Android Binder机制 基础(一)
前言
Linus Benedict Torvalds : RTFSC – Read The Fucking Source Code
Dianne Hackborn 记住这家伙，这系统他做的。
Binder 简要
Binder：用来实现不用进程间通信。Binder属于一个驱动，工作在linux层，运行在kernel。服务端，客户端处在用户空间，binder驱动处在内核空间。
binder
Binder机制有两个重要的类：IBinder和Binder。通过分析这两个主要的类来浅析Binder的使用。
1、IBinder
Base interface for a remotable object, the core part of a lightweight remote procedure call mechanism designed for high performance when performing in-process and cross-process calls. This interface describes the abstract protocol for interacting with a remotable object. Do not implement this interface directly, instead extend from Binder.
翻译：
一个远程对象的基本接口，一个轻量级的远程程序的调用机制的核心部分专为高性能设计在执行程序和跨进程调用。该接口描述了与远程对象交互的抽象协议。注意：不要直接实现这个接口，而不是从“Binder”中扩展。
IBinder的核心API是transact transact()匹配在Binder类的onTransact Binder.onTransact()函数中。这方法允许你发送一个调用到一个IBinder对象和从IBinder对象中收到一个调用。这个API是同步的。
通过transact()传输的数据是一个Parcel，一个通用的缓存数据，同时也有一些元数据。在buffer中元数据使用来管理IBinder对象标记的，这样的引用可以在buffer穿越过程中得以保留。这种机制确保IBinder被写入Parcel发送到另一个进程，如果其他进程发送一个标记到同一个IBinder回到原来的过程，那么原始的过程中会接收到同一个IBinder对象回来。这些语义允许IBinder/Binder对象作为一个独特的身份（作为一个记号或作其他用途），可以在过程管理。
The system maintains a pool of transaction threads in each process that it runs in. These threads are used to dispatch all IPCs coming in from other processes.
The Binder system also supports recursion across processes.
1.1、transact()方法
- IBinder核心接口方法：
- 允许你可以分别发送一个消息到一个Binder对象和接收一个从Binder对象发出的消息。
/*
Perform a generic operation with the object.
@param code The action to perform. This should be a number between FIRST_CALL_TRANSACTION and LAST_CALL_TRANSACTION.
@param data Marshalled data to send to the target. Must not be null. If you are not sending any data, you must create an empty Parcel that is given here.
@param reply Marshalled data to be received from the target. May be null if you are not interested in the return value.
@param flags Additional operation flags. Either 0 for a normal RPC, or FLAG_ONEWAY for a one-way RPC.
*/
public boolean transact(int code, Parcel data, Parcel reply, int flags) throws RemoteException;
- 确认远程对象是否有效有三个方法：
- The transact(); 如果对象被销毁而你尝试去调用这个函数，它会抛出一个异常。
- The pingBinder(); 如果对象被销毁，它会返回false。
- The linkToDeath(); 用来注册一个DeathRecipient，当被销毁的时候将会监听到。
2、Binder
Base class for a remotable object, the core part of a lightweight remote procedure call mechanism defined by IBinder. This class is an implementation of IBinder that provides standard local implementation of such an object.
翻译：
基类可远程对象，由定义的IBinder一个轻量级的远程过程调用机制的核心组成部分。这个类是的IBinder的实现，提供了标准的地方实现这样一个目标的。
大多数开发者不应该直接实现这个类，作为代替应该使用AIDl工具去描述你想要的接口，产生合适的Binder子类。但是，你可以直接使用Binder来实现自定义的RPC协议或简单地实例化一个Binder的对象来直接使用作为一个记号可以共享的过程。
2.1、transact()方法
/**Default implementation rewinds the parcels and calls onTransact. On the remote side, transact calls into the binder to do the IPC.*/
public final boolean transact(int code, Parcel data, Parcel reply,
int flags) throws RemoteException {
if (false) Log.v("Binder", "Transact: " + code + " to " + this);
if (data != null) {
data.setDataPosition(0);
}
boolean r = onTransact(code, data, reply, flags);
if (reply != null) {
reply.setDataPosition(0);
}
return r;
}
2.2、onTransact()方法
/*Default implementation is a stub that returns false. You will want to override this to do the appropriate unmarshalling of transactions. If you want to call this, call transact().*/
protected boolean onTransact(int code, Parcel data, Parcel reply,
int flags) throws RemoteException {
if (code == INTERFACE_TRANSACTION) {
...
return true;
} else if (code == DUMP_TRANSACTION) {
...
return true;
}
return false;
}
2.3、execTransact()方法
// Entry point from android_util_Binder.cpp's onTransact
private boolean execTransact(int code, long dataObj, long replyObj,
int flags) {
Parcel data = Parcel.obtain(dataObj);
Parcel reply = Parcel.obtain(replyObj);
// theoretically, we should call transact, which will call onTransact,
// but all that does is rewind it, and we just got these from an IPC,
// so we'll just call it directly.
boolean res;
// Log any exceptions as warnings, don't silently suppress them.
// If the call was FLAG_ONEWAY then these exceptions disappear into the ether.
try {
res = onTransact(code, data, reply, flags);
} catch (RemoteException e) {
if ((flags & FLAG_ONEWAY) != 0) {
} else {
reply.setDataPosition(0);
reply.writeException(e);
}
res = true;
} catch (RuntimeException e) {
if ((flags & FLAG_ONEWAY) != 0) {
} else {
reply.setDataPosition(0);
reply.writeException(e);
}
res = true;
} catch (OutOfMemoryError e) {
// Unconditionally log this, since this is generally unrecoverable.
RuntimeException re = new RuntimeException("Out of memory", e);
reply.setDataPosition(0);
reply.writeException(re);
res = true;
}
checkParcel(this, code, reply, "Unreasonably large binder reply buffer");
reply.recycle();
data.recycle();
// Just in case -- we are done with the IPC, so there should be no more strict
// mode violations that have gathered for this thread. Either they have been
// parceled and are now in transport off to the caller, or we are returning back
// to the main transaction loop to wait for another incoming transaction. Either
// way, strict mode begone!
StrictMode.clearGatheredViolations();
return res;
}
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