Android进程间通信深入浅出AIDL(二)

短命女 2022-12-29 02:17 194阅读 0赞

本篇文章为Android进程间通信 深入浅出AIDL系列的第二篇,建议配合[AIDL第一篇][AIDL]食用

## 3. AIDL原理 ##

### 3.1 AIDL是怎么工作的? ###

我们编写了aidl文件之后,啥也没干就自动拥有了跨进程通信的能力.这一切得归功于Android Studio根据aidl文件生成的`IPersonManager.java`文件(生成的这个文件通过双击Shift输入`IPersonManager.java`即可找到),它里面已经帮我们封装好了跨进程通信这块的逻辑(最终是通过Binder来完成的),所以这个`IPersonManager.java`文件最终会打包进apk里面,我们才得以方便地进行跨进程通信.

### 3.2 详细解读 ###

先来简单看下`IPersonManager.java`的大致结构,为了更方便阅读,我将`IPersonManager.aidl`文件中多余的方法全部删除,只剩下`List<Person> getPersonList();`和`void addPersonIn(in Person person);`:

/* * This file is auto-generated. DO NOT MODIFY. */
    package com.xfhy.allinone.ipc.aidl;
    
    public interface IPersonManager extends android.os.IInterface { 
        /** * Default implementation for IPersonManager. */
        public static class Default implements com.xfhy.allinone.ipc.aidl.IPersonManager { 
            @Override
            public java.util.List<com.xfhy.allinone.ipc.aidl.Person> getPersonList() throws android.os.RemoteException { 
                return null;
            }
    
            @Override
            public void addPersonIn(com.xfhy.allinone.ipc.aidl.Person person) throws android.os.RemoteException { 
            }
    
            @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.xfhy.allinone.ipc.aidl.IPersonManager { 
            private static final java.lang.String DESCRIPTOR = "com.xfhy.allinone.ipc.aidl.IPersonManager";
    
            /** * Construct the stub at attach it to the interface. */
            public Stub() { 
                this.attachInterface(this, DESCRIPTOR);
            }
    
            /** * Cast an IBinder object into an com.xfhy.allinone.ipc.aidl.IPersonManager interface, * generating a proxy if needed. */
            public static com.xfhy.allinone.ipc.aidl.IPersonManager asInterface(android.os.IBinder obj) { 
                if ((obj == null)) { 
                    return null;
                }
                android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
                if (((iin != null) && (iin instanceof com.xfhy.allinone.ipc.aidl.IPersonManager))) { 
                    return ((com.xfhy.allinone.ipc.aidl.IPersonManager) iin);
                }
                return new com.xfhy.allinone.ipc.aidl.IPersonManager.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_getPersonList: { 
                        data.enforceInterface(descriptor);
                        java.util.List<com.xfhy.allinone.ipc.aidl.Person> _result = this.getPersonList();
                        reply.writeNoException();
                        reply.writeTypedList(_result);
                        return true;
                    }
                    case TRANSACTION_addPersonIn: { 
                        data.enforceInterface(descriptor);
                        com.xfhy.allinone.ipc.aidl.Person _arg0;
                        if ((0 != data.readInt())) { 
                            _arg0 = com.xfhy.allinone.ipc.aidl.Person.CREATOR.createFromParcel(data);
                        } else { 
                            _arg0 = null;
                        }
                        this.addPersonIn(_arg0);
                        reply.writeNoException();
                        return true;
                    }
                    default: { 
                        return super.onTransact(code, data, reply, flags);
                    }
                }
            }
    
            private static class Proxy implements com.xfhy.allinone.ipc.aidl.IPersonManager { 
                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.xfhy.allinone.ipc.aidl.Person> getPersonList() throws android.os.RemoteException { 
                    android.os.Parcel _data = android.os.Parcel.obtain();
                    android.os.Parcel _reply = android.os.Parcel.obtain();
                    java.util.List<com.xfhy.allinone.ipc.aidl.Person> _result;
                    try { 
                        _data.writeInterfaceToken(DESCRIPTOR);
                        boolean _status = mRemote.transact(Stub.TRANSACTION_getPersonList, _data, _reply, 0);
                        if (!_status && getDefaultImpl() != null) { 
                            return getDefaultImpl().getPersonList();
                        }
                        _reply.readException();
                        _result = _reply.createTypedArrayList(com.xfhy.allinone.ipc.aidl.Person.CREATOR);
                    } finally { 
                        _reply.recycle();
                        _data.recycle();
                    }
                    return _result;
                }
    
                @Override
                public void addPersonIn(com.xfhy.allinone.ipc.aidl.Person person) 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 ((person != null)) { 
                            _data.writeInt(1);
                            person.writeToParcel(_data, 0);
                        } else { 
                            _data.writeInt(0);
                        }
                        boolean _status = mRemote.transact(Stub.TRANSACTION_addPersonIn, _data, _reply, 0);
                        if (!_status && getDefaultImpl() != null) { 
                            getDefaultImpl().addPersonIn(person);
                            return;
                        }
                        _reply.readException();
                    } finally { 
                        _reply.recycle();
                        _data.recycle();
                    }
                }
    
                public static com.xfhy.allinone.ipc.aidl.IPersonManager sDefaultImpl;
            }
    
            static final int TRANSACTION_getPersonList = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
            static final int TRANSACTION_addPersonIn = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);
    
            public static boolean setDefaultImpl(com.xfhy.allinone.ipc.aidl.IPersonManager impl) { 
                // Only one user of this interface can use this function
                // at a time. This is a heuristic to detect if two different
                // users in the same process use this function.
                if (Stub.Proxy.sDefaultImpl != null) { 
                    throw new IllegalStateException("setDefaultImpl() called twice");
                }
                if (impl != null) { 
                    Stub.Proxy.sDefaultImpl = impl;
                    return true;
                }
                return false;
            }
    
            public static com.xfhy.allinone.ipc.aidl.IPersonManager getDefaultImpl() { 
                return Stub.Proxy.sDefaultImpl;
            }
        }
    
        public java.util.List<com.xfhy.allinone.ipc.aidl.Person> getPersonList() throws android.os.RemoteException;
    
        public void addPersonIn(com.xfhy.allinone.ipc.aidl.Person person) throws android.os.RemoteException;
    }

这块代码看起来很长.咱依次来看,首先IPersonManager是一个接口,然后它继承自IInterface接口.IInterface接口是Binder接口的基类,要通过Binder传输的接口都必须继承自IInterface.它里面的方法就是我们在aidl文件中声明的2个方法.然后用2个整型的id用于标识在transact过程中客户端所请求的是哪个方法.IPersonManager.Stub继承自Binder并实现了IPersonManager接口.当客户端与服务端都位于同一个进程时,方法调用不会走跨进程的transact过程,而当两者位于不同进程时,方法调用需要走transact过程,这个逻辑是由Stub的内部代理Proxy完成的.

Default类就只是IPersonManager的默认实现,可以不用在意.

下面来单独分析一下Stub类

**IPersonManager.Stub**

public static abstract class Stub extends android.os.Binder implements com.xfhy.allinone.ipc.aidl.IPersonManager { 
        private static final java.lang.String DESCRIPTOR = "com.xfhy.allinone.ipc.aidl.IPersonManager";
    
        /** * Construct the stub at attach it to the interface. */
        public Stub() { 
            this.attachInterface(this, DESCRIPTOR);
        }
        
        /** * Cast an IBinder object into an com.xfhy.allinone.ipc.aidl.IPersonManager interface, * generating a proxy if needed. */
        public static com.xfhy.allinone.ipc.aidl.IPersonManager asInterface(android.os.IBinder obj) { 
            if ((obj == null)) { 
                return null;
            }
            android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
            if (((iin != null) && (iin instanceof com.xfhy.allinone.ipc.aidl.IPersonManager))) { 
                return ((com.xfhy.allinone.ipc.aidl.IPersonManager) iin);
            }
            return new com.xfhy.allinone.ipc.aidl.IPersonManager.Stub.Proxy(obj);
        }
    }

*  DESCRIPTOR是Binder的唯一标识,一般用当前Binder的类名表示.
 *  attachInterface()是将Binder对象转成客户端需要的AIDL接口类型对象.如果需要跨进程,则还需要封装一个Stub.Proxy对象再返回;如果不需要跨进程,那么直接将Service端的Stub直接返回就行.

接着看Stub剩余的方法:

@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_getPersonList: { 
                data.enforceInterface(descriptor);
                java.util.List<com.xfhy.allinone.ipc.aidl.Person> _result = this.getPersonList();
                reply.writeNoException();
                reply.writeTypedList(_result);
                return true;
            }
            case TRANSACTION_addPersonIn: { 
                data.enforceInterface(descriptor);
                //从data中序列化一个Person出来,然后调用addPersonIn()去添加这个Person
                com.xfhy.allinone.ipc.aidl.Person _arg0;
                if ((0 != data.readInt())) { 
                    _arg0 = com.xfhy.allinone.ipc.aidl.Person.CREATOR.createFromParcel(data);
                } else { 
                    _arg0 = null;
                }
                this.addPersonIn(_arg0);
                reply.writeNoException();
                return true;
            }
            default: { 
                return super.onTransact(code, data, reply, flags);
            }
        }
    }

*  asBinder()就是将当前的Binder对象返回
 *  onTransact()方法是运行在服务端的线程池中的.这里看起来就像是同一个进程里面的调用一样,但其实已经涉及到跨进程通信了.当客户端跨进程请求服务端时,远程请求会通过系统底层封装后交由此方法来处理.服务端通过code来确定客户端请求的目标方法是什么,然后从data中取出目标方法所需的参数,然后执行目标方法.当目标方法执行完毕之后,向reply中写入返回值.

**IPersonManager.Stub.Proxy**

private static class Proxy implements com.xfhy.allinone.ipc.aidl.IPersonManager { 
        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;
        }
    }

这里的mRemote是用于远程请求的Binder.如果需要跨进程,那么客户端就是通过这个Proxy代理来进行远程调用的.来看看调用方法具体是怎么实现的

static final int TRANSACTION_getPersonList = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
    static final int TRANSACTION_addPersonIn = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);
    
    @Override
    public java.util.List<com.xfhy.allinone.ipc.aidl.Person> getPersonList() throws android.os.RemoteException { 
        android.os.Parcel _data = android.os.Parcel.obtain();
        android.os.Parcel _reply = android.os.Parcel.obtain();
        java.util.List<com.xfhy.allinone.ipc.aidl.Person> _result;
        try { 
            _data.writeInterfaceToken(DESCRIPTOR);
            boolean _status = mRemote.transact(Stub.TRANSACTION_getPersonList, _data, _reply, 0);
            if (!_status && getDefaultImpl() != null) { 
                return getDefaultImpl().getPersonList();
            }
            _reply.readException();
            _result = _reply.createTypedArrayList(com.xfhy.allinone.ipc.aidl.Person.CREATOR);
        } finally { 
            _reply.recycle();
            _data.recycle();
        }
        return _result;
    }
    
    @Override
    public void addPersonIn(com.xfhy.allinone.ipc.aidl.Person person) 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 ((person != null)) { 
                _data.writeInt(1);
                person.writeToParcel(_data, 0);
            } else { 
                _data.writeInt(0);
            }
            boolean _status = mRemote.transact(Stub.TRANSACTION_addPersonIn, _data, _reply, 0);
            if (!_status && getDefaultImpl() != null) { 
                getDefaultImpl().addPersonIn(person);
                return;
            }
            _reply.readException();
        } finally { 
            _reply.recycle();
            _data.recycle();
        }
    }

*  这2个方法都是运行在客户端的,当客户端调用此方法时: 首先创建该方法所需要的输入型Parcel对象`_data`,输出型Parcel对象`_reply`和返回值(如果有).然后将客户端方法的入参写入`_data`里面,通过序列化的方式.接着调用transact方法发起RPC远程调用,当前线程会被挂起,然后服务端的onTransact方法会被调用,直到RPC过程返回后,当前线程才能继续执行.然后将方法的返回值写入到`_reply`中,也是通过序列化的方式,最后返回`_reply`中的数据.
 *  Parcel对象是用来进行客户端与服务端进行数据传输的,只能传输可序列化的数据

应用层面的原理基本就这些了,如果再往里层探究的话,就会涉及到Binder机制里面比较底层的东西了.这里暂时不做分析.

下面这张图刚好总结上面的流程(来自Android开发艺术探索):

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3hmaHlf_size_16_color_FFFFFF_t_70_pic_center]

因为上面的`IPersonManager.java`文件是自动生成的,所以是很有规律的.即使我们不用AIDL也完全可以自定义Binder,从而实现跨进程通信.

## 资料 ##

*  [文中代码仓库][Link 1]
 *  [Android 接口定义语言 (AIDL)][Android _ _AIDL]
 *  [Android中AIDL的工作原理][Android_AIDL]
 *  [你真的理解AIDL中的in，out，inout么？][AIDL_in_out_inout]
 *  [Android 深入浅出AIDL（一）][Android _AIDL]
 *  [RemoteCallbackList][]
 *  [Android：学习AIDL，这一篇文章就够了(下)][Android_AIDL 1]
 *  Android开发艺术探索

[AIDL]: https://xfhy666.blog.csdn.net/article/details/111399814
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3hmaHlf_size_16_color_FFFFFF_t_70_pic_center]: /images/20221120/7bb10124b6c34dbf92a8e0be574e4f5e.png
[Link 1]: https://github.com/xfhy/AllInOne/blob/master/app/src/main/java/com/xfhy/allinone/ipc/aidl/AidlActivity.kt
[Android _ _AIDL]: https://developer.android.com/guide/components/aidl?hl=zh-cn
[Android_AIDL]: https://www.jianshu.com/p/e0c583ea9289
[AIDL_in_out_inout]: https://www.jianshu.com/p/ddbb40c7a251
[Android _AIDL]: https://blog.csdn.net/qian520ao/article/details/78072250
[RemoteCallbackList]: https://developer.android.com/reference/android/os/RemoteCallbackList
[Android_AIDL 1]: https://www.jianshu.com/p/0cca211df63c