使用AIDL来分析Binder的工作机制

最近一直在看Android开发艺术探索的IPC机制，做下笔记。

首先新建三个文件，Book.java，Book.aidl，IBookManager.aidl

然后Rebuild Project一下，我们可以看到系统为IBookManager.aidl生产的类，通过Ctrl+N查找IBookManager.java

package com.ffm.bindertest;

public interface IBookManager extends android.os.IInterface {
    /**
     * Local-side IPC implementation stub class.
     */
    public static abstract class Stub extends android.os.Binder implements com.ffm.bindertest.IBookManager {
        private static final java.lang.String DESCRIPTOR = "com.ffm.bindertest.IBookManager";

        /**
         * Construct the stub at attach it to the interface.
         */
        public Stub() {
            this.attachInterface(this, DESCRIPTOR);
        }

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

        @Override
        public android.os.IBinder asBinder() {
            return this;
        }

        @Override
        public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException {
            switch (code) {
                case INTERFACE_TRANSACTION: {
                    reply.writeString(DESCRIPTOR);
                    return true;
                }
                case TRANSACTION_getBookList: {
                    data.enforceInterface(DESCRIPTOR);
                    java.util.List _result = this.getBookList();
                    reply.writeNoException();
                    reply.writeTypedList(_result);
                    return true;
                }
                case TRANSACTION_addBook: {
                    data.enforceInterface(DESCRIPTOR);
                    com.ffm.bindertest.Book _arg0;
                    if ((0 != data.readInt())) {
                        _arg0 = com.ffm.bindertest.Book.CREATOR.createFromParcel(data);
                    } else {
                        _arg0 = null;
                    }
                    this.addBook(_arg0);
                    reply.writeNoException();
                    return true;
                }
            }
            return super.onTransact(code, data, reply, flags);
        }

        private static class Proxy implements com.ffm.bindertest.IBookManager {
            private android.os.IBinder mRemote;

            Proxy(android.os.IBinder remote) {
                mRemote = remote;
            }

            @Override
            public android.os.IBinder asBinder() {
                return mRemote;
            }

            public java.lang.String getInterfaceDescriptor() {
                return DESCRIPTOR;
            }

            @Override
            public java.util.List getBookList() throws android.os.RemoteException {
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                java.util.List _result;
                try {
                    _data.writeInterfaceToken(DESCRIPTOR);
                    mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0);
                    _reply.readException();
                    _result = _reply.createTypedArrayList(com.ffm.bindertest.Book.CREATOR);
                } finally {
                    _reply.recycle();
                    _data.recycle();
                }
                return _result;
            }

            @Override
            public void addBook(com.ffm.bindertest.Book book) throws android.os.RemoteException {
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                try {
                    _data.writeInterfaceToken(DESCRIPTOR);
                    if ((book != null)) {
                        _data.writeInt(1);
                        book.writeToParcel(_data, 0);
                    } else {
                        _data.writeInt(0);
                    }
                    mRemote.transact(Stub.TRANSACTION_addBook, _data, _reply, 0);
                    _reply.readException();
                } finally {
                    _reply.recycle();
                    _data.recycle();
                }
            }
        }

        static final int TRANSACTION_getBookList = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
        static final int TRANSACTION_addBook = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);
    }

    public java.util.List getBookList() throws android.os.RemoteException;

    public void addBook(com.ffm.bindertest.Book book) throws android.os.RemoteException;
}

DESCRIPTOR

Binder的唯一标识

asInterface(android.os.IBinder obj)

用于将服务端的Binder对象转换成客户端所需的AIDL借口类型的对象，如果客户端和服务端是同一进程，返回的是服务端的Stub对象本身；否则返回的是系统封装后的Stub.proxy对象。

asBinder

返回当前Binder对象。

onTransact

这个方法运行在服务端的Binder线程池中，当客户端发起跨进程请求时，远程请求会通过系统底层封装后交由此方法来处理。

Proxy中的getBookList和addBook方法都运行在客户端。

再看下客户端代码

public class MainActivity extends AppCompatActivity {


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Intent intent = new Intent(this, BookManagerService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
    }

    private ServiceConnection mConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {

            IBookManager manager = IBookManager.Stub.asInterface(service);
            try {
                manager.getBookList();
                manager.asBinder();
            } catch (RemoteException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {

        }
    };

    @Override
    protected void onDestroy() {
        unbindService(mConnection);
        super.onDestroy();
    }
}

bindService成功后在onServiceConnected方法中，可以拿到服务端的Binder对象。通过asInterface(android.os.IBinder obj)方法，可以将服务端的Binder对象转换成Stub.proxy对象，这个对象是AIDL接口类型的对象。通过这个对象调用getBookList()方法，此方法是运行在客户端的。然后看下IBookManager.Sutb.proxy中的getBookList()方法，mRemote调用transact方法，mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0);     这个时候客户端的线程挂起，然后会执行服务端的onTransact()方法。在onTransact()方法中，java.util.List _result = this.getBookList();是服务端进程的方法实现

public class BookManagerService extends Service {

    @Override
    public void onCreate() {
        super.onCreate();
        mBookList.add(new Book(1, "Android"));
        mBookList.add(new Book(2, "Ios"));
    }

    @Override
    public IBinder onBind(Intent intent) {

        return mBinder;
    }

    private CopyOnWriteArrayList mBookList = new CopyOnWriteArrayList();

    private Binder mBinder = new IBookManager.Stub() {
        @Override
        public List getBookList() throws RemoteException {
            return mBookList;
        }

        @Override
        public void addBook(Book book) throws RemoteException {
            mBookList.add(book);
        }
    };
}

拿到数据后将结果写在_reply中，回到Stub.Proxy的getBookList()方法中，客户端就能拿到数据了。