Service

Service 概述

Service 特点
Service 也是 AMS 所管理的对象
Service可以单独启动，而不用跟随Activity一起启动 （但同样要启动该Service所属的进程，也就是 ActivityThread）
AMS会通知ActivityThread来启动Service
一般 App 启动Service 有两种方式
通过 Context#startService 来启动指定 Service
通过 Context#bindService 绑定 Service

绑定 Service

public class TestService extends AppCompatActivity {
    // 匿名类
    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) {
            // 向下转型（当然你也可以用Binder提供的方法）
            MyService.MyBindClass bindClass = (MyService.MyBindClass) iBinder;    
        }

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

ContextImpl - bindService 调用 AMS 注册 Receiver

Context的实作类是 ContextImpl 类 （原因可以看看Activity#attach函数，这里就不细讲），所以在使用 bindService 的时候，实作类就是 ContextImpl
取得 IServiceConnection 对象：透过 LoadApk#getServiceDispatcher取得 IServiceConnection 对象，IServiceConnection是APP端IBinder Server对象，将来让AMS做回调用 （下个小节说明）
取得Binder Proxy访问AMS:ActivityManager#getService取得Binder代理类，透过代理类呼叫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 类型 LoadedApk
        if (mPackageInfo != null) {

            // @ 1. 主要是要透过 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());

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

IServiceConnection 创建 Binder回调 - Service 连线

分析 LoadedApk#getServiceDispatcherCommon 方法：可以发现它主要在创建IServiceConnection接口，该接口之后会让AMS做回调
检查Cache：首先检查本地缓存是否有该Service的Binder回调（Context作为Key），如果有的话直接返回，否则创建ServiceDispatcher对象
如果没有就创建LoadedApk.ServiceDispatcher对象
透过getIServiceConnection取得Binder Server回调接口

// LoadedApk.java

public final class LoadedApk {

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

    public final IServiceConnection getServiceDispatcher(ServiceConnection c,
            Context context, Handler handler, int flags) {
        // @ 追踪 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. 判断缓存
            if (map != null) {
                ...

                sd = map.get(c);    // 有缓存
            }

            if (sd == null) {
                // @ 2. 没缓存，直接创建 (根据回传的接口进行创建)
                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 类：是LoadedApk的静态内部类，真正给AMS回调的Binder是 InnerConnection类，当AMS回调时会呼叫 connected 方法
创建IServiceConnection.Stub给AMS回调
AMS回调实，会创建 RunConnection 对象，让 Handler or Executor 呼叫
App端IServiceConnection服务实作，类关系图

// 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 在呼叫时会使用 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);
                }
            }
        }

        // 建构式
        ServiceDispatcher(ServiceConnection conn,
                    Context context, Handler activityThread, int flags) {

            // 创建 InnerConnection 对象
            mIServiceConnection = new InnerConnection(this);

            ... 省略部分參數

        }

        IServiceConnection getIServiceConnection() {
            // 实现类实作 IServiceConnection.Stub 
            return mIServiceConnection;
        }

        public void connected(ComponentName name, IBinder service, boolean dead) {
            // 如果有设定Executor就通过Executor通知
            if (mActivityExecutor != null) {
                // 通过 Executor
                mActivityExecutor.execute(new RunConnection(name, service, 0, dead));
            } else if (mActivityThread != null) {
                // @ 透过 Handler (ActivityThread#H 类)
                mActivityThread.post(new RunConnection(name, service, 0, dead));
            } else {
                doConnected(name, service, dead);
            }
        }

    }
}

AMS 透过 Binder 回调 - ServiceConnection

RunConnection类：是一个Runnable，以目前状况来说是使用Handler，这个Runnable会传到ActivityThread#H类（也是个Handler）让它回调

// LoadedApk#ServiceDispatcher#RunConnection.java

    private final class RunConnection implements Runnable {

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

        public void run() {
            // 目前传入的 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：对App传入的ServiceConnection接口进行回调，并有多个状况如下`

// 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. 判断是否已经绑定&&是同一个Binder
            if (old != null && old.binder == service) {
                // Huh, already have this one.  Oh well!
                return;
            }

            if (service != null) {
                // 开始联机新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 从缓存中移除
                mActiveConnections.remove(name);
            }

            if (old != null) {
                old.binder.unlinkToDeath(old.deathMonitor, 0);
            }
        }
        // 2. 如果是旧连接，呼叫onServiceDisconnected断开但不同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 - 启动、绑定Service过程

客户端APP透过ContextImpl#bindService，来触发AMS#bindIsolatedService方法，绑定APP & Service
检查Service Intent必须要有FileDescriptors
呼叫 ActiveServices#bindServiceLocked 方法

// ActivityManagerService.java

    // 存活的 Servcie 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. 检查 intent
        // intent 必须要有 FileDescriptors
        if (service != null && service.hasFileDescriptors() == true) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
        }

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

        ... 省略部分

        // 函数取名中有 Lock 的原因，对呼叫线程堵塞
        synchronized(this) {

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

    }

ActiveServices#bindServiceLocked 方法
检查呼叫者进程是否存在（可能已被kill），若不存在则抛出
透过 retrieveServiceLocked方法:去PKMS中找到对应的Service信息
App端设定的 BIND_AUTO_CREATE Flag若有设置该Flag，就会透过 bringUpServiceLocked 方法 自动创建该Service
透过 requestServiceBindingLocked 方法：要求绑定 Service，这个函数包含了第一次绑定 & 重新绑定的功能 (差异在最后一个参数)

// 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. 透过AMS寻找呼叫者进程
        final ProcessRecord callerApp = mAm.getRecordForAppLOSP(caller);

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

        ...

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

        // 若没有res代表没有该Service
        if (res == null) {
            return 0;
        }

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

        ServiceRecord s = res.record;

        ...

        try {
            ...

            // 自动启动 Service
            if ((flags&Context.BIND_AUTO_CREATE) != 0) {
                s.lastActivity = SystemClock.uptimeMillis();
                needOomAdj = true;

                // 自动启动 Service
                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);

            // Service 连接纪录
            ConnectionRecord c = new ConnectionRecord(b, activity,
                connection, flags, clientLabel, clientIntent,
                callerApp.uid, callerApp.processName, callingPackage);

            ... 調整 OomAdj 數值

            // 判斷 Service 進程是否啟動 (app 就是 ProcessReocrd)
            if (s.app != null && b.intent.received) {

                try {
                   // 透过使用者传入的接口进行进行回掉
                   // 上个小节有分析过（conn就是IServiceConnect），最终会呼叫到onServiceConnected
                    c.conn.connected(s.name, b.intent.binder, false);
                } /* 省略 catch */


                // 如果当前应用进程是第一个与Service进程绑定的
                //  &&有调用过doRebind方法
                if (b.intent.apps.size() == 1 && b.intent.doRebind) {
                    // 最后一个参数表示，是否重新绑定
                    //  @分析requestServiceBindingLocked方法
                    requestServiceBindingLocked(s, b.intent, callerFg, true);
                }

            } else if (!b.intent.requested) {    // 应用端没有绑定过
                // 最后一个参数表示，是否重新绑定
                requestServiceBindingLocked(s, b.intent, callerFg, false);
            }

            ...

        } finally {
            Binder.restoreCallingIdentity(origId);
        }

        return 1;
    }

ActiveServices#requestServiceBindingLocked 方法：针对传入的 ServiceRecord 来进行判断、操作
检查ServiceRecord指定的进程是否起动（检查ProcessRecord & IApplicationThread是否存在），也就是检查Service是否启动
目前假设Service目标进程已经启动，那就会透过该进程内实现的IApplicationThread#requestServiceBindingLocked回到目标进程内

// ActiveServices.java

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

        // 检查 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;
        }

        ... Bebug 訊息


        // 客户端没绑定过or呼叫Service要重新绑定
        if ((!i.requested || rebind) && i.apps.size() > 0) {

            try {
                // 通知 IApplicationThread (App端实线) 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 进程 - Service#onBind 呼叫

ActivityThread - scheduleBindService 进入 Service 所在进程做绑定
创建BindServiceData对象，储存 ServiceRecord & Intent 信息
Server进程收到Binder通信通知后，透过Handler对主线程发送BIND_SERVICE信息
主线程接收到 BIND_SERVICE 消息后，呼叫handleBindService方法，如果是第一次绑定就 1. 呼叫该Service的 onBind 方法，再接着 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. 创建BindServiceData对象，储存ServiceRecord & Intent
            BindServiceData s = new BindServiceData();
            // token 就是 ServiceRecord
            s.token = token;
            s.intent = intent;
            s.rebind = rebind;

            ... Debug 訊息

            // 2. 对主线程发送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);

        // 查看缓存
        // mServices在handleCreateService时就会被添加
        Service s = mServices.get(data.token);

        // 若取不到，则代表Service尚未启动
        if (s != null) {
            try {
                ...

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

                        // publishService代表Server已经启动
                        ActivityManager.getService().publishService(
                                data.token, data.intent, binder);
                    } else {
                        // 重新绑定则呼叫onRebind
                        s.onRebind(data.intent);
                        ActivityManager.getService().serviceDoneExecuting(
                                data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
                    }
                try {
                    // 通知AMS该Service正在运行中
                    ActivityManager.getService().serviceDoneExecuting(
                            data.token, SERVICE_DONE_EXECUTING_START, data.startId, res);
                }  /* 省略 catch */
            } /* 省略 catch */
        }
    }

Service 发布 - 通知所有使用者

若Service进程启动第一次启动，就会呼叫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 透过ServiceRecord取得所有与该Serice有连接的APP端，并逐一进行通知

// 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) {
                ...

                // 取得该Service的绑定信息
                IntentBindRecord b = r.bindings.get(filter);

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

                    // 透过ServiceRecord取得，所有与该Service联机的纪录
                    ArrayMap<IBinder, ArrayList<ConnectionRecord>> connections = r.getConnections();

                    for (int conni = connections.size() - 1; conni >= 0; conni--) {
                        ArrayList<ConnectionRecord> clist = connections.valueAt(conni);
                        // 遍历每个联机纪录
                        for (int i=0; i<clist.size(); i++) {
                            // 当前联机纪录
                            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);
        }


    }

启动 Service

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

透过ActivityManager作为代理类访问AMS的startService方法，这边可以看出来startService并没有回调，但是 可以通过返回ComponentName判断是否启动成功

// 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);

            // 透过代理类访问AMS#startService
            ComponentName cn = ActivityManager.getService().startService(
                    mMainThread.getApplicationThread(), service,
                    service.resolveTypeIfNeeded(getContentResolver()), requireForeground,
                    getOpPackageName(), getAttributionTag(), user.getIdentifier());
            
            if (cn != null) {
                // 没有权限去启动Service
                if (cn.getPackageName().equals("!")) {
                    throw new SecurityException(
                            "Not allowed to start service " + service
                            + " without permission " + cn.getClassName());
                } else if (cn.getPackageName().equals("!!")) {
                    // 无法启动Service
                    throw new SecurityException(
                            "Unable to start service " + service
                            + ": " + cn.getClassName());
                } else if (cn.getPackageName().equals("?")) {
                    // 不允许启动Service
                    throw ServiceStartNotAllowedException.newInstance(requireForeground,
                            "Not allowed to start service " + service + ": " + cn.getClassName());
                }
            }
            
            ... 省略部分
            
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

AMS - 启动 Service 过程

ActivityManagerService#startService
检查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 {

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

        // 检查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类型是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
1.检查目前装置（同样是透过PKMS）是否有指定Service，查看retrieveServiceLocked方法
2.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. 检查目前装置是否有指定Service，查看retrieveServiceLocked方法
        ServiceLookupResult res =
            retrieveServiceLocked(service, null, resolvedType, callingPackage,
                    callingPid, callingUid, userId, true, callerFg, false, false);
        
        // 找不到 Service
        if (res == null) {
            return null;
        }

        ...
        
        ServiceRecord r = res.record;
        
        ... 省略部分
        
        // 2. 找到的ServiceRecord就是用于描述一个Service
        return startServiceInnerLocked(r, service, callingUid, callingPid, fgRequired, callerFg,
                allowBackgroundActivityStarts, backgroundActivityStartsToken);

    }

ServiceRecord用于描述一个Service，类似于ActivityRecord描述一个Activity

ActiveServices - 透过 PKMS 找 Service

AMS 寻找 Service 的方法是 ServiceLoopupResult，目的是要找到 ServiceRecord，而 找到 ServiceRecord 主要透过 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) {

        // 每个Service都会转为ServiceRecord
        ServiceRecord r = null;

        ServiceMap smap = getServiceMapLocked(userId);

        // 组装ComponentName
        final ComponentName comp;

        if (instanceName == null) {    // 目前传入的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);
        }
        
        // 判断 ComponentName 结果
        if (comp != null) {
            r = smap.mServicesByInstanceName.get(comp);
        }
        
        ... 省略部分
            
        if (r == null) {
            try {
                
                ...
                
                // 透过 PackageManager 找对对应的 Service
                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) {
                // 在同进程，所以一定不会发生
            } 
        }
        
        
        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) {
        // 尝试取得缓存
        ServiceMap smap = mServiceMap.get(callingUser);
        if (smap == null) {
            // 重新创建一个Service纪录对象
            smap = new ServiceMap(mAm.mHandler.getLooper(), callingUser);
            mServiceMap.put(callingUser, smap);
        }
        return smap;
    }

    final class ServiceMap extends Handler {
        final int mUserId;
        // 呼叫过的Service都会记录起来
        final ArrayMap<ComponentName, ServiceRecord> mServicesByInstanceName = new ArrayMap<>();
    
        ... 省略部分
    
    }

mServiceMap主要是透过User作为Key，储存相对应的ServiceMap，内部又有储存ComponentName & ServiceRecord，当User呼叫过相同的Service就可以透过它取得Service的缓存

Service 启动入口 - startServiceInnerLocked

// ActiveServices.java
    final ActivityManagerService mAm;

    ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
            boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
    
        // 这也是Function有Locked关键字的关系
        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
        
        
        // 透过bringUpServiceLocked启动Service
        String error = bringUpServiceLocked(r, service.getFlags(), callerFg,
                false /* whileRestarting */,
                false /* permissionsReviewRequired */,
                false /* packageFrozen */,
                true /* enqueueOomAdj */);
        
        ...
        
        return r.name;
    
    }

// 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

        // 该Service是否隔离
        final boolean isolated = (r.serviceInfo.flags&ServiceInfo.FLAG_ISOLATED_PROCESS) != 0;

        // 获取Service要运行在哪个进程
        final String procName = r.processName;
        HostingRecord hostingRecord = new HostingRecord("service", r.instanceName);
        ProcessRecord app;

        if (!isolated) {
            // 1. 尝试透过AMS取得ProcessRecord对象
            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 真正启动 Service
                        realStartServiceLocked(r, app, thread, pid, uidRecord, execInFg,
                                enqueueOomAdj);
                        
                        return null;
                    } /* 省略 catch*/
                    
                }
            }
        } else {
            ... 省略部分
        }
        
        
        if (app == null && !permissionsReviewRequired && !packageFrozen) {
            // 目标process不存在就透过AMS#startProcessLocked启动
            if ((app = mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
                        hostingRecord, ZYGOTE_POLICY_FLAG_EMPTY, false, isolated)) == null) {
                ... 啟動失敗
                
                // 关闭Servcie
                bringDownServiceLocked(r, enqueueOomAdj);
                return msg;
            }
            if (isolated) {
                r.isolatedProc = app;
            }
        }
        
        ... 省略部分
    }

真正创建 Service - realStartServiceLocked

透过ProcessRecord中的thread（也就是AcitivtyThread内的ApplicationThread对象）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 {

        // 设定目标Process
        r.setProcess(app, thread, pid, uidRecord);

        ... 省略部分

        // 取得目标进程内所有的Services
        final ProcessServiceRecord psr = app.mServices;

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

        // 透过AMS调整OOM_ADJ的数值
        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端处理scheduleCreateService，会透过MainHandler传送CREATE_SERVICE到主线程，之后就会呼叫到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);

            // 将需要的数据封装成CreateServiceData类
            CreateServiceData s = new CreateServiceData();
            s.token = token;    // Token 就是ServiceRecord类
            s.info = info;        // 包有 Service 的 class name ... 等等信息
            s.compatInfo = compatInfo;

            // 透过Hander传送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);

            ...

            // 透过LoadedApk创建Servcie对象
            // getAppFactory（）=>创建的是AppComponentFactory对象
            service = packageInfo.getAppFactory()
                    .instantiateService(cl, data.info.name, data.intent);

            // 创建对应的ContextImpl对象
            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，Service创建成功
                // 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 */
    }

透过 LoadedApk 创建Service对象

// 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();

        ... 

        // 透过createAppFactory方法创建Factory
        mAppComponentFactory = createAppFactory(mApplicationInfo, mBaseClassLoader);
    }

    public AppComponentFactory getAppFactory() {
        return mAppComponentFactory;
    }

    private AppComponentFactory createAppFactory(ApplicationInfo appInfo, ClassLoader cl) {

        // 若有自定义Factory则透过反射创建
        if (mIncludeCode && appInfo.appComponentFactory != null && cl != null) {
            try {
                return (AppComponentFactory)
                        cl.loadClass(appInfo.appComponentFactory).newInstance();
            } /* 省略部分 */
        }

        // 预设Factory
        return AppComponentFactory.DEFAULT;
    }

预设工厂 AppComponentFactory：透过ClassLoader#loadClass读取指定Class，并透过反射创建指定类

// 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 {
        // 透过ClassLoader#loadClass读取指定Class，并透过反射创建该类
        return (Service) cl.loadClass(className).newInstance();
    }

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