Android 存储设备管理－－ IMountService （二）

这里我们主要是梳理下图中的架构，以IMountService为例。

在StorageManager的构造函数中，用到了IMountService

    /**     * Constructs a StorageManager object through which an application can     * can communicate with the systems mount service.     *      * @param tgtLooper The {@android.os.Looper} which events will be received on.     *      * Applications can get instance of this class by calling     * {@link android.content.Context#getSystemService(java.lang.String)} with an argument     * of {@link android.content.Context#STORAGE_SERVICE}.     *      * @hide     */    public StorageManager(Looper tgtLooper) throws RemoteException {        mMountService = IMountService.Stub.asInterface(ServiceManager.getService("mount"));        if (mMountService == null) {               Log.e(TAG, "Unable to connect to mount service! - is it running yet?");            return;        }        mTgtLooper = tgtLooper;                mBinderListener = new MountServiceBinderListener();        mMountService.registerListener(mBinderListener);    }

我们看一下IMountService.java中asInterface()的定义

        /**           * Cast an IBinder object into an IMountService interface, generating a         * proxy if needed.         */        public static IMountService asInterface(IBinder obj) {            if (obj == null) {                return null;            }                IInterface iin = obj.queryLocalInterface(DESCRIPTOR);            if (iin != null && iin instanceof IMountService) {                return (IMountService) iin;             }                return new IMountService.Stub.Proxy(obj);        }

它new了一个Proxy，很明显StorageManager的构造函数中得到的只是一个Proxy，也就是代理。这个可以看一下声明

/** * WARNING! Update IMountService.h and IMountService.cpp if you change this * file. In particular, the ordering of the methods below must match the * _TRANSACTION enum in IMountService.cpp * * @hide - Applications should use android.os.storage.StorageManager to access *       storage functions. */public interface IMountService extends IInterface {    /** Local-side IPC implementation stub class. */    public static abstract class Stub extends Binder implements IMountService {        private static class Proxy implements IMountService {            private final IBinder mRemote;            Proxy(IBinder remote) {                mRemote = remote;            }            public IBinder asBinder() {                return mRemote;            }            public String getInterfaceDescriptor() {                return DESCRIPTOR;            }            /**               * Registers an IMountServiceListener for receiving async             * notifications.             */            public void registerListener(IMountServiceListener listener) throws RemoteException {                Parcel _data = Parcel.obtain();                Parcel _reply = Parcel.obtain();                try {                    _data.writeInterfaceToken(DESCRIPTOR);                    _data.writeStrongBinder((listener != null ? listener.asBinder() : null));                    mRemote.transact(Stub.TRANSACTION_registerListener, _data, _reply, 0);                    _reply.readException();                } finally {                    _reply.recycle();                    _data.recycle();                }                }

从中可以看出，Proxy包含于Stub中，而Stub包含于IMountService接口类中。

图中的Proxy和Stub都在IMountService.java中实现。

Proxy中有的函数，在Stub中也有对应的函数，这是因为它们都implements IMountService。

我们以registerListener()为例，从上面的代码可以看出其在Proxy中的定义，它调用了mRemote.transact()，其实它是跟Stub的onTransact()相对应，我们看看它的定义

        /** Construct the stub at attach it to the interface. */        public Stub() {            attachInterface(this, DESCRIPTOR);        }            public IBinder asBinder() {            return this;        }            @Override        public boolean onTransact(int code, Parcel data, Parcel reply,                int flags) throws RemoteException {            switch (code) {                case INTERFACE_TRANSACTION: {                    reply.writeString(DESCRIPTOR);                    return true;                }                    case TRANSACTION_registerListener: {                    data.enforceInterface(DESCRIPTOR);                    IMountServiceListener listener;                    listener = IMountServiceListener.Stub.asInterface(data.readStrongBinder());                    registerListener(listener);                    reply.writeNoException();                    return true;                }

可以看到，在onTransact()中，也调到了registerListener()，但是Stub没有实现这个函数，这样就会调用它的实例中的实现。

而这个实例就是MountService。我们看一下MountService的声明

/** * MountService implements back-end services for platform storage * management. * @hide - Applications should use android.os.storage.StorageManager * to access the MountService. */class MountService extends IMountService.Stub        implements INativeDaemonConnectorCallbacks, Watchdog.Monitor {    private static final boolean LOCAL_LOGD = false;    private static final boolean DEBUG_UNMOUNT = true;    private static final boolean DEBUG_EVENTS = true;    private static final boolean DEBUG_OBB = false;

可以看出，MountService继承自IMountService.Stub，它才是上图中真正的Service。它在SystemServer.java中被调用，并实例化。

而IMountService.Stub就是图中的Stub。它应该会在MountService启动时被实例化。

IMountService.Stub.Proxy是图中的Proxy，它在StorageManager.java中被实例化。

StorageManager就是图中的Client。

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