AIDL 结构

android中的进程，有的是有init.rc中启动，有的是通过ams调用fork来启动。ams当然是在init.rc中启动的。aidl用来定义进程之间的接口，由于android应用进程与他进程频繁通讯，所以有大量的aidl文件。在分析ams的aidl之前，先以一个简单的例子来看看其结构：

// IText.aidl
package com.example.aidl;

// Declare any non-default types here with import statements

interface IText {
	//仅定义一个方法
            int add(int a, int b);
}

编译时生成java文件

public interface IText extends android.os.IInterface
{
/** Local-side IPC implementation stub class. */
public static abstract class Stub extends android.os.Binder implements com.example.aidl.IText
{
private static final java.lang.String DESCRIPTOR = "com.example.aidl.IText";
/** Construct the stub at attach it to the interface. */
public Stub()
{
this.attachInterface(this, DESCRIPTOR);
}
/**
 * Cast an IBinder object into an com.example.aidl.IText interface,
 * generating a proxy if needed.
 */
public static com.example.aidl.IText asInterface(android.os.IBinder obj)
{
if ((obj==null)) {
return null;
}
android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
if (((iin!=null)&&(iin instanceof com.example.aidl.IText))) {
return ((com.example.aidl.IText)iin);
}
return new com.example.aidl.IText.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_add:
{
data.enforceInterface(descriptor);
int _arg0;
_arg0 = data.readInt();
int _arg1;
_arg1 = data.readInt();
int _result = this.add(_arg0, _arg1);
reply.writeNoException();
reply.writeInt(_result);
return true;
}
default:
{
return super.onTransact(code, data, reply, flags);
}
}
}
private static class Proxy implements com.example.aidl.IText
{
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 int add(int a, int b) throws android.os.RemoteException
{
android.os.Parcel _data = android.os.Parcel.obtain();
android.os.Parcel _reply = android.os.Parcel.obtain();
int _result;
try {
_data.writeInterfaceToken(DESCRIPTOR);
_data.writeInt(a);
_data.writeInt(b);
mRemote.transact(Stub.TRANSACTION_add, _data, _reply, 0);
_reply.readException();
_result = _reply.readInt();
}
finally {
_reply.recycle();
_data.recycle();
}
return _result;
}
}
static final int TRANSACTION_add = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
}
public int add(int a, int b) throws android.os.RemoteException;
}

编译后，aidl文件将生成一个接口，它本身也继续自IInterface，IInterface,仅有一个返回IBinder的方法.

/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

/**
 * Base class for Binder interfaces.  When defining a new interface,
 * you must derive it from IInterface.
 */
public interface IInterface
{
    /**
     * Retrieve the Binder object associated with this interface.
     * You must use this instead of a plain cast, so that proxy objects
     * can return the correct result.
     */
    public IBinder asBinder();
}

IText.aidl文件生成IText.java文件之后，会生成一个Stub抽象嵌套类，而Stub包含一个Proxy的静态内部类,Stub和Proxy均继承IText，Proxy实现了在aidl中定义的add方法，实现的方式是通过远程调用。