Android P系统服务接口调用原理

1、调用系统服务方式
通过以下方式获取StorageManager的实例对象，通过StorageManager调用系统存储服务。
StorageManager是系统存储服务的接口。
getVolumes()可返回系统上可用卷的集合，例如USB设备。

StorageManager storageManager = (StorageManager) context.getSystemService(Context.STORAGE_SERVICE);
List<VolumeInfo> volInfoList = storageManager.getVolumes();

2、getSystemService()方法解析
/framework/base/core/java/android/content/Context.java
从下述的注释中可以看到各种系统级服务的Manager。通过名称可获取对应的系统级服务句柄。

    /**
     * Return the handle to a system-level service by name. The class of the
     * returned object varies by the requested name. Currently available names
     * are:
     *
     * <dl>
     *  <dt> {@link #WINDOW_SERVICE} ("window")
     *  <dd> The top-level window manager in which you can place custom
     *  windows.  The returned object is a {@link android.view.WindowManager}.
     *  <dt> {@link #LAYOUT_INFLATER_SERVICE} ("layout_inflater")
     *  <dd> A {@link android.view.LayoutInflater} for inflating layout resources
     *  in this context.
     *  <dt> {@link #ACTIVITY_SERVICE} ("activity")
     *  <dd> A {@link android.app.ActivityManager} for interacting with the
     *  global activity state of the system.
     *  <dt> {@link #POWER_SERVICE} ("power")
     *  <dd> A {@link android.os.PowerManager} for controlling power
     *  management.
     *  <dt> {@link #ALARM_SERVICE} ("alarm")
     *  <dd> A {@link android.app.AlarmManager} for receiving intents at the
     *  time of your choosing.
     *  <dt> {@link #NOTIFICATION_SERVICE} ("notification")
     *  <dd> A {@link android.app.NotificationManager} for informing the user
     *   of background events.
     *  <dt> {@link #KEYGUARD_SERVICE} ("keyguard")
     *  <dd> A {@link android.app.KeyguardManager} for controlling keyguard.
     *  <dt> {@link #LOCATION_SERVICE} ("location")
     *  <dd> A {@link android.location.LocationManager} for controlling location
     *   (e.g., GPS) updates.
     *  <dt> {@link #SEARCH_SERVICE} ("search")
     *  <dd> A {@link android.app.SearchManager} for handling search.
     *  <dt> {@link #VIBRATOR_SERVICE} ("vibrator")
     *  <dd> A {@link android.os.Vibrator} for interacting with the vibrator
     *  hardware.
     *  <dt> {@link #CONNECTIVITY_SERVICE} ("connection")
     *  <dd> A {@link android.net.ConnectivityManager ConnectivityManager} for
     *  handling management of network connections.
     *  <dt> {@link #IPSEC_SERVICE} ("ipsec")
     *  <dd> A {@link android.net.IpSecManager IpSecManager} for managing IPSec on
     *  sockets and networks.
     *  <dt> {@link #WIFI_SERVICE} ("wifi")
     *  <dd> A {@link android.net.wifi.WifiManager WifiManager} for management of Wi-Fi
     *  connectivity.  On releases before NYC, it should only be obtained from an application
     *  context, and not from any other derived context to avoid memory leaks within the calling
     *  process.
     *  <dt> {@link #WIFI_AWARE_SERVICE} ("wifiaware")
     *  <dd> A {@link android.net.wifi.aware.WifiAwareManager WifiAwareManager} for management of
     * Wi-Fi Aware discovery and connectivity.
     *  <dt> {@link #WIFI_P2P_SERVICE} ("wifip2p")
     *  <dd> A {@link android.net.wifi.p2p.WifiP2pManager WifiP2pManager} for management of
     * Wi-Fi Direct connectivity.
     * <dt> {@link #INPUT_METHOD_SERVICE} ("input_method")
     * <dd> An {@link android.view.inputmethod.InputMethodManager InputMethodManager}
     * for management of input methods.
     * <dt> {@link #UI_MODE_SERVICE} ("uimode")
     * <dd> An {@link android.app.UiModeManager} for controlling UI modes.
     * <dt> {@link #DOWNLOAD_SERVICE} ("download")
     * <dd> A {@link android.app.DownloadManager} for requesting HTTP downloads
     * <dt> {@link #BATTERY_SERVICE} ("batterymanager")
     * <dd> A {@link android.os.BatteryManager} for managing battery state
     * <dt> {@link #JOB_SCHEDULER_SERVICE} ("taskmanager")
     * <dd>  A {@link android.app.job.JobScheduler} for managing scheduled tasks
     * <dt> {@link #NETWORK_STATS_SERVICE} ("netstats")
     * <dd> A {@link android.app.usage.NetworkStatsManager NetworkStatsManager} for querying network
     * usage statistics.
     * <dt> {@link #HARDWARE_PROPERTIES_SERVICE} ("hardware_properties")
     * <dd> A {@link android.os.HardwarePropertiesManager} for accessing hardware properties.
     * </dl>
     *
     * <p>Note:  System services obtained via this API may be closely associated with
     * the Context in which they are obtained from.  In general, do not share the
     * service objects between various different contexts (Activities, Applications,
     * Services, Providers, etc.)
     *
     * <p>Note: Instant apps, for which {@link PackageManager#isInstantApp()} returns true,
     * don't have access to the following system services: {@link #DEVICE_POLICY_SERVICE},
     * {@link #FINGERPRINT_SERVICE}, {@link #SHORTCUT_SERVICE}, {@link #USB_SERVICE},
     * {@link #WALLPAPER_SERVICE}, {@link #WIFI_P2P_SERVICE}, {@link #WIFI_SERVICE},
     * {@link #WIFI_AWARE_SERVICE}. For these services this method will return <code>null</code>.
     * Generally, if you are running as an instant app you should always check whether the result
     * of this method is null.
     *
     * @param name The name of the desired service.
     *
     * @return The service or null if the name does not exist.
     *
     * @see #WINDOW_SERVICE
     * @see android.view.WindowManager
     * @see #LAYOUT_INFLATER_SERVICE
     * @see android.view.LayoutInflater
     * @see #ACTIVITY_SERVICE
     * @see android.app.ActivityManager
     * @see #POWER_SERVICE
     * @see android.os.PowerManager
     * @see #ALARM_SERVICE
     * @see android.app.AlarmManager
     * @see #NOTIFICATION_SERVICE
     * @see android.app.NotificationManager
     * @see #KEYGUARD_SERVICE
     * @see android.app.KeyguardManager
     * @see #LOCATION_SERVICE
     * @see android.location.LocationManager
     * @see #SEARCH_SERVICE
     * @see android.app.SearchManager
     * @see #SENSOR_SERVICE
     * @see android.hardware.SensorManager
     * @see #STORAGE_SERVICE
     * @see android.os.storage.StorageManager
     * @see #VIBRATOR_SERVICE
     * @see android.os.Vibrator
     * @see #CONNECTIVITY_SERVICE
     * @see android.net.ConnectivityManager
     * @see #WIFI_SERVICE
     * @see android.net.wifi.WifiManager
     * @see #AUDIO_SERVICE
     * @see android.media.AudioManager
     * @see #MEDIA_ROUTER_SERVICE
     * @see android.media.MediaRouter
     * @see #TELEPHONY_SERVICE
     * @see android.telephony.TelephonyManager
     * @see #TELEPHONY_SUBSCRIPTI