Android -- 网络相关的系统服务启动简要分析

Android --  网络相关的系统服务启动简要分析

Android中众多的系统服务都是在SystemServer中启动的，一般有两种方式：

1. SystemServiceManager.startServcie()
   
2. ServiceManageraddService()
   

前一种方式也是通过后者将一个服务添加到Android的服务体系中的。我们知道，Android的服务体系都是基于Binder来实现的；Java代码里调用的这些启动、添加服务的方法，实际上是对底层C++实现的服务体系的代码封装，这里我们不做深入探讨。

我们以EthernetService为例，简单介绍下该服务的启动过程。进入SystemServer：

private static final String ETHERNET_SERVICE_CLASS =
            "com.android.server.ethernet.EthernetService";

if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_ETHERNET) ||
                    mPackageManager.hasSystemFeature(PackageManager.FEATURE_USB_HOST)) {
                    mSystemServiceManager.startService(ETHERNET_SERVICE_CLASS);
}

如果支持Ethernet，则会调用SystemServiceManager.startService()方法开启服务：

 /**
     * Starts a service by class name.
     *
     * @return The service instance.
     */
    @SuppressWarnings("unchecked")
    public SystemService startService(String className) {
        final Class serviceClass;
        try {
            serviceClass = (Class)Class.forName(className);
        } catch (ClassNotFoundException ex) {
            Slog.i(TAG, "Starting " + className);
            throw new RuntimeException("Failed to create service " + className
                    + ": service class not found, usually indicates that the caller should "
                    + "have called PackageManager.hasSystemFeature() to check whether the "
                    + "feature is available on this device before trying to start the "
                    + "services that implement it", ex);
        }
        return startService(serviceClass);
    }

    /**
     * Creates and starts a system service. The class must be a subclass of
     * {@link com.android.server.SystemService}.
     *
     * @param serviceClass A Java class that implements the SystemService interface.
     * @return The service instance, never null.
     * @throws RuntimeException if the service fails to start.
     */
    @SuppressWarnings("unchecked")
    public  T startService(Class serviceClass) {
        final String name = serviceClass.getName();
        Slog.i(TAG, "Starting " + name);

        // Create the service.
        if (!SystemService.class.isAssignableFrom(serviceClass)) {
            throw new RuntimeException("Failed to create " + name
                    + ": service must extend " + SystemService.class.getName());
        }
        final T service;
        try {
            Constructor constructor = serviceClass.getConstructor(Context.class);
            service = constructor.newInstance(mContext);
        } catch (InstantiationException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service could not be instantiated", ex);
        } catch (IllegalAccessException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service must have a public constructor with a Context argument", ex);
        } catch (NoSuchMethodException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service must have a public constructor with a Context argument", ex);
        } catch (InvocationTargetException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service constructor threw an exception", ex);
        }

        // Register it.
        mServices.add(service);

        // Start it.
        try {
            service.onStart();
        } catch (RuntimeException ex) {
            throw new RuntimeException("Failed to start service " + name
                    + ": onStart threw an exception", ex);
        }
        return service;
    }

startService()通过反射，调用构造函数得到一个EthernetService的实例化对象；同时，EthernetService中会创建一个EthernetServiceImpl对象，它是EthernetManager的服务端。调用EthernetService的onStart()方法，注册该服务：

public final class EthernetService extends SystemService {

    private static final String TAG = "EthernetService";
    final EthernetServiceImpl mImpl;

    public EthernetService(Context context) {
        super(context);
        mImpl = new EthernetServiceImpl(context);
    }

    @Override
    public void onStart() {
        Log.i(TAG, "Registering service " + Context.ETHERNET_SERVICE);
        publishBinderService(Context.ETHERNET_SERVICE, mImpl);
    }

    @Override
    public void onBootPhase(int phase) {
        if (phase == SystemService.PHASE_SYSTEM_SERVICES_READY) {
            mImpl.start();
        }
    }
}

EthernetService继承自SystemService。SystemService是一个抽象类，它的设计初衷是用来表示一个运行在system进程中的服务。根据生命周期阶段的不同，它提供了很多系统回调方法，我们可以对它们进行重写，以在生命周期的不同阶段完成某些工作。这里主要看两个方法：

    /**
     * Called when the dependencies listed in the @Service class-annotation are available
     * and after the chosen start phase.
     * When this method returns, the service should be published.
     */
    public abstract void onStart();

    /**
     * Called on each phase of the boot process. Phases before the service's start phase
     * (as defined in the @Service annotation) are never received.
     *
     * @param phase The current boot phase.
     */
    public void onBootPhase(int phase) {}

当我们需要启动一个服务时，调用onStart()方法将该服务注册到ServiceManager中；

 /**
     * Publish the service so it is accessible to other services and apps.
     */
    protected final void publishBinderService(String name, IBinder service) {
        publishBinderService(name, service, false);
    }

    /**
     * Publish the service so it is accessible to other services and apps.
     */
    protected final void publishBinderService(String name, IBinder service,
            boolean allowIsolated) {
        ServiceManager.addService(name, service, allowIsolated);
    }

onBootPhase()可以在boot的各个阶段被调用。在EthernetService中，当系统处于PHASE_SYSTEM_SERVICES_READY阶段时，会调用EthernetServiceImpl.start()方法，做一些Ethernet的初始化工作：

 public void start() {
        Log.i(TAG, "Starting Ethernet service");

        HandlerThread handlerThread = new HandlerThread("EthernetServiceThread");
        handlerThread.start();
        mHandler = new Handler(handlerThread.getLooper());

        mTracker.start(mContext, mHandler);//EthernetNetworkFactory

        mStarted.set(true);
    }

EthernetManager对外暴露接口，EthernetServiceImpl向EthernetManager提供服务，EthernetNetworkFactory包揽所有的网络管理操作。

这样，Ethernet服务的启动过程就结束了。