【内核研究】ServiceManager管理的服务

系统服务中的Binder对象

在应用程序编程时，经常使用getSystemService（String serviceName）方法获取一个系统服务，那么，这些系统服务的Binder引用是如何传递给客户端的呢？须知系统服务并不是通过startService()启动的。

getSystemService()函数的实现是在ContextImpl类中，该函数所返回的Service比较多，具体可参照源码。这些Service一般都由ServiceManager管理。

@Override
    public Object getSystemService(String name) {
        if (WINDOW_SERVICE.equals(name)) {
            return WindowManagerImpl.getDefault();
        } else if (LAYOUT_INFLATER_SERVICE.equals(name)) {
            synchronized (mSync) {
                LayoutInflater inflater = mLayoutInflater;
                if (inflater != null) {
                    return inflater;
                }
                mLayoutInflater = inflater =
                    PolicyManager.makeNewLayoutInflater(getOuterContext());
                return inflater;
            }
        } else if (ACTIVITY_SERVICE.equals(name)) {
            return getActivityManager();
        } else if (INPUT_METHOD_SERVICE.equals(name)) {
            return InputMethodManager.getInstance(this);
        } else if (ALARM_SERVICE.equals(name)) {
            return getAlarmManager();
        } else if (ACCOUNT_SERVICE.equals(name)) {
            return getAccountManager();
        } else if (POWER_SERVICE.equals(name)) {
            return getPowerManager();
        } else if (CONNECTIVITY_SERVICE.equals(name)) {
            return getConnectivityManager();
        } else if (THROTTLE_SERVICE.equals(name)) {
            return getThrottleManager();
        } else if (WIFI_SERVICE.equals(name)) {
            return getWifiManager();
        } else if (NOTIFICATION_SERVICE.equals(name)) {
            return getNotificationManager();
        } else if (KEYGUARD_SERVICE.equals(name)) {
            return new KeyguardManager();
        } else if (ACCESSIBILITY_SERVICE.equals(name)) {
            return AccessibilityManager.getInstance(this);
        } else if (LOCATION_SERVICE.equals(name)) {
            return getLocationManager();
        } else if (SEARCH_SERVICE.equals(name)) {
            return getSearchManager();
        } else if (SENSOR_SERVICE.equals(name)) {
            return getSensorManager();
        } else if (STORAGE_SERVICE.equals(name)) {
            return getStorageManager();
        } else if (VIBRATOR_SERVICE.equals(name)) {
            return getVibrator();
        } else if (STATUS_BAR_SERVICE.equals(name)) {
            synchronized (mSync) {
                if (mStatusBarManager == null) {
                    mStatusBarManager = new StatusBarManager(getOuterContext());
                }
                return mStatusBarManager;
            }
        } else if (AUDIO_SERVICE.equals(name)) {
            return getAudioManager();
        } else if (TELEPHONY_SERVICE.equals(name)) {
            return getTelephonyManager();
        } else if (CLIPBOARD_SERVICE.equals(name)) {
            return getClipboardManager();
        } else if (WALLPAPER_SERVICE.equals(name)) {
            return getWallpaperManager();
        } else if (DROPBOX_SERVICE.equals(name)) {
            return getDropBoxManager();
        } else if (DEVICE_POLICY_SERVICE.equals(name)) {
            return getDevicePolicyManager();
        } else if (UI_MODE_SERVICE.equals(name)) {
            return getUiModeManager();
        } else if (DOWNLOAD_SERVICE.equals(name)) {
            return getDownloadManager();
        } else if (NFC_SERVICE.equals(name)) {
            return getNfcManager();
        }

        return null;
    }

    // activity manager
    private ActivityManager getActivityManager() {
        synchronized (mSync) {
            if (mActivityManager == null) {
                mActivityManager = new ActivityManager(getOuterContext(),
                        mMainThread.getHandler());
            }
        }
        return mActivityManager;
    }
    // alarm manager
    private AlarmManager getAlarmManager() {
        synchronized (sSync) {
            if (sAlarmManager == null) {
                IBinder b = ServiceManager.getService(ALARM_SERVICE);
                IAlarmManager service = IAlarmManager.Stub.asInterface(b);
                sAlarmManager = new AlarmManager(service);
            }
        }
        return sAlarmManager;
    }

ServiceManager管理的服务

ServiceManager是一个独立进程，其作用如名称所示，管理各种系统服务，管理的逻辑如下所示。

ServiceManager

AlarmService

InputMethodManagerService

VibrationService

AccountManagerService

WIFIService

LOcationService

...

ServiceManager本身也是一个Service，Framework提供了一个系统函数，可以获取该Service对应的Binder引用，那就是BinderInternal.getContextObject()。该静态函数返回ServiceManager后，就可以通过ServiceManager提供的方法获取其他系统Service的Binder引用。这种设计模式在日常生活中到处可见，ServiceManager就像是一个公司的总机，这个总机号码是公开的，系统中任何进程都可以使用BinderInternal.getContextObject()获取该总机的Binder对象，而当用户想联系公司中的其他人（服务）时，则要经过总机再获得分机号码。这种设计的好处是系统中仅暴露一个全局Binder引用，那就是ServiceManager，而其他系统服务则可以隐藏起来，从而有助于系统服务的扩展，以及调用系统服务的安全检查。其他系统服务在启动时，首先把自己的Binder对象传递给ServiceManager，即所谓的注册（addService）。

下面从代码实现来看以上逻辑。可以查看ContextImpl.getSystemService()中各种Service的具体获取方式，比如INPUT_METHOD_SERVICE，代码如下：

 } else if (INPUT_METHOD_SERVICE.equals(name)) {
            return InputMethodManager.getInstance(this);
        }

而InputMethodManager.getInstance（this）的关键代码如下：

    /**
     * Retrieve the global InputMethodManager instance, creating it if it
     * doesn't already exist.
     * @hide
     */
    static public InputMethodManager getInstance(Context context) {
        return getInstance(context.getMainLooper());
    }

    /**
     * Internally, the input method manager can't be context-dependent, so
     * we have this here for the places that need it.
     * @hide
     */
    static public InputMethodManager getInstance(Looper mainLooper) {
        synchronized (mInstanceSync) {
            if (mInstance != null) {
                return mInstance;
            }
            IBinder b = ServiceManager.getService(Context.INPUT_METHOD_SERVICE);
            IInputMethodManager service = IInputMethodManager.Stub.asInterface(b);
            mInstance = new InputMethodManager(service, mainLooper);
        }
        return mInstance;
    }

即通过ServiceManager获取InputMethod Service对应的Binder对象b，然后再将该Binder对象作为IInputMethodManager.Stub.asInterface()的参数，返回一个IInputMethodManager的统一接口。

ServiceManager.getService()的代码如下：

/**
     * Returns a reference to a service with the given name.
     * 
     * @param name the name of the service to get
     * @return a reference to the service, or <code>null</code> if the service doesn't exist
     */
    public static IBinder getService(String name) {
        try {
            IBinder service = sCache.get(name);
            if (service != null) {
                return service;
            } else {
                return getIServiceManager().getService(name);
            }
        } catch (RemoteException e) {
            Log.e(TAG, "error in getService", e);
        }
        return null;
    }

即首先从sCache缓存中查看是否有对应的Binder对象，有则返回，没有则调用getIServiceManager().getService(name)，第一个函数getIServiceManager()即用于返回系统中唯一的ServiceManager对应的Binder，其代码如下：

    private static IServiceManager getIServiceManager() {
        if (sServiceManager != null) {
            return sServiceManager;
        }

        // Find the service manager
        sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject());
        return sServiceManager;
    }

以上代码中，BinderInternal.getContextObject()静态函数即用于返回ServiceManager对应的全局Binder对象，该函数不需要任何参数，因为它的作用是固定的。从这个角度来看，这个函数的命名似乎更应该明确一些，比如，可以命名为getServiceManager()。

其他所有通过ServiceManager获取的系统服务的过程与以上基本类似，所不同的就是传递给ServiceManager的服务名称不同，因为ServiceManager正是按照服务的名称（String类型）来保存不同的Binder对象的。

关于使用addService()向ServiceManager中添加一个服务一般是在SystemService进程启动时完成的，具体见关于Framework启动过程的描述。

系统服务列表

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.os.PowerManager} for controlling power management,
     * including "wake locks," which let you keep the device on while
     * you're running long tasks.
     */
    public static final String POWER_SERVICE = "power";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.view.WindowManager} for accessing the system's window
     * manager.
     *
     * @see #getSystemService
     * @see android.view.WindowManager
     */
    public static final String WINDOW_SERVICE = "window";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.view.LayoutInflater} for inflating layout resources in this
     * context.
     *
     * @see #getSystemService
     * @see android.view.LayoutInflater
     */
    public static final String LAYOUT_INFLATER_SERVICE = "layout_inflater";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.accounts.AccountManager} for receiving intents at a
     * time of your choosing.
     *
     * @see #getSystemService
     * @see android.accounts.AccountManager
     */
    public static final String ACCOUNT_SERVICE = "account";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.app.ActivityManager} for interacting with the global
     * system state.
     *
     * @see #getSystemService
     * @see android.app.ActivityManager
     */
    public static final String ACTIVITY_SERVICE = "activity";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.app.AlarmManager} for receiving intents at a
     * time of your choosing.
     *
     * @see #getSystemService
     * @see android.app.AlarmManager
     */
    public static final String ALARM_SERVICE = "alarm";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.app.NotificationManager} for informing the user of
     * background events.
     *
     * @see #getSystemService
     * @see android.app.NotificationManager
     */
    public static final String NOTIFICATION_SERVICE = "notification";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.view.accessibility.AccessibilityManager} for giving the user
     * feedback for UI events through the registered event listeners.
     *
     * @see #getSystemService
     * @see android.view.accessibility.AccessibilityManager
     */
    public static final String ACCESSIBILITY_SERVICE = "accessibility";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.app.NotificationManager} for controlling keyguard.
     *
     * @see #getSystemService
     * @see android.app.KeyguardManager
     */
    public static final String KEYGUARD_SERVICE = "keyguard";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.location.LocationManager} for controlling location
     * updates.
     *
     * @see #getSystemService
     * @see android.location.LocationManager
     */
    public static final String LOCATION_SERVICE = "location";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.app.SearchManager} for handling searches.
     *
     * @see #getSystemService
     * @see android.app.SearchManager
     */
    public static final String SEARCH_SERVICE = "search";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.hardware.SensorManager} for accessing sensors.
     *
     * @see #getSystemService
     * @see android.hardware.SensorManager
     */
    public static final String SENSOR_SERVICE = "sensor";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.os.storage.StorageManager} for accessing system storage
     * functions.
     *
     * @see #getSystemService
     * @see android.os.storage.StorageManager
     */
    public static final String STORAGE_SERVICE = "storage";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * com.android.server.WallpaperService for accessing wallpapers.
     *
     * @see #getSystemService
     */
    public static final String WALLPAPER_SERVICE = "wallpaper";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.os.Vibrator} for interacting with the vibration hardware.
     *
     * @see #getSystemService
     * @see android.os.Vibrator
     */
    public static final String VIBRATOR_SERVICE = "vibrator";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.app.StatusBarManager} for interacting with the status bar.
     *
     * @see #getSystemService
     * @see android.app.StatusBarManager
     * @hide
     */
    public static final String STATUS_BAR_SERVICE = "statusbar";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.net.ConnectivityManager} for handling management of
     * network connections.
     *
     * @see #getSystemService
     * @see android.net.ConnectivityManager
     */
    public static final String CONNECTIVITY_SERVICE = "connectivity";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.net.ThrottleManager} for handling management of
     * throttling.
     *
     * @hide
     * @see #getSystemService
     * @see android.net.ThrottleManager
     */
    public static final String THROTTLE_SERVICE = "throttle";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.net.NetworkManagementService} for handling management of
     * system network services
     *
     * @hide
     * @see #getSystemService
     * @see android.net.NetworkManagementService
     */
    public static final String NETWORKMANAGEMENT_SERVICE = "network_management";

    /**
     * Use with {@link #getSystemService} to retrieve a {@link
     * android.net.wifi.WifiManager} for handling management of
     * Wi-Fi access.
     *
     * @see #getSystemService
     * @see android.net.wifi.WifiManager
     */
    public static final String WIFI_SERVICE = "wifi";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.media.AudioManager} for handling management of volume,
     * ringer modes and audio routing.
     *
     * @see #getSystemService
     * @see android.media.AudioManager
     */
    public static final String AUDIO_SERVICE = "audio";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.telephony.TelephonyManager} for handling management the
     * telephony features of the device.
     *
     * @see #getSystemService
     * @see android.telephony.TelephonyManager
     */
    public static final String TELEPHONY_SERVICE = "phone";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.text.ClipboardManager} for accessing and modifying
     * the contents of the global clipboard.
     *
     * @see #getSystemService
     * @see android.text.ClipboardManager
     */
    public static final String CLIPBOARD_SERVICE = "clipboard";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.view.inputmethod.InputMethodManager} for accessing input
     * methods.
     *
     * @see #getSystemService
     */
    public static final String INPUT_METHOD_SERVICE = "input_method";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.appwidget.AppWidgetManager} for accessing AppWidgets.
     *
     * @hide
     * @see #getSystemService
     */
    public static final String APPWIDGET_SERVICE = "appwidget";

    /**
     * Use with {@link #getSystemService} to retrieve an
     * {@link android.app.backup.IBackupManager IBackupManager} for communicating
     * with the backup mechanism.
     * @hide
     *
     * @see #getSystemService
     */
    public static final String BACKUP_SERVICE = "backup";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.os.DropBoxManager} instance for recording
     * diagnostic logs.
     * @see #getSystemService
     */
    public static final String DROPBOX_SERVICE = "dropbox";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.app.admin.DevicePolicyManager} for working with global
     * device policy management.
     *
     * @see #getSystemService
     */
    public static final String DEVICE_POLICY_SERVICE = "device_policy";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.app.UiModeManager} for controlling UI modes.
     *
     * @see #getSystemService
     */
    public static final String UI_MODE_SERVICE = "uimode";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.app.DownloadManager} for requesting HTTP downloads.
     *
     * @see #getSystemService
     */
    public static final String DOWNLOAD_SERVICE = "download";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.nfc.NfcManager} for using NFC.
     *
     * @see #getSystemService
     */
    public static final String NFC_SERVICE = "nfc";

    /**
     * Use with {@link #getSystemService} to retrieve a
     * {@link android.net.sip.SipManager} for accessing the SIP related service.
     *
     * @see #getSystemService
     */
    /** @hide */
    public static final String SIP_SERVICE = "sip";