Android中的IPC方式——Binder（二）

在开发过程中，实现进程间通信用的最多的就是 AIDL。

AIDL(Android Interface Definition Language)，也就是接口定义语言，提供接口给远程调用者。

当我们定义好 AIDL 文件，在编译时编译器会帮我们生成代码实现 IPC 通信。为了可以更好的理解Binder的过程，从AIDL入手。

服务端

先创建一个服务端，创建一个IStudentManager.aidl文件,声明2个方法getStudentList()以及addStudent()

IStudentManager.aidl

// IStudentManager.aidl
package com.golden.aidlserver;
import com.golden.aidlserver.Student;
// Declare any non-default types here with import statements

interface IStudentManager {
    List<Student> getStudentList();
    void addStudent(in Student student);
}

IStudentManager.java

Build一下工程，android studio会自动为我们生成一个java类：IStudentManager.java,大体结构如下。

package com.golden.aidlserver;
// Declare any non-default types here with import statements

public interface IStudentManager extends android.os.IInterface {
    public static abstract class Stub extends android.os.Binder 
            implements com.golden.aidlserver.IStudentManager {
        private static final java.lang.String DESCRIPTOR = "com.golden.aidlserver.IStudentManager";

        public Stub() {
            this.attachInterface(this, DESCRIPTOR);
        }
        public static com.golden.aidlserver.IStudentManager asInterface(android.os.IBinder 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 {...}

        private static class Proxy implements com.golden.aidlserver.IStudentManager {
            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<com.golden.aidlserver.Student> getStudentList() throws android.os.RemoteException {...}

            @Override
            public void addStudent(com.golden.aidlserver.Student student) throws android.os.RemoteException {...}

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

    public java.util.List<com.golden.aidlserver.Student> getStudentList() throws android.os.RemoteException;

    public void addStudent(com.golden.aidlserver.Student student) throws android.os.RemoteException;
}

该类首先包含了一个抽象内部类：Stub, 该类继承自Binder并实现了IStudentManager接口。在Stub的内部，又包含了一个静态内部类：Proxy，Proxy类同样实现了IStudentManager接口。

Sercvice

接下来创建一个StudentManagerService,需要实现刚刚我们定义的两个方法,并且在AndroidManifest注册。

public class StudentManagerService extends Service {
    private CopyOnWriteArrayList<Student>  mStudentList = new CopyOnWriteArrayList<>();
    @Override
    public void onCreate() {
        super.onCreate();
        mStudentList.add(1,"张三");
    }

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

    class MyBinder extends IStudentManager.Stub {
        @Override
        public List<Student> getStudentList() throws RemoteException {
            return mStudentList;
        }

        @Override
        public void addStudent(Student student) throws RemoteException {
            mStudentList.add(student);
        }
    }
}

 <service android:name=".StudentManagerService"
            android:process=":remote">
      <intent-filter>
          <!--是客户端用于访问AIDL服务的ID  -->
          <action android:name="com.golden.server.StudentManagerService" />
      </intent-filter>
</service>

客户端

为了逻辑上区分清晰，另外重新创建一个客户端的应用。需要将服务端的aidl以及Student.java拷贝到客户端注意与服务端的包名保持一致

public class ClientMainActivity extends AppCompatActivity {
    private static final String TAG = "AIDL Client";
    private IStudentManager mRemoteStudentManager;

    private int size = 1;
    private Button mAddBtn;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mAddBtn = findViewById(R.id.mButton);

        Intent intent = new Intent();
        intent.setComponent(new ComponentName("com.golden.aidlserver", "com.golden.aidlserver.StudentManagerService"));
        bindService(intent, mConnection, BIND_AUTO_CREATE);
        initView();
    }

    private void initView() {
        mAddBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                try {
                    if (mRemoteStudentManager != null) {
                        mRemoteStudentManager.addStudent(new Student(size+1,"李四"+String.valueOf(size+1)));
                        List<Student> studentList = mRemoteStudentManager.getStudentList();
                        size = studentList.size();
                        Log.e(TAG,studentList.toString());
                    }
                } catch (RemoteException e) {

                    e.printStackTrace();
                }
            }
        });
    }

    private ServiceConnection mConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            Log.e(TAG, "onServiceConnected");
            //获得IStudentManager对象
            mRemoteStudentManager = IStudentManager.Stub.asInterface(service);
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
            Log.e(TAG, "onServiceDisconnected");
            mRemoteStudentManager = null;
        }
    };
}

client中去bindservice在onServiceConnected()中得到服务端获得IStudentManager对象mRemoteStudentManager,可以通过mRemoteStudentManager来调用服务端service的方法。实现点击button之后服务端添加一个学生然后getStudentList()打印出来。

Stub

public static abstract class Stub extends android.os.Binder implements com.golden.aidlserver.IStudentManager {
    private static final java.lang.String DESCRIPTOR = "com.golden.aidlserver.IStudentManager";
    /**
     * Construct the stub at attach it to the interface.
     */
    public Stub() {
        this.attachInterface(this, DESCRIPTOR);
    }
    /**
     * Cast an IBinder object into an com.golden.aidlserver.IStudentManager interface,
     * generating a proxy if needed.
     */
    public static com.golden.aidlserver.IStudentManager asInterface(android.os.IBinder obj) {
        if ((obj == null)) {
            return null;
        }
        android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
        if (((iin != null) && (iin instanceof com.golden.aidlserver.IStudentManager))) {
            return ((com.golden.aidlserver.IStudentManager) iin);
        }
        return new com.golden.aidlserver.IStudentManager.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 {
        java.lang.String descriptor = DESCRIPTOR;
        switch (code) {
            case INTERFACE_TRANSACTION: {
                reply.writeString(descriptor);
                return true;
            }
            case TRANSACTION_getStudentList: {
                data.enforceInterface(descriptor);
                java.util.List<com.golden.aidlserver.Student> _result = this.getStudentList();
                reply.writeNoException();
                reply.writeTypedList(_result);
                return true;
            }
            case TRANSACTION_addStudent: {
                data.enforceInterface(descriptor);
                com.golden.aidlserver.Student _arg0;
                if ((0 != data.readInt())) {
                    _arg0 = com.golden.aidlserver.Student.CREATOR.createFromParcel(data);
                } else {
                    _arg0 = null;
                }
                this.addStudent(_arg0);
                reply.writeNoException();
                return true;
            }
            default: {
                return super.onTransact(code, data, reply, flags);
            }
        }
    static final int TRANSACTION_getStudentList = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
    static final int TRANSACTION_addStudent = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);
}

其中有一个DESCRIPTOR，它在Stub初始化的时候会绑定这个标识符，就是前面曾经提到的安全性方面。

client与server拥有一样的IStudentManager.java文件,在client端我们可以看到

//client端
private ServiceConnection mConnection = new ServiceConnection() {
	@Override
	public void onServiceConnected(ComponentName name, IBinder service) {
		Log.e(TAG, "onServiceConnected");
		//获得IStudentManager对象
		mRemoteStudentManager = IStudentManager.Stub.asInterface(service);
	}

	@Override
	public void onServiceDisconnected(ComponentName name) {
		Log.e(TAG, "onServiceDisconnected");
		mRemoteStudentManager = null;
	}
};

我们在 client端中bindservice中在onServiceConnected()会传给我们一个IBinder，这个 IBinder类型的参数是Binder驱动传给我们的，后面在framework的会说到。方法中会去调用 binder.queryLocalInterface() 去查找 Binder 本地对象，如果找到了就说明 Client 和 Server 在同一进程，那么这个 binder 本身就是 Binder 本地对象，可以直接使用。否则说明是 IBinder是个远程对象，也就是 BinderProxy。因此需要我们创建一个代理对象 Proxy，通过这个代理对象来是实现远程访问。

在server端中我们只是创建了stub的对象并且实现了其中定义的两个方法，等待调用onTransact()方法。

//server端    
class MyBinder extends IStudentManager.Stub {
        @Override
        public List<Student> getStudentList() throws RemoteException {
            return mStudentList;
        }

        @Override
        public void addStudent(Student student) throws RemoteException {
            mStudentList.add(student);
        }
}

Proxy

在client中当client 和server处于不同的进程的情况下，client使用的是Proxy对象mRemote。

private static class Proxy implements com.golden.aidlserver.IStudentManager {
    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<com.golden.aidlserver.Student> getStudentList() throws android.os.RemoteException {
        android.os.Parcel _data = android.os.Parcel.obtain();
        android.os.Parcel _reply = android.os.Parcel.obtain();
        java.util.List<com.golden.aidlserver.Student> _result;
        try {
            _data.writeInterfaceToken(DESCRIPTOR);
            mRemote.transact(Stub.TRANSACTION_getStudentList, _data, _reply, 0);
            _reply.readException();
            _result = _reply.createTypedArrayList(com.golden.aidlserver.Student.CREATOR);
        } finally {
            _reply.recycle();
            _data.recycle();
        }
        return _result;
    }

    @Override
    public void addStudent(com.golden.aidlserver.Student student) 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 ((student != null)) {
                _data.writeInt(1);
                student.writeToParcel(_data, 0);
            } else {
                _data.writeInt(0);
            }
            mRemote.transact(Stub.TRANSACTION_addStudent, _data, _reply, 0);
            _reply.readException();
        } finally {
            _reply.recycle();
            _data.recycle();
        }
    }
}

单独看下 client调用getStudentList()过程就是 proxy中调用。数据包_data需要将标识符DESCRIPTOR写入，远程调用mRemote.transact(Stub.TRANSACTION_getStudentList, _data, _reply, 0);（因性能考虑需要将方法以int值来标识），然后server端会调用到stub中的ontranscat()，最后client中的proxy得到结果。

@Override
public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, 
 int flags) throws android.os.RemoteException {
	java.lang.String descriptor = DESCRIPTOR;
	switch (code) {
        case INTERFACE_TRANSACTION: {
            reply.writeString(descriptor);
            return true;
        }
        case TRANSACTION_getStudentList: {
            data.enforceInterface(descriptor);
            java.util.List<com.golden.aidlserver.Student> _result = this.getStudentList();
            reply.writeNoException();
            reply.writeTypedList(_result);
            return true;
        }
        case TRANSACTION_addStudent: {
            data.enforceInterface(descriptor);
            com.golden.aidlserver.Student _arg0;
            if ((0 != data.readInt())) {
            _arg0 = com.golden.aidlserver.Student.CREATOR.createFromParcel(data);
            } else {
            _arg0 = null;
            }
            this.addStudent(_arg0);
            reply.writeNoException();
            return true;
        }
        default: {
            return super.onTransact(code, data, reply, flags);
        }
	}
}

将_result 写入 _replay中返回给客户端。如果是在同一个进程中，就将直接调用stub中的getStudent()方法不会走transact()和onTransact()逻辑。

特别注意的是在mRemote.transact()之后会挂起等待服务端的reply，所以如果是在UI线程中调用并且是耗时操作需要特殊处理。

以上就是一个简单的进程通信的流程，在不同进程的前提下，在理解的时候可以把client的所有操作都是通过proxy来实现，server端就是stub的具体实现。

下一篇会讲framework层源码。