;

        synchronized (this) {
            ...

            old = mActiveConnections.get(name);
            // 1. 判斷是否已經綁定&&是同一個 Binder
            if (old != null && old.binder == service) {
                // Huh, already have this one.  Oh well!
                return;
            }

            if (service != null) {
                // 開始聯機新 Service
                info = new ConnectionInfo();
                info.binder = service;
                info.deathMonitor = new DeathMonitor(name, service);
                try {
                    service.linkToDeath(info.deathMonitor, 0);
                    mActiveConnections.put(name, info);
                } catch (RemoteException e) {
                    // This service was dead before we got it...  just
                    // don't do anything with it.
                    mActiveConnections.remove(name);
                    return;
                }
            } else {
                // Service disconnect 從緩存中移除
                mActiveConnections.remove(name);
            }

            if (old != null) {
                old.binder.unlinkToDeath(old.deathMonitor, 0);
            }
        }
        // 2. 如果是舊連接，呼叫 onServiceDisconnected 斷開但不同 IBinder
        if (old != null) {
            mConnection.onServiceDisconnected(name);
        }

        if (dead) {
            mConnection.onBindingDied(name);
        } else {
            // If there is a new viable service, it is now connected.
            if (service != null) {

                // 3. 呼叫 onServiceConnected (回到使用者接口)
                mConnection.onServiceConnected(name, service);
            } else {
                mConnection.onNullBinding(name);
            }
        }
    }

AMS - 啟動、綁定服務過程#

客戶端 APP 透過 ContextImpl#bindService，來觸發 AMS#bindIsolatedService 方法，綁定 APP & 服務
檢查服務 Intent 必須要有 FileDescriptors
呼叫 ActiveServices#bindServiceLocked 方法

// ActivityManagerService.java

    // 存活的服務列表
    final ActiveServices mServices;

    public int bindIsolatedService(IApplicationThread caller, IBinder token, Intent service,
            String resolvedType, IServiceConnection connection, int flags, String instanceName,
            String callingPackage, int userId) throws TransactionTooLargeException {
        enforceNotIsolatedCaller("bindService");

        // 1. 檢查 intent
        // intent 必須要有 FileDescriptors
        if (service != null && service.hasFileDescriptors() == true) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
        }

        // 檢查客戶端包名
        if (callingPackage == null) {
            throw new IllegalArgumentException("callingPackage cannot be null");
        }

        ... 省略部分

        // 函數取名中有 Lock 的原因，對呼叫線程堵塞
        synchronized(this) {

            // @ 2. 呼叫 ActiveServices#bindServiceLocked
            return mServices.bindServiceLocked(caller, token, service,
                    resolvedType, connection, flags, instanceName, callingPackage, userId);
        }

    }

ActiveServices#bindServiceLocked 方法
檢查呼叫者進程是否存在（可能已被 kill），若不存在則拋出
透過 retrieveServiceLocked 方法：去 PKMS 中找到對應的服務信息
App 端設定的 BIND_AUTO_CREATE Flag 若有設定該 Flag，就會透過 bringUpServiceLocked 方法 自動創建該服務
透過 requestServiceBindingLocked 方法：要求綁定服務，這個函數包含了第一次綁定 & 重新綁定的功能 (差異在最後一個參數)

// ActiveServices.java

    int bindServiceLocked(IApplicationThread caller, IBinder token, Intent service,
            String resolvedType, final IServiceConnection connection, int flags,
            String instanceName, String callingPackage, final int userId)
            throws TransactionTooLargeException {

        // 呼叫者 Pid、Uid
        final int callingPid = Binder.getCallingPid();
        final int callingUid = Binder.getCallingUid();

        // 1. 透過 AMS 尋找呼叫者進程
        final ProcessRecord callerApp = mAm.getRecordForAppLOSP(caller);

        if (callerApp == null) {
            // 拋出錯誤
            throw new SecurityException(
                    "Unable to find app for caller " + caller
                    + " (pid=" + callingPid
                    + ") when binding service " + service);
        }

        ...

        // 2. 搜尋 PKMS 中的服務
        ServiceLookupResult res =
            retrieveServiceLocked(service, instanceName, resolvedType, callingPackage,
                    callingPid, callingUid, userId, true,
                    callerFg, isBindExternal, allowInstant);

        // 若沒有 res 代表沒有該服務
        if (res == null) {
            return 0;
        }

        // 判斷其中的 ServiceRecord
        if (res.record == null) {
            return -1;
        }

        ServiceRecord s = res.record;

        ...

        try {
            ...

            // 自動啟動服務
            if ((flags&Context.BIND_AUTO_CREATE) != 0) {
                s.lastActivity = SystemClock.uptimeMillis();
                needOomAdj = true;

                // 自動啟動服務
                if (bringUpServiceLocked(s, service.getFlags(), callerFg, false,
                        permissionsReviewRequired, packageFrozen, true) != null) {
                    mAm.updateOomAdjPendingTargetsLocked(OomAdjuster.OOM_ADJ_REASON_BIND_SERVICE);
                    return 0;
                }
            }

            ...

            AppBindRecord b = s.retrieveAppBindingLocked(service, callerApp);

            // 服務連接紀錄
            ConnectionRecord c = new ConnectionRecord(b, activity,
                connection, flags, clientLabel, clientIntent,
                callerApp.uid, callerApp.processName, callingPackage);

            ... 調整 OomAdj 數值

            // 判斷服務進程是否啟動 (app 就是 ProcessRecord)
            if (s.app != null && b.intent.received) {

                try {
                   // 透過使用者傳入的接口進行進行回調
				   // 上個小節有分析過（conn 就是 IServiceConnect），最終會呼叫到 onServiceConnected
                    c.conn.connected(s.name, b.intent.binder, false);
                } /* 省略 catch */


                // 如果當前應用進程是第一個與服務進程綁定的
				//  && 有呼叫過 doRebind 方法
                if (b.intent.apps.size() == 1 && b.intent.doRebind) {
                    // 最後一個參數表示，是否重新綁定
					//  @分析 requestServiceBindingLocked 方法
                    requestServiceBindingLocked(s, b.intent, callerFg, true);
                }

            } else if (!b.intent.requested) {    // 應用端沒有綁定過
                // 最後一個參數表示，是否重新綁定
                requestServiceBindingLocked(s, b.intent, callerFg, false);
            }

            ...

        } finally {
            Binder.restoreCallingIdentity(origId);
        }

        return 1;
    }

ActiveServices#requestServiceBindingLocked 方法：針對傳入的 ServiceRecord 來進行判斷、操作
檢查 ServiceRecord 指定的進程是否啟動（檢查 ProcessRecord & IApplicationThread 是否存在），也就是檢查服務是否啟動
目前假設服務目標進程已經啟動，那就會透過該進程內實現的 IApplicationThread#requestServiceBindingLocked 回到目標進程內

// ActiveServices.java

    private final boolean requestServiceBindingLocked(ServiceRecord r, 
          IntentBindRecord i,
          boolean execInFg, 
          boolean rebind) throws TransactionTooLargeException {

        // 檢查 ProcessRecord & 其中的 Service#IApplicationThread 是否存在
        if (r.app == null || r.app.getThread() == null) {
            // If service is not currently running, can't yet bind.
            return false;
        }

        ... Debug 訊息


        // 客戶端沒綁定過或呼叫服務要重新綁定
        if ((!i.requested || rebind) && i.apps.size() > 0) {

            try {
                // 通知 IApplicationThread (App 端實線) 的 scheduleBindService 方法
                
                // 分析 scheduleBindService 方法
                r.app.getThread().scheduleBindService(r, i.intent.getIntent(), rebind,
                        r.app.mState.getReportedProcState());

                if (!rebind) {
                    i.requested = true;
                }

                i.hasBound = true;
                i.doRebind = false;
            } /* 省略 catch */
        }

        return true;
    }

App 進程 - `Service#onBind` 呼叫#

ActivityThread - scheduleBindService 進入服務所在進程做綁定
創建 BindServiceData 對象，儲存 ServiceRecord & Intent 信息
Server 進程收到 Binder 通信通知後，透過 Handler 對主線程發送 BIND_SERVICE 信息
主線程接收到 BIND_SERVICE 消息後，呼叫 handleBindService 方法，如果是第一次綁定就 1. 呼叫該服務的 onBind 方法，再接著 2. 呼叫 AMS#publishService 方法

// ActivityThread.java

    private class ApplicationThread extends IApplicationThread.Stub {

        ... 省略其他方法

        public final void scheduleBindService(IBinder token, Intent intent,
                boolean rebind, int processState) {
            ...

            // 1. 創建 BindServiceData 對象，儲存 ServiceRecord & Intent
            BindServiceData s = new BindServiceData();
            // token 就是 ServiceRecord
            s.token = token;
            s.intent = intent;
            s.rebind = rebind;

            ... Debug 訊息

            // 2. 對主線程發送 BIND_SERVICE 信息
            sendMessage(H.BIND_SERVICE, s);
        }
    }


    class H extends Handler {

        public static final int BIND_SERVICE = 121;

        public void handleMessage(Message msg) {

            ... 省略其他 case

            case BIND_SERVICE:
                ... 省略 trace

                handleBindService((BindServiceData)msg.obj);
                break;
        }
    }

    private void handleBindService(BindServiceData data) {
        // token 就是 ServiceRecord
        CreateServiceData createData = mServicesData.get(data.token);

        // 查看緩存
        // mServices 在 handleCreateService 時就會被添加
        Service s = mServices.get(data.token);

        // 若取不到，則代表服務尚未啟動
        if (s != null) {
            try {
                ...

                try {
                    if (!data.rebind) {
                        // 3. 第一次綁定就呼叫 onBind、AMS#publishService 方法
                        IBinder binder = s.onBind(data.intent);

                        // publishService 代表 Server 已經啟動
                        ActivityManager.getService().publishService(
                                data.token, data.intent, binder);
                    } else {
                        // 重新綁定則呼叫 onRebind
                        s.onRebind(data.intent);
                        ActivityManager.getService().serviceDoneExecuting(
                                data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
                    }
                try {
                    // 通知 AMS 該服務正在運行中
                    ActivityManager.getService().serviceDoneExecuting(
                            data.token, SERVICE_DONE_EXECUTING_START, data.startId, res);
                }  /* 省略 catch */
            } /* 省略 catch */
        }
    }

服務發布 - 通知所有使用者#

若服務進程啟動第一次啟動，就會呼叫 AMS#publishService 方法

// ActivityManagerService.java

    // token 就是 BindServiceData
    public void publishService(IBinder token, Intent intent, IBinder service) {
        // Refuse possible leaked file descriptors
        if (intent != null && intent.hasFileDescriptors() == true) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
        }

        synchronized(this) {
            if (!(token instanceof ServiceRecord)) {
                throw new IllegalArgumentException("Invalid service token");
            }
            // 分析 publishServiceLocked 方法
            mServices.publishServiceLocked((ServiceRecord)token, intent, service);
        }
    }

ActiveServices)#publishServiceLocked 透過 ServiceRecord 取得所有與該服務有連接的 APP 端，並逐一進行通知

// ActiveServices.java

    // token 就是 BindServiceData
    void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {

        final long origId = Binder.clearCallingIdentity();
        try {
            ... debug 訊息

            if (r != null) {
                ...

                // 取得該服務的綁定信息
                IntentBindRecord b = r.bindings.get(filter);

                if (b != null && !b.received) {
                    b.binder = service;
                    b.requested = true;
                    b.received = true;

                    // 透過 ServiceRecord 取得，所有與該服務聯機的紀錄
                    ArrayMap<IBinder, ArrayList<ConnectionRecord>> connections = r.getConnections();

                    for (int conni = connections.size() - 1; conni >= 0; conni--) {
                        ArrayList<ConnectionRecord> clist = connections.valueAt(conni);
                        // 遍歷每個聯機紀錄
                        for (int i=0; i<clist.size(); i++) {
                            // 當前聯機紀錄
                            ConnectionRecord c = clist.get(i);
                            ...
                            try {
                                // conn 是 IServiceConnect
                                // 分析 IServiceConnect#connected 方法
                                c.conn.connected(r.name, service, false);
                            } /* 省略 catch */
                        }
                    }
                }

                ...
            }
        } finally {
            Binder.restoreCallingIdentity(origId);
        }


    }

啟動服務#

public class TestService extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        
        Intent intent = new Intent(this, MyService.class);
        startService(intent);
    }
}

ContextImpl startService 呼叫 AMS#

透過 ActivityManager 作為代理類訪問 AMS 的 startService 方法，這邊可以看出 startService 並沒有回調，但是 可以通過返回 ComponentName 判斷是否啟動成功

// ContextImpl.java

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

    private ComponentName startServiceCommon(Intent service, boolean requireForeground,
            UserHandle user) {

        try {
            validateServiceIntent(service);
            service.prepareToLeaveProcess(this);

            // 透過代理類訪問 AMS#startService
            ComponentName cn = ActivityManager.getService().startService(
                    mMainThread.getApplicationThread(), service,
                    service.resolveTypeIfNeeded(getContentResolver()), requireForeground,
                    getOpPackageName(), getAttributionTag(), user.getIdentifier());
            
            if (cn != null) {
                // 沒有權限去啟動服務
                if (cn.getPackageName().equals("!")) {
                    throw new SecurityException(
                            "Not allowed to start service " + service
                            + " without permission " + cn.getClassName());
                } else if (cn.getPackageName().equals("!!")) {
                    // 無法啟動服務
                    throw new SecurityException(
                            "Unable to start service " + service
                            + ": " + cn.getClassName());
                } else if (cn.getPackageName().equals("?")) {
                    // 不允許啟動服務
                    throw ServiceStartNotAllowedException.newInstance(requireForeground,
                            "Not allowed to start service " + service + ": " + cn.getClassName());
                }
            }
            
            ... 省略部分
            
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

AMS - 啟動服務過程#

ActivityManagerService#startService
檢查 Intent 是否包含 FileDescriptors
呼叫 ActiveServices#startServiceLocked 方法

// ActivityManagerService.java

    final ActiveServices mServices;

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

        // 檢查 Intent
        if (service != null && service.hasFileDescriptors() == true) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
        }

        // 檢查 Package
        if (callingPackage == null) {
            throw new IllegalArgumentException("callingPackage cannot be null");
        }

        ... debug 訊息

        synchronized(this) {
            // 取得呼叫者的 pid & uid
            final int callingPid = Binder.getCallingPid();
            final int callingUid = Binder.getCallingUid();

            final long origId = Binder.clearCallingIdentity();


            ComponentName res;
            try {
                // mServices 類型是 ActiveServices
                // 分析 startServiceLocked 方法
                res = mServices.startServiceLocked(caller, service,
                        resolvedType, callingPid, callingUid,
                        requireForeground, callingPackage, callingFeatureId, userId);
            } finally {
                Binder.restoreCallingIdentity(origId);
            }
            return res;
        }
    }

ActiveServices

檢查目前裝置（同樣是透過 PKMS）是否有指定服務，查看 retrieveServiceLocked 方法
找到指定 ServiceRecord

// ActiveServices.java

    // caller 是呼叫者
    ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
            int callingPid, int callingUid, boolean fgRequired,
            String callingPackage, @Nullable String callingFeatureId, final int userId,
            boolean allowBackgroundActivityStarts, @Nullable IBinder backgroundActivityStartsToken)
            throws TransactionTooLargeException {

        ... 省略部分

        // 1. 檢查目前裝置是否有指定服務，查看 retrieveServiceLocked 方法
        ServiceLookupResult res =
            retrieveServiceLocked(service, null, resolvedType, callingPackage,
                    callingPid, callingUid, userId, true, callerFg, false, false);
        
        // 找不到服務
        if (res == null) {
            return null;
        }

        ...
        
        ServiceRecord r = res.record;
        
        ... 省略部分
        
        // 2. 找到的 ServiceRecord 就是用於描述一個服務
        return startServiceInnerLocked(r, service, callingUid, callingPid, fgRequired, callerFg,
                allowBackgroundActivityStarts, backgroundActivityStartsToken);

    }

ServiceRecord 用於描述一個服務，類似於 ActivityRecord 描述一個 Activity

APP 端描述	AMS 中描述
Activity	ActivityRecord
Service	ServiceRecord

ActiveServices - 透過 PKMS 找服務#

AMS 尋找服務的方法是 ServiceLoopupResult，目的是要找到 ServiceRecord，而 找到 ServiceRecord 主要透過 PKMS

// ActiveServices.java

    private ServiceLookupResult retrieveServiceLocked(Intent service,
            String instanceName, String resolvedType, String callingPackage,
            int callingPid, int callingUid, int userId,
            boolean createIfNeeded, boolean callingFromFg, boolean isBindExternal,
            boolean allowInstant) {

        // 每個服務都會轉為 ServiceRecord
        ServiceRecord r = null;

        ServiceMap smap = getServiceMapLocked(userId);

        // 組裝 ComponentName
        final ComponentName comp;

        if (instanceName == null) {    // 目前傳入的 instanceName 是 null
            comp = service.getComponent();
        } else {

            // 透過 Component 組裝 ComponentName
            final ComponentName realComp = service.getComponent();
            if (realComp == null) {
                throw new IllegalArgumentException("Can't use custom instance name '" + instanceName
                        + "' without expicit component in Intent");
            }
            comp = new ComponentName(realComp.getPackageName(),
                    realComp.getClassName() + ":" + instanceName);
        }
        
        // 判斷 ComponentName 結果
        if (comp != null) {
            r = smap.mServicesByInstanceName.get(comp);
        }
        
        ... 省略部分
            
        if (r == null) {
            try {
                
                ...
                
                // 透過 PackageManager 找對應的服務
                ResolveInfo rInfo = mAm.getPackageManagerInternal().resolveService(service,
                    resolvedType, flags, userId, callingUid);
                
                ServiceInfo sInfo = rInfo != null ? rInfo.serviceInfo : null;
                if (sInfo == null) {
                    // 如果 PKMS 都找不到就直接返回 null
                    Slog.w(TAG_SERVICE, "Unable to start service " + service + " U=" + userId +
                          ": not found");
                    return null;
                }
                ... 省略部分
                
            }  catch (RemoteException ex) {
                // 在同進程，所以一定不會發生
            } 
        }
        
        
        if (r != null) {
            ... 省略部分
                
            return new ServiceLookupResult(r, null);
        }
        
        return null;

    }


    // Key 是 callingUser(int)
    final SparseArray<ServiceMap> mServiceMap = new SparseArray<>();

    private ServiceMap getServiceMapLocked(int callingUser) {
        // 嘗試取得緩存
        ServiceMap smap = mServiceMap.get(callingUser);
        if (smap == null) {
            // 重新創建一個 Service 紀錄對象
            smap = new ServiceMap(mAm.mHandler.getLooper(), callingUser);
            mServiceMap.put(callingUser, smap);
        }
        return smap;
    }

    final class ServiceMap extends Handler {
        final int mUserId;
        // 呼叫過的服務都會記錄起來
        final ArrayMap<ComponentName, ServiceRecord> mServicesByInstanceName = new ArrayMap<>();
    
        ... 省略部分
    
    }

mServiceMap 主要是透過 User 作為 Key，儲存相對應的 ServiceMap，內部又有儲存 ComponentName & ServiceRecord，當 User 呼叫過相同的服務就可以透過它取得服務的緩存

服務啟動入口 - startServiceInnerLocked#

// ActiveServices.java
    final ActivityManagerService mAm;

    ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
            boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
    
        // 這也是 Function 有 Locked 關鍵字的關係
        synchronized (mAm.mProcessStats.mLock) {
            final ServiceState stracker = r.getTracker();
            if (stracker != null) {
                stracker.setStarted(true, mAm.mProcessStats.getMemFactorLocked(), r.lastActivity);
            }
        }
        r.callStart = false;
        
        final int uid = r.appInfo.uid;
        final String packageName = r.name.getPackageName();
        final String serviceName = r.name.getClassName();
        
        ... 省略部分
        
        
        // 透過 bringUpServiceLocked 啟動服務
        String error = bringUpServiceLocked(r, service.getFlags(), callerFg,
                false /* whileRestarting */,
                false /* permissionsReviewRequired */,
                false /* packageFrozen */,
                true /* enqueueOomAdj */);
        
        ...
        
        return r.name;
    
    }

函數名	功能
bringUpServiceLocked	判斷 Process，若設為啟動則透過 AMS 喚醒
bringDownServiceLocked	取得連接該服務的所有聯機，關閉服務
bringUpServiceLocked 判斷 Process 狀態，若服務應在要所在的進程沒有啟動則透過 AMS 啟動目標 Process

// ActiveServices.java

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

        ... 省略判空、Log 訊息

        // 該服務是否隔離
        final boolean isolated = (r.serviceInfo.flags&ServiceInfo.FLAG_ISOLATED_PROCESS) != 0;

        // 獲取服務要運行在哪個進程
        final String procName = r.processName;
        HostingRecord hostingRecord = new HostingRecord("service", r.instanceName);
        ProcessRecord app;

        if (!isolated) {
            // 1. 嘗試透過 AMS 取得 ProcessRecord 對象
            app = mAm.getProcessRecordLocked(procName, r.appInfo.uid);
            
            if (app != null) { // ProcessRecord 存在
                
                final IApplicationThread thread = app.getThread();
                final int pid = app.getPid();
                final UidRecord uidRecord = app.getUidRecord();
                
                
                if (thread != null) {
                    try {
                        app.addPackage(r.appInfo.packageName, r.appInfo.longVersionCode,
                                mAm.mProcessStats);
                        
                        // 2. realStartServiceLocked 真正啟動服務
                        realStartServiceLocked(r, app, thread, pid, uidRecord, execInFg,
                                enqueueOomAdj);
                        
                        return null;
                    } /* 省略 catch*/
                    
                }
            }
        } else {
            ... 省略部分
        }
        
        
        if (app == null && !permissionsReviewRequired && !packageFrozen) {
            // 目標 process 不存在就透過 AMS#startProcessLocked 啟動
            if ((app = mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
                        hostingRecord, ZYGOTE_POLICY_FLAG_EMPTY, false, isolated)) == null) {
                ... 啟動失敗
                
                // 關閉服務
                bringDownServiceLocked(r, enqueueOomAdj);
                return msg;
            }
            if (isolated) {
                r.isolatedProc = app;
            }
        }
        
        ... 省略部分
    }

真正創建服務 - realStartServiceLocked#

透過 ProcessRecord 中的 thread（也就是 ActivityThread 內的 ApplicationThread 對象）呼叫 scheduleCreateService 方法

// ActiveServices.java

    private void realStartServiceLocked(ServiceRecord r, ProcessRecord app,
            IApplicationThread thread, int pid, UidRecord uidRecord, boolean execInFg,
            boolean enqueueOomAdj) throws RemoteException {

        // 設定目標 Process
        r.setProcess(app, thread, pid, uidRecord);

        ... 省略部分

        // 取得目標進程內所有的 Services
        final ProcessServiceRecord psr = app.mServices;

        final boolean newService = psr.startService(r);
        ...

        // 透過 AMS 調整 OOM_ADJ 的數值
        mAm.enqueueOomAdjTargetLocked(app);
        mAm.updateOomAdjLocked(app, OomAdjuster.OOM_ADJ_REASON_START_SERVICE);

        boolean created = false;
        try {

            ...

            final int uid = r.appInfo.uid;
            final String packageName = r.name.getPackageName();
            final String serviceName = r.name.getClassName();

            ...

            // thread 是 App 端的 IApplicationThread
            thread.scheduleCreateService(r, 
                                         r.serviceInfo,
                    mAm.compatibilityInfoForPackage(r.serviceInfo.applicationInfo),
                    app.mState.getReportedProcState());

            ...

            created = true;
        } /* 省略 catch、finally */

        ... 省略部分

    }

ActivityThread - APP 端處理 scheduleCreateService，會透過 MainHandler 傳送 CREATE_SERVICE 到主線程，之後就會呼叫到 handleCreateService 方法

// ActivityThread.java

    private class ApplicationThread extends IApplicationThread.Stub {

        public final void scheduleCreateService(IBinder token,
                ServiceInfo info, CompatibilityInfo compatInfo, int processState) {

            updateProcessState(processState, false);

            // 將需要的數據封裝成 CreateServiceData 類
            CreateServiceData s = new CreateServiceData();
            s.token = token;    // Token 就是 ServiceRecord 類
            s.info = info;        // 包有 Service 的 class name ... 等等信息
            s.compatInfo = compatInfo;

            // 透過 Handler 傳送 CREATE_SERVICE 信息
            sendMessage(H.CREATE_SERVICE, s);
        }

    }

    class H extends Handler {

        public static final int CREATE_SERVICE  = 114;

        public void handleMessage(Message msg) {
            switch (msg.what) {

                case CREATE_SERVICE:
                    ...

                    handleCreateService((CreateServiceData)msg.obj);
                    break;
            }
        }
    }


    @UnsupportedAppUsage
    private void handleCreateService(CreateServiceData data) {
        Service service = null;


        LoadedApk packageInfo = getPackageInfoNoCheck(
                data.info.applicationInfo, data.compatInfo);

        try {

            Application app = packageInfo.makeApplication(false, mInstrumentation);

            ...

            // 透過 LoadedApk 創建服務對象
			// getAppFactory（）=>創建的是 AppComponentFactory 對象
            service = packageInfo.getAppFactory()
                    .instantiateService(cl, data.info.name, data.token);

            // 創建對應的 ContextImpl 對象
            ContextImpl context = ContextImpl.getImpl(service
                    .createServiceBaseContext(this, packageInfo));

            // 呼叫 service#attach 方法
            service.attach(context, this, data.info.name, data.token, app,
                    ActivityManager.getService());

            // 呼叫 onCreate 方法
            service.onCreate();

            try {
                // 通知 AMS，服務創建成功
				// data.token 就是 ServiceRecord
                ActivityManager.getService().serviceDoneExecuting(
                        data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }

        } /* 省略 catch */
    }

透過 LoadedApk 創建服務對象

// LoadedApk.java
    private AppComponentFactory mAppComponentFactory;

    public LoadedApk(ActivityThread activityThread, ApplicationInfo aInfo,
            CompatibilityInfo compatInfo, ClassLoader baseLoader,
            boolean securityViolation, boolean includeCode, boolean registerPackage) {
        mActivityThread = activityThread;
        // Default ApplicationInfo
        mApplicationInfo = new ApplicationInfo();

        ... 

        // 透過 createAppFactory 方法創建 Factory
        mAppComponentFactory = createAppFactory(mApplicationInfo, mBaseClassLoader);
    }

    public AppComponentFactory getAppFactory() {
        return mAppComponentFactory;
    }

    private AppComponentFactory createAppFactory(ApplicationInfo appInfo, ClassLoader cl) {

        // 若有自定義 Factory 則透過反射創建
        if (mIncludeCode && appInfo.appComponentFactory != null && cl != null) {
            try {
                return (AppComponentFactory)
                        cl.loadClass(appInfo.appComponentFactory).newInstance();
            } /* 省略部分 */
        }

        // 預設 Factory
        return AppComponentFactory.DEFAULT;
    }

預設工廠 AppComponentFactory：透過 ClassLoader#loadClass 讀取指定 Class，並透過反射創建指定類

// AppComponentFactory.java

public class AppComponentFactory {

    ... 省略部分

    public @NonNull Service instantiateService(@NonNull ClassLoader cl,
            @NonNull String className, @Nullable Intent intent)
            throws InstantiationException, IllegalAccessException, ClassNotFoundException {
        // 透過 ClassLoader#loadClass 讀取指定 Class，並透過反射創建該類
        return (Service) cl.loadClass(className).newInstance();
    }

    // 預設 Factory
    public static final AppComponentFactory DEFAULT = new AppComponentFactory();
}

服務