Android進程間通信之AIDL工作原理初探

我們新建了AIDL文件後，再build一下項目，就會生成編譯後的AIDL，如：

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

    // 服務端數據
    String getString();
}

編譯後得到的 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
  {
    // 服務端數據

    @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;
      }
      // 服務端數據

      @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;
    }
  }
  // 服務端數據

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

我們來仔細分析一下，這個編譯後的AIDL。整體結構是這樣的：

IMyAidlInterface繼承了IInterface接口，IInterface接口：

IBinder是用於遠程對象的基礎接口。輕量級遠程調用機制的核心部分，爲提高在進程內和跨進程調用的性能而設計。這個接口描述了與遠程對象交互使用的抽象協議，不要直接實現這個接口，而是通過繼承Binder來完成。在Android中，實現遠程對象（服務）的調用，就是靠Binder來實現的。
（1）Default類，提供了默認實現。

（2）我們自己定義的接口getString，這個接口要在服務端實現，最終也是通過這個接口向外提供服務的。
（3）爲了獲得實現了getString接口的對象，我們現在要介紹最重要的Ｓtub類：

 /** 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;
      }
      // 服務端數據

      @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的結構如下：

Stub類是一個抽象類實現了IMyAidlInterface接口，但Stub並不實現getString這個方法，留給真正要對外服務的子類來實現。
Stub類繼承了Binder，使得Stub具有訪問遠程對象的能力。
asInterface接口：

在客戶端獲取服務端的數據是這樣獲得的：

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) {

        }
    };

由調用方式IMyAidlInterface.Stub.asInterface(service)可知，Stub沒有被初始化，因此執行了：

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

返回了一個代理Proxy對象，那麼我們看看定義在Stub類裏的內部類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;
      }
      // 服務端數據

      @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;
    }

從這個代理可知，當我們要調用getString方法獲得服務器數據時，它就會通過Binder對象的transact，向服務端請求數據。通過這種方式來完成跨進程的通信。

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

以上就是關於AIDL的通信原理的一部分知識。

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