5.2 Context.getSystemService() 源码分析

追踪 ContextImpl getSystemService() 源代码

  @Override
  public Object getSystemService(String name) {
    return SystemServiceRegistry.getSystemService(this, name);
  }

继续追踪 SystemServiceRegistry 源代码

  /**
   * Gets a system service from a given context.
   */
  public static Object getSystemService(ContextImpl ctx, String name) {
    ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
    return fetcher != null ? fetcher.getService(ctx) : null;
  }

追踪 SYSTEM_SERVICE_FETCHERS 可以发现在 SystemServiceRegistry 静态区中注册了几乎所有的系统服务

  registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
      new CachedServiceFetcher<LayoutInflater>() {
    @Override
    public LayoutInflater createService(ContextImpl ctx) {
      return new PhoneLayoutInflater(ctx.getOuterContext());
    }});

  registerService(Context.LOCATION_SERVICE, LocationManager.class,
      new CachedServiceFetcher<LocationManager>() {
    @Override
    public LocationManager createService(ContextImpl ctx) {
      IBinder b = ServiceManager.getService(Context.LOCATION_SERVICE);
      return new LocationManager(ctx, ILocationManager.Stub.asInterface(b));
    }});

上面代码片断中， PhoneLayoutInflater 最终回到了 LayoutInflater 。而 LocationManager 则是对 ILocationManager 的封装。可以发现，在 frameworks/base/location/java/android/location 包下含有大量的 AIDL 文件。

继续追踪 ServiceManager.getService(Context.LOCATION_SERVICE)

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

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

  /**
   * 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;
  }

从上面代码片断可以看出， ServiceManager 会从 sCache 缓存或 IServiceManager 中查找服务并返回一个 IBinder 对象。这个 IBinder 就是一个远程对象，可以通过它与其他进程交互。

继续深入 getIServiceManager().getService(name) , 进入 ServiceManagerNative

  /**
   * Cast a Binder object into a service manager interface, generating
   * a proxy if needed.
   */
  static public IServiceManager asInterface(IBinder obj)
  {
    if (obj == null) {
      return null;
    }
    IServiceManager in =
      (IServiceManager)obj.queryLocalInterface(descriptor);
    if (in != null) {
      return in;
    }
    
    return new ServiceManagerProxy(obj);
  }
  
  class ServiceManagerProxy implements IServiceManager {
    public ServiceManagerProxy(IBinder remote) {
      mRemote = remote;
    }
    
    public IBinder asBinder() {
      return mRemote;
    }
    
    public IBinder getService(String name) throws RemoteException {
      Parcel data = Parcel.obtain();
      Parcel reply = Parcel.obtain();
      data.writeInterfaceToken(IServiceManager.descriptor);
      data.writeString(name);
      mRemote.transact(GET_SERVICE_TRANSACTION, data, reply, 0);
      IBinder binder = reply.readStrongBinder();
      reply.recycle();
      data.recycle();
      return binder;
    }
    
    private IBinder mRemote;
  }

从上边代码片断可以看到， ServiceManager.getIServiceManager() 返回的是一个 ServiceManagerProxy , 而 ServiceManager.getService() 则是在 ServiceManagerProxy 中通过 ServiceManager 的远程 Binder 对象 mRemote ，操作 Parcel 数据，调用 IBinder.transact(int code, Parcel data, Parcel reply, int flags) 方法来发送请求，并通过 reply.readStrongBinder() 返回了要查找的服务的远程对象。

可以看到，系统服务的获取方式也是通过 AIDL 的方式实现的。