Android进阶-IPC机制

一.背景：

一个应用默认只在一个进程中运行，当两个进程需要进行通信时不能像平常一样传递数据，因为每个进程都有它自己的虚拟地址空间，两个进程虽然有相同的虚拟地址，但是这两个虚拟地址被映射到的实际的物理地址却不尽相同，这时就需要有进程间通信机制。在Android下，常见的IPC机制有：
1. Bundle
2. 文件共享
3. Messager
4. AIDL
5. ContentProvider
6. Socket
这里主要记录AIDL的一些知识。

二.AIDL的使用：

1. 创建接口
在/src/main/aidl/pacage name/下新建IBookManager.aidl

package com.dhn.learnaidl;

interface IBookManager {
List<String> getBookList();
void addBook(String book);
}

sync工程，
自动生成IBookManager.java：

package com.dhn.learnaidl;

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

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

        /**
         * Cast an IBinder object into an com.dhn.learnaidl.IBookManager interface,
         * generating a proxy if needed.
         */
        public static com.dhn.learnaidl.IBookManager asInterface(android.os.IBinder obj) {
            if ((obj == null)) {
                return null;
            }
            android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
            if (((iin != null) && (iin instanceof com.dhn.learnaidl.IBookManager))) {
                return ((com.dhn.learnaidl.IBookManager) iin);
            }
            return new com.dhn.learnaidl.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<java.lang.String> _result = this.getBookList();
                    reply.writeNoException();
                    reply.writeStringList(_result);
                    return true;
                }
                case TRANSACTION_addBook: {
                    data.enforceInterface(DESCRIPTOR);
                    java.lang.String _arg0;
                    _arg0 = data.readString();
                    this.addBook(_arg0);
                    reply.writeNoException();
                    return true;
                }
            }
            return super.onTransact(code, data, reply, flags);
        }

        private static class Proxy implements com.dhn.learnaidl.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<java.lang.String> getBookList() throws android.os.RemoteException {
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                java.util.List<java.lang.String> _result;
                try {
                    _data.writeInterfaceToken(DESCRIPTOR);
                    mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0);
                    _reply.readException();
                    _result = _reply.createStringArrayList();
                } finally {
                    _reply.recycle();
                    _data.recycle();
                }
                return _result;
            }

            @Override
            public void addBook(java.lang.String 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);
                    _data.writeString(book);
                    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<java.lang.String> getBookList() throws android.os.RemoteException;

    public void addBook(java.lang.String book) throws android.os.RemoteException;
}

将该类画成UML图：

2.完成服务端代码：

public class BookManagerService extends Service {

    private List<String> mBookList = new ArrayList<String>();
    private Binder mBinder = new IBookManager.Stub(){

        @Override
        public List<String> getBookList() throws RemoteException {
            return mBookList;
        }

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

    public BookManagerService() {
    }

    @Override
    public void onCreate() {
        super.onCreate();
        mBookList.add("book1");
        mBookList.add("book2");
        mBookList.add("book3");
    }

    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }
}

3.客户端代码：

public class MainActivity extends AppCompatActivity implements ServiceConnection{

    private final String TAG = "MAIN";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

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


    @Override
    public void onServiceConnected(ComponentName name, IBinder service) {
        //将IBookManaver.Stub对象转换为IBookManager.Stub.Proxy对象
        IBookManager bookManager = IBookManager.Stub.asInterface(service);

        try {
            //从Service所在的进程返回数据
            List<String> list = bookManager.getBookList();
            Log.e(TAG, "从Service进程返回的书单：" + list.toString());
            bookManager.addBook("book4");
            list = bookManager.getBookList();
            Log.e(TAG, "添加一本书后返回的书单：" + list.toString());


        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onServiceDisconnected(ComponentName name) {

    }
}

4.运行结果：

E/MAIN: 从Service进程返回的书单：[book1, book2, book3]
E/MAIN: 添加一本书后返回的书单：[book1, book2, book3, book4]

三.原理分析：

客户端调用BindService()，在onServiceConnected()中获得从服务端Service返回的IBookManager.Stub对象（客户端和服务端各有一个IBookManager.Stub对象），调用IBookManager.Stub.asInterface(service)，在不同进程的情况下该方法返回的是IBookManager.Stub.Proxy对象（即IBookManager.Stub的代理类）。然后使用该对象调用bookManager.getBookList()获取服务端提供的服务，从生成的代码中看出该方法调用mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0)方法，mRemote是IBookManager.Stub对象（由UML图，IBookManager.Stub继承Binder，重写了Binder.onTranscat()），该方法接着调用IBookManager.Stub.onTransect()方法，该方法挂起客户端进程，根据传入的方法代号在服务端的Binder线程池中选择运行指定的服务，这里是自己实现的getBookList()方法。运行结束后结果从mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0)返回，当前线程继续运行。这样就完成了一次进程间通信。