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android分析之Binder 02
分析Java层的ServiceManager，看看Binder在Java层是如何实现的。
public final class ServiceManager { private static final String TAG = "ServiceManager"; private static IServiceManager sServiceManager;//IserviceManager是一个接口，定义了通用（公共）方法。 private static HashMap<String, IBinder> sCache = new HashMap<String, IBinder>();//缓存，其值是IBinder... public static IBinder getService(String name) { try { IBinder service = sCache.get(name);//先从缓存中查找 if (service != null) { return service; } else { return getIServiceManager().getService(name);//生成新的IBinder } } catch (RemoteException e) { Log.e(TAG, "error in getService", e); } return null; }
...这里的ServiceManager仅仅是一种封装，其成员变量和方法都是static。从上面的sCache保存的键值对和getService的返回值类型为IBinder（与C++层的IBinder不同）可以看出，通过ServiceManager得到的是一个IBinder类型对象，通过后面的分析，实际可以看出它是BpBinder对象。
private static IServiceManager getIServiceManager() { if (sServiceManager != null) { return sServiceManager; } // Find the service manager sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject());//找到IServiceManager对象，并返回 return sServiceManager; }... /** * Return the global "context object" of the system. This is usually * an implementation of IServiceManager, which you can use to find * other services. */ public static final native IBinder getContextObject();//BinderInternale:调用native函数，返回IBinder对象... static public IServiceManager asInterface(IBinder obj)//传入的就是上面的那个IBinder对象，记住这是Java对象 { if (obj == null) { return null; } IServiceManager in = (IServiceManager)obj.queryLocalInterface(descriptor);//从IBinder对象里查找，并转换为IServiceManager对象 if (in != null) { return in; } return new ServiceManagerProxy(obj);//如果找不到，则使用传入的IBinder对象生成一个ServiceManagePorxy对象 }上面绕了一圈，直接从ServiceManagerProxy来看，有两方面：1，得到native层的一个某对象，并转换为IBinder，即上面的native IBinder getContextObject()；2，使用获得的IBinder来生成一个ServiceManagerProxy对象。下面来看看ServiceManagerProxy的构造方法：
public ServiceManagerProxy(IBinder remote) { mRemote = remote;//mRemote是IBinder类型，这里将传入的IBinder对象保存在mRemote } public IBinder getService(String name) throws RemoteException { Parcel data = http://www.mamicode.com/Parcel.obtain();>小结：调用ServiceManager的getService()，会生成一个ServiceManagerProxy对象，该对象持有一个mRemote(IBinder)，通过该mRemote（它是BpBinder在Java层的代表）可以向下层发送数据。
getContextObject分析：
public static final native IBinder getContextObject();//BinderInternale:调用native函数，返回IBinder对象——对应为下面这个函数：
static jobject android_os_BinderInternal_getContextObject(JNIEnv* env, jobject clazz){ sp<IBinder> b = ProcessState::self()->getContextObject(NULL);//走到这里：b为一个BpBinder return javaObjectForIBinder(env, b);//返回一个Java层的BinderProxy到这里明白了，上面的mRemote持有的就是BinderProxy对象。而上面调用mRemote对象的transact()方法就是调用BinderProxy的transact方法：
public native boolean transact(int code, Parcel data, Parcel reply, int flags) throws RemoteException;...static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj, jint code, jobject dataObj, jobject replyObj, jint flags) // throws RemoteException{ if (dataObj == NULL) { jniThrowNullPointerException(env, NULL); return JNI_FALSE; } Parcel* data = http://www.mamicode.com/parcelForJavaObject(env, dataObj);>...
status_t err = target->transact(code, *data, reply, flags);//调用BpBinder来与更下层通信...走到这里：Java层主要就是获取Native层的BpBinder，并使用它来与下层通信。
android分析之Binder 02