Android进程间通信之AIDL工作原理初探

我们新建了AIDL文件后，再build一下项目，就会生成编译后的AIDL，如：

// IMyAidlInterface.aidl
package com.wong.remoteservice;
// Declare any non-default types here with import statements
interface IMyAidlInterface {

    // 服务端数据
    String getString();
}

编译后得到的 IMyAidlInterface.java:

/*
 * This file is auto-generated.  DO NOT MODIFY.
 */
package com.wong.remoteservice;
// Declare any non-default types here with import statements

public interface IMyAidlInterface extends android.os.IInterface
{
  /** Default implementation for IMyAidlInterface. */
  public static class Default implements com.wong.remoteservice.IMyAidlInterface
  {
    // 服务端数据

    @Override public java.lang.String getString() throws android.os.RemoteException
    {
      return null;
    }
    @Override
    public android.os.IBinder asBinder() {
      return null;
    }
  }
  /** Local-side IPC implementation stub class. */
  public static abstract class Stub extends android.os.Binder implements com.wong.remoteservice.IMyAidlInterface
  {
    private static final java.lang.String DESCRIPTOR = "com.wong.remoteservice.IMyAidlInterface";
    /** Construct the stub at attach it to the interface. */
    public Stub()
    {
      this.attachInterface(this, DESCRIPTOR);
    }
    /**
     * Cast an IBinder object into an com.wong.remoteservice.IMyAidlInterface interface,
     * generating a proxy if needed.
     */
    public static com.wong.remoteservice.IMyAidlInterface asInterface(android.os.IBinder obj)
    {
      if ((obj==null)) {
        return null;
      }
      android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
      if (((iin!=null)&&(iin instanceof com.wong.remoteservice.IMyAidlInterface))) {
        return ((com.wong.remoteservice.IMyAidlInterface)iin);
      }
      return new com.wong.remoteservice.IMyAidlInterface.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_getString:
        {
          data.enforceInterface(descriptor);
          java.lang.String _result = this.getString();
          reply.writeNoException();
          reply.writeString(_result);
          return true;
        }
        default:
        {
          return super.onTransact(code, data, reply, flags);
        }
      }
    }
    private static class Proxy implements com.wong.remoteservice.IMyAidlInterface
    {
      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;
      }
      // 服务端数据

      @Override public java.lang.String getString() throws android.os.RemoteException
      {
        android.os.Parcel _data = android.os.Parcel.obtain();
        android.os.Parcel _reply = android.os.Parcel.obtain();
        java.lang.String _result;
        try {
          _data.writeInterfaceToken(DESCRIPTOR);
          boolean _status = mRemote.transact(Stub.TRANSACTION_getString, _data, _reply, 0);
          if (!_status && getDefaultImpl() != null) {
            return getDefaultImpl().getString();
          }
          _reply.readException();
          _result = _reply.readString();
        }
        finally {
          _reply.recycle();
          _data.recycle();
        }
        return _result;
      }
      public static com.wong.remoteservice.IMyAidlInterface sDefaultImpl;
    }
    static final int TRANSACTION_getString = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
    public static boolean setDefaultImpl(com.wong.remoteservice.IMyAidlInterface impl) {
      if (Stub.Proxy.sDefaultImpl == null && impl != null) {
        Stub.Proxy.sDefaultImpl = impl;
        return true;
      }
      return false;
    }
    public static com.wong.remoteservice.IMyAidlInterface getDefaultImpl() {
      return Stub.Proxy.sDefaultImpl;
    }
  }
  // 服务端数据

  public java.lang.String getString() throws android.os.RemoteException;
}

我们来仔细分析一下，这个编译后的AIDL。整体结构是这样的：

IMyAidlInterface继承了IInterface接口，IInterface接口：

IBinder是用于远程对象的基础接口。轻量级远程调用机制的核心部分，为提高在进程内和跨进程调用的性能而设计。 这个接口描述了与远程对象交互使用的抽象协议，不要直接实现这个接口，而是通过继承Binder来完成。在Android中，实现远程对象（服务）的调用，就是靠Binder来实现的。

（1）Default类，提供了默认实现。

（2）我们自己定义的接口getString，这个接口要在服务端实现，最终也是通过这个接口向外提供服务的。
（3）为了获得实现了getString接口的对象，我们现在要介绍最重要的Ｓtub类：

/** Local-side IPC implementation stub class. */
  public static abstract class Stub extends android.os.Binder implements com.wong.remoteservice.IMyAidlInterface
  {
    private static final java.lang.String DESCRIPTOR = "com.wong.remoteservice.IMyAidlInterface";
    /** Construct the stub at attach it to the interface. */
    public Stub()
    {
      this.attachInterface(this, DESCRIPTOR);
    }
    /**
     * Cast an IBinder object into an com.wong.remoteservice.IMyAidlInterface interface,
     * generating a proxy if needed.
     */
    public static com.wong.remoteservice.IMyAidlInterface asInterface(android.os.IBinder obj)
    {
      if ((obj==null)) {
        return null;
      }
      android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
      if (((iin!=null)&&(iin instanceof com.wong.remoteservice.IMyAidlInterface))) {
        return ((com.wong.remoteservice.IMyAidlInterface)iin);
      }
      return new com.wong.remoteservice.IMyAidlInterface.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_getString:
        {
          data.enforceInterface(descriptor);
          java.lang.String _result = this.getString();
          reply.writeNoException();
          reply.writeString(_result);
          return true;
        }
        default:
        {
          return super.onTransact(code, data, reply, flags);
        }
      }
    }
    private static class Proxy implements com.wong.remoteservice.IMyAidlInterface
    {
      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;
      }
      // 服务端数据

      @Override public java.lang.String getString() throws android.os.RemoteException
      {
        android.os.Parcel _data = android.os.Parcel.obtain();
        android.os.Parcel _reply = android.os.Parcel.obtain();
        java.lang.String _result;
        try {
          _data.writeInterfaceToken(DESCRIPTOR);
          boolean _status = mRemote.transact(Stub.TRANSACTION_getString, _data, _reply, 0);
          if (!_status && getDefaultImpl() != null) {
            return getDefaultImpl().getString();
          }
          _reply.readException();
          _result = _reply.readString();
        }
        finally {
          _reply.recycle();
          _data.recycle();
        }
        return _result;
      }
      public static com.wong.remoteservice.IMyAidlInterface sDefaultImpl;
    }
    static final int TRANSACTION_getString = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
    public static boolean setDefaultImpl(com.wong.remoteservice.IMyAidlInterface impl) {
      if (Stub.Proxy.sDefaultImpl == null && impl != null) {
        Stub.Proxy.sDefaultImpl = impl;
        return true;
      }
      return false;
    }
    public static com.wong.remoteservice.IMyAidlInterface getDefaultImpl() {
      return Stub.Proxy.sDefaultImpl;
    }
  }

Stub的结构如下：

• Stub类是一个抽象类实现了IMyAidlInterface接口，但Stub并不实现getString这个方法，留给真正要对外服务的子类来实现。
• Stub类继承了Binder，使得Stub具有访问远程对象的能力。
• asInterface接口：



• 在客户端获取服务端的数据是这样获得的：

private IMyAidlInterface myBind;
private ServiceConnection serviceConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            myBind = IMyAidlInterface.Stub.asInterface(service);
        }
        @Override
        public void onServiceDisconnected(ComponentName name) {

        }
    };

由调用方式IMyAidlInterface.Stub.asInterface(service)可知，Stub没有被初始化，因此执行了：

return new com.wong.remoteservice.IMyAidlInterface.Stub.Proxy(obj);

返回了一个代理Proxy对象，那么我们看看定义在Stub类里的内部类Proxy：

private static class Proxy implements com.wong.remoteservice.IMyAidlInterface
    {
      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;
      }
      // 服务端数据

      @Override public java.lang.String getString() throws android.os.RemoteException
      {
        android.os.Parcel _data = android.os.Parcel.obtain();
        android.os.Parcel _reply = android.os.Parcel.obtain();
        java.lang.String _result;
        try {
          _data.writeInterfaceToken(DESCRIPTOR);
          boolean _status = mRemote.transact(Stub.TRANSACTION_getString, _data, _reply, 0);
          if (!_status && getDefaultImpl() != null) {
            return getDefaultImpl().getString();
          }
          _reply.readException();
          _result = _reply.readString();
        }
        finally {
          _reply.recycle();
          _data.recycle();
        }
        return _result;
      }
      public static com.wong.remoteservice.IMyAidlInterface sDefaultImpl;
    }

从这个代理可知，当我们要调用getString方法获得服务器数据时，它就会通过Binder对象的transact，向服务端请求数据。通过这种方式来完成跨进程的通信。

boolean _status = mRemote.transact(Stub.TRANSACTION_getString, _data, _reply, 0);

以上就是关于AIDL的通信原理的一部分知识。

谢谢阅读