通过Android Binder拓展系统服务
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?如图所示，ServiceManager中有个getIServiceManager方法可以返回ServiceManager的Java运程接口，利用返回的对象就可以获得ServiceManager对系统服务管理的服务了。
private static IServiceManager getIServiceManager() { if (sServiceManager != null) { return sServiceManager; } // Find the service manager sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject()); return sServiceManager; }?跟踪代码可以看到最终要返回IServiceManager，需要往ServiceManagerProxy的构造函数中传入一个IBinder对象，这个对象是由BinderInternal.getContextObject()返回的。public static final native IBinder getContextObject();?而getContextObject是个native方法，通过JNI返回了一个IBinder对象。具体的C++层的代码这里暂时不做分析。这样获取ServiceManager的Java远程接口就结束了。?2.?系统服务HawkingService接口的定义HawkingService提供的服务就是say，如下：
interface IHawkingService{ void say(String sth);}?我利用AIDL生成了HawkingService相关的proxy，stub代码，如下：public interface IHawkingService extends IInterface {//Hawking Serverpublic static abstract class Stub extends Binder implements IHawkingService {private static final java.lang.String DESCRIPTOR = "com.test.IHawkingService";public Stub() {this.attachInterface(this, DESCRIPTOR);}/** * 如果是本地服务接口，将IBinder对象cast为IHawkingService并返回 * 如果不是本地服务(可能是远程服务)接口，返回IHawkingService的代理对象 */public static IHawkingService asInterface(IBinder obj) {if ((obj == null)) {return null;}IInterface iin = (IInterface) obj.queryLocalInterface(DESCRIPTOR);if (((iin != null) && (iin instanceof IHawkingService))) {return ((IHawkingService) iin);}return new IHawkingService.Stub.Proxy(obj);}public IBinder asBinder() {return this;}@Overridepublic boolean onTransact(int code, Parcel data,Parcel reply, int flags)throws RemoteException {switch (code) {case INTERFACE_TRANSACTION: {reply.writeString(DESCRIPTOR);return true;}case TRANSACTION_say: {data.enforceInterface(DESCRIPTOR);String _arg0 = data.readString();this.say(_arg0);reply.writeNoException();return true;}}return super.onTransact(code, data, reply, flags);}//Hawking Proxyprivate static class Proxy implements IHawkingService {private IBinder mRemote;Proxy(IBinder remote) {mRemote = remote;}public IBinder asBinder() {return mRemote;}public void say(String sth) throws RemoteException {Parcel _data = Parcel.obtain();Parcel _reply = Parcel.obtain();try {_data.writeInterfaceToken(DESCRIPTOR);_data.writeString(sth);mRemote.transact(Stub.TRANSACTION_say, _data, _reply, 0);_reply.readException();} finally {_reply.recycle();_data.recycle();}}}static final int TRANSACTION_say = (IBinder.FIRST_CALL_TRANSACTION + 0);}public void say(String sth) throws RemoteException;}?IHawkingService的服务端实现类时HawkingService，代码如下public class HawkingService extends IHawkingService.Stub{@Overridepublic void say(String sth) throws RemoteException {System.out.println("hawking say: "+sth);}}?3. HawkingService服务的启动过程开机时，系统服务的启动都是交给SystemServer进行处理的，SystemServer会启动一个ServerThread的线程用于启动系统服务并把启动的服务添加到ServiceManager中。public class SystemServer{ native public static void init1(String[] args); public static void main(String[] args) { ....... } public static final void init2() { Slog.i(TAG, "Entered the Android system server!"); Thread thr = new ServerThread(); thr.setName("android.server.ServerThread"); thr.start(); }} class ServerThread extends Thread { ...... @Override public void run() { ..... Looper.prepare(); ...... try { Slog.i(TAG, "HawkingService"); ServiceManager.addService("hawking", new HawkingService()); } catch (Throwable e) { Slog.e(TAG, "Failure starting Hawking Service", e); } ...... Looper.loop(); ...... } } ?如上述代码ServiceManager.addService("hawking", new HawkingService());，new HawkingService()是创建了一个Binder对象，下面我们就来看一下，如何创建一个Binder对象以及如何将这个进行add Service。创建Binder对象过程：public class Binder implements IBinder { ...... private int mObject; ...... public Binder() { init(); ...... } private native final void init(); ...... } ?new HawkingService()会调用Binder的构造函数，如上述代码，Binder的构造函数会调用一个native方法init，这个方法定义在frameworks/base/core/jni/android_util_Binder.cpp中。static void android_os_Binder_init(JNIEnv* env, jobject clazz) { JavaBBinderHolder* jbh = new JavaBBinderHolder(env, clazz); if (jbh == NULL) { jniThrowException(env, "java/lang/OutOfMemoryError", NULL); return; } LOGV("Java Binder %p: acquiring first ref on holder %p", clazz, jbh); jbh->incStrong(clazz); env->SetIntField(clazz, gBinderOffsets.mObject, (int)jbh); }?可以看出init做的事情就是创建一个JavaBBinderHolder对象，并将对象地址返回给mObject。ServiceManager.addService过程class ServiceManagerProxy implements IServiceManager { public ServiceManagerProxy(IBinder remote) { mRemote = remote; } ...... public void addService(String name, IBinder service) throws RemoteException { Parcel data = Parcel.obtain(); Parcel reply = Parcel.obtain(); data.writeInterfaceToken(IServiceManager.descriptor); data.writeString(name); data.writeStrongBinder(service); mRemote.transact(ADD_SERVICE_TRANSACTION, data, reply, 0); reply.recycle(); data.recycle(); } ...... private IBinder mRemote; } ?addService最终的实现在ServiceManagerProxy中。上述代码?data.writeStrongBinder(service) 是将Binder对象写入到Parcel包裹中，这个写入的对象service就是刚才生成的mObject地址转化过来的。具体的实现在writeStrongBinder的JNI实现中。?4.Client获取HawkingService的Java远程接口的过程下面编写一个客户端通过ServiceManager的getService方法去获取系统服务HawkingService。public class HawkingActivity extends Activity{ private IHawkingService hawkingService; @Override public void onCreate(Bundle savedInstanceState) { hawkingService= IHawkingService.Stub.asInterface( ServiceManager.getService("hawking")); hawkingService.say("Hello everyone"); } } ?ServiceManager.getService实际上是调用了ServiceManagerProxy.getService函数。class ServiceManagerProxy implements IServiceManager { public ServiceManagerProxy(IBinder remote) { mRemote = remote; } 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; } .......}?