Android系统服务调用分析

      在Android系统中,framework向应用层提供了很多系统服务,例如InputManager、ActivityManager、WindowManager等,方便的实现软件需求的开发。

一.系统服务使用

      一般而言,在使用时,通过以下方式来获取对应的系统服务管理者:

WindowManager mWindowManager;
mWindowManager = (WindowManager) mContext.getSystemService(Context.WINDOW_SERVICE);
mWindowManager.addView();

      即:调用 Context 类的 getSystemService(String name) 方法,通过一步一步调用,该方法的最终实现是在 ContextImpl 类中,通过查看 ContextImpl 的源码,发现对应的实现如下:

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

      从ContextImpl里面的实现来看,会调用到SystemServiceRegistry类里面的getSystemService方法,再进去看一下SystemServiceRegistry.java的内部实现:

public static Object getSystemService(ContextImpl ctx, String name) {
    SystemServiceRegistry.ServiceFetcher fetcher = (SystemServiceRegistry.ServiceFetcher)SYSTEM_SERVICE_FETCHERS.get(name);
    return fetcher != null ? fetcher.getService(ctx) : null;
}

      从以上方法可以看到:从SYSTEM_SERVICE_FETCHERS 中获取对应名称的系统服务管理者提取器,然后从中得到对应的服务器管理者。看一下SYSTEM_SERVICE_FETCHERS:

private static final HashMap> SYSTEM_SERVICE_FETCHERS = new HashMap();

      SYSTEM_SERVICE_FETCHERS是一个HashMap,存放着String为key,ServiceFetcher为value的键值对:

private static  void registerService(String serviceName, Class serviceClass, SystemServiceRegistry.ServiceFetcher serviceFetcher) {
    SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
    SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
}

      在执行registerService方法时,将serviceName对应的serviceFetcher存入HashMap,接着看一下ServiceFetcher的实现:

abstract static class CachedServiceFetcher implements SystemServiceRegistry.ServiceFetcher {
    private final int mCacheIndex;

    public CachedServiceFetcher() {
        mCacheIndex = SystemServiceRegistry.sServiceCacheSize++;
    }

    public final T getService(ContextImpl ctx) {
        final Object[] cache = ctx.mServiceCache;
        synchronized(cache) {
            Object service = cache[mCacheIndex];
            if (service == null) {
                try {
                    service = createService(ctx);
                    cache[mCacheIndex] = service;
                } catch (ServiceNotFoundException e) {
                    onServiceNotFound(e);
               }
            }

            return (T)service;
        }
    }

    public abstract T createService(ContextImpl ctx) throws ServiceNotFoundException;
}
interface ServiceFetcher {
    T getService(ContextImpl var1);
}

      从以上可以看到,ServiceFetcher是一个接口,通过CachedServiceFetcher来实现,里面有一个抽象方法createService()需要实现,在getService()方法里面有调用到createService(),接下来看一下系统服务是如何注册的:

二.系统服务注册

      要想使用系统服务,肯定是需要系统服务先注册后使用,看一下系统服务是什么时候注册的:

static {
    ....
    registerService("rttmanager", RttManager.class, new SystemServiceRegistry.CachedServiceFetcher() {
        public RttManager createService(ContextImpl ctx) throws ServiceNotFoundException {
            IBinder b = ServiceManager.getServiceOrThrow("rttmanager");
            IRttManager service = android.net.wifi.IRttManager.Stub.asInterface(b);
            return new RttManager(ctx.getOuterContext(), service, ConnectivityThread.getInstanceLooper());
        }
    });

    registerService("ethernet", EthernetManager.class, new SystemServiceRegistry.CachedServiceFetcher() {
        public EthernetManager createService(ContextImpl ctx) throws ServiceNotFoundException {
            IBinder b = ServiceManager.getServiceOrThrow("ethernet");
            IEthernetManager service = android.net.IEthernetManager.Stub.asInterface(b);
            return new EthernetManager(ctx.getOuterContext(), service);
        }
    });

    registerService("window", WindowManager.class, new SystemServiceRegistry.CachedServiceFetcher() {
        public WindowManager createService(ContextImpl ctx) {
            return new WindowManagerImpl(ctx);
        }
    });
}
....

      registerService()是在static代码块里面,createService()会返回对应的Manager,那么在getSystemService()内部执行fetcher.getService(ctx)就获取到了manager,在返回前有以下逻辑:

IBinder b = ServiceManager.getServiceOrThrow("ethernet");
IEthernetManager service = android.net.IEthernetManager.Stub.asInterface(b);

      先通过ServiceManager获取了“ethernet”对应的IBinder,然后获取到service,最后把service作为参数传递给EthernetManager。看一下EthernetManager的实现:

public class EthernetManager {
    private final IEthernetManager mService;
    public EthernetManager(Context context, IEthernetManager service) {
        mContext = context;
        mService = service;
    }

    public IpConfiguration getConfiguration() {
        try {
            return mService.getConfiguration();
        }
        .....
    }

    public void setConfiguration(IpConfiguration config) {
        try {
            mService.setConfiguration(config);
        } 
        .....
    }

    public boolean isAvailable() {
        try {
            return mService.isAvailable();
        }
        ......
    }

    public void addListener(Listener listener) {
        mListeners.add(listener);
        if (mListeners.size() == 1) {
            try {
                mService.addListener(mServiceListener);
            }
            ......
        }
    }

    public void removeListener(Listener listener) {
        mListeners.remove(listener);
        if (mListeners.isEmpty()) {
            try {
                mService.removeListener(mServiceListener);
            }
            ......
        }
    }

      通过以上看到,通过EthernetManager来调用framework接口,其实是通过IEthernetManager来执行的,IEthernetManager是aidl接口,而前面看到,IEthernetManager是通过Stub.asInterface()来获取的,肯定有对应Binder实现,接下来看一下该Binder的实现及Binder是如何暴露出来的:

三.系统服务实现

      本文拿EthernetManager来举例,看一下对应的EthernetService的内部逻辑是如何实现的:

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);
        //公开Binder
        publishBinderService(Context.ETHERNET_SERVICE, mImpl);
    }
}

      在构造方法中创建EthernetServiceImpl()实例,然后在onStart()中执行publishBinderService(),该方法是父类SystemService.java的方法,实现如下:

protected final void publishBinderService(String name, IBinder service) {
    publishBinderService(name, service, false);
}

protected final void publishBinderService(String name, IBinder service,
            boolean allowIsolated) {
    //最终会通过ServiceManager.addService来注册Binder
    ServiceManager.addService(name, service, allowIsolated);
}

      通过以上可以看到,在EthernetService启动时,会执行publishBinderService()来将EthernetServiceImpl来作为实名的Binder来暴露出来,在publishBinderService()内部会通过ServiceManager.addService()将实名Binder统一给ServiceManager来管理。

public class EthernetServiceImpl extends IEthernetManager.Stub {
    //实现IEthernetManager里面的方法
    IpConfiguration getConfiguration() {
    }

    void setConfiguration(in IpConfiguration config) {
    }

    boolean isAvailable(){
    }

    void addListener(in IEthernetServiceListener listener){
   }
}

      EthernetServiceImpl继承IEthernetManager.Stub,来实现IEthernetManager.aidl的方法,供远端的Client来最终调用。

interface IEthernetManager
{
    IpConfiguration getConfiguration();
    void setConfiguration(in IpConfiguration config);
    boolean isAvailable();
    void addListener(in IEthernetServiceListener listener);
}

四.系统服务启动

      要想获取IEthernetManager的Binder,必须要先把Binder公开出来,该Binder公开是在EthernetService启动后执行的,那么需要有个地方来启动EthernetService,该启动入口是在哪的?在SystemServer.java中:

    private void run() {
        // Start services.
        try {
            traceBeginAndSlog("StartServices");
            startBootstrapServices();
            startCoreServices();
            startOtherServices();
            SystemServerInitThreadPool.shutdown();
        }
        ......
    }

      在SystemServer启动的时候,去在main()中调用run()方法,然后在run()中会去启动系统的一些公共service,该service在startOtherServices()中启动的:

private static final String ETHERNET_SERVICE_CLASS ="com.android.server.ethernet.EthernetService";
private void startOtherServices() {
    ......
    //其他service
    ......
    if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_ETHERNET) ||
        mPackageManager.hasSystemFeature(PackageManager.FEATURE_USB_HOST)) {
        traceBeginAndSlog("StartEthernet");
        mSystemServiceManager.startService(ETHERNET_SERVICE_CLASS);
        traceEnd();
     }
    ....
    //其他service
    ......
}

      EthernetService是在startOtherServices()通过SystemServiceManager.startService()去启动的,传入对应的系统服务类名,看一下SystemServiceManager内部startService的实现逻辑:

    public SystemService startService(String className) {
        final Class serviceClass;
        try {
            serviceClass = (Class)Class.forName(className);
        } 
        ......
        return startService(serviceClass);
    }

      先通过传入的字符串找到对应的类,然后传入类名调用startService():

    public  T startService(Class serviceClass) {
        try {
            final String name = serviceClass.getName();
            .......
            final T service;
            try {
                Constructor constructor = serviceClass.getConstructor(Context.class);
                service = constructor.newInstance(mContext);
            } 
            .......
            startService(service);
            return service;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
        }
    }

      通过反射找到构造方法,然后通过newInstance()来创造了实例,然后传入该实例调用startService():

    public void startService(@NonNull final SystemService service) {
        // Register it.
        mServices.add(service);
        // Start it.
        long time = SystemClock.elapsedRealtime();
        try {
            service.onStart();
        }
        ......
    }

      调用service的onStart()方法,即调用了EthernetService的onStart()方法,将Binder暴露出来供调用;注意:SystemService是一个抽象类,不是Service。
      简单总结一下:系统服务首先通过SystemServer内部调用SystemServiceManager来进行启动,启动后会将自己注册到ServiceManager作为实名服务,系统服务对应的Manager在SystemServiceRegistry内部注册时就通过ServiceManager来获取到该服务,从而供上层应用使用;当使用系统服务时,首先通过 Context 类的 getSystemService方法获取到服务对应的Manager,Manager会通过对应服务公开的Binder最终与服务进行通信,然后来执行上层的逻辑。

五.系统服务实现及使用流程

      1.创建xx.aidl文件,加入系统服务能提供的方法;
      2.创建类文件来实现xx.Stub,来实现服务提供的方法;
      3.将实现xx.Stub的类对象Binder通过ServiceManager.addService()注册【SystemServer作为入口触发】;
      4.创建服务对应的Manager,封装对应服务提供方法的方法来供app调用;
      5.SystemServiceProxy通过registerService来注册对应的服务【Manager与对应的服务建立联系】;
      6.app通过getSystemService()来获取对应服务的Manager,然后调用方法;

六.其他

      系统服务是供上层app来使用的,在rom定制化开发时,如果使用某个服务的应用很多时,比如数据上报,那么可以将数据上报作为一个系统服务来跟后台进行交互;
      系统服务暴露对应的门面类Manager供应用来调用,这样的话系统服务只跟Manager打交道就行了,不需要跟所有的应用来维持关系。

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