Android消息机制（基于源码解析)

1. 消息机制概述

Android中的消息机制主要指的是Handler的运行机制，Handler的运行需要底层的MessageQueue和Looper、Message的支撑，下文会逐一分析。

2. 为什么需要消息机制

Android中的消息机制主要是为了满足线程间通信而设计的，最重要的应用场景应该在于更新UI

Android规定访问UI只能在主线程中进行，如果在子线程中访问UI，那么程序就会抛出异常

系统为什么不允许在自线程中访问UI呢？这是因为Android的UI控件不是线程安全的，如果在多线程中并发访问可能会导致UI控件处于不可预期的状态。

那为什么不对UI控件的访问加上锁机制呢？缺点有两个：

首先加上锁机制会让UI访问的逻辑变得复杂；
其次锁机制会降低访问UI的效率，因为锁机制会阻塞某些现成的执行

鉴于这两个缺点，最简单且最高效的方法就是采用单线程模型来处理UI操作，对于开发者来说也不是很麻烦，只是需要通过Handler切换下UI的访问执行线程即可

3. Android中线程的分类

带有消息队列，用来执行循环型任务（例如主线程ActivityThread，Android.os.HandlerThread）有消息时就处理，没有消息时就睡眠
没有消息队列，用来执行一次性任务（例如java.lang.Thread）
任务一旦执行完成便退出

4. 带有消息队列的线程具体实现

主要涉及4个方面

Message（消息）
MessageQueue（消息队列）
Looper（消息循环）
Handler （消息发送和处理）

4.1 消息队列

说到MessageQueue，我们来看下它是干什么的

/**
* Low-level class holding the list of messages to be dispatched by a
* {@link Looper}. Messages are not added directly to a MessageQueue,
* but rather through {@link Handler} objects associated with the Looper.
*
*You can retrieve the MessageQueue for the current thread with
* {@link Looper#myQueue() Looper.myQueue()}.
*/

它是一个低等级的持有Messages集合的类，被Looper分发。Messages并不是直接加到MessageQueue的，而是通过Handler对象和Looper关联到一起。我们可以通过Looper.myQueue()方法来检索当前线程的MessageQueue。

4.2 Message

在整个消息处理机制中，message又叫task，封装了任务携带的信息和处理该任务的handler。我们看下这个类的注释

/**
*
* Defines a message containing a description and arbitrary data object that can be
* sent to a {@link Handler}. This object contains two extra int fields and an
* extra object field that allow you to not do allocations in many cases.
*
* While the constructor of Message is public, the best way to get
* one of these is to call {@link #obtain Message.obtain()} or one of the
* {@link Handler#obtainMessage Handler.obtainMessage()} methods, which will pull
* them from a pool of recycled objects.
*/

这个类定义了一个包含描述和一个任意类型对象的对象，它可以被发送给Handler。
从注释里我们还可以了解到以下几点：

尽管Message有public的默认构造方法，但是你应该通过Message.obtain()来从消息池中获得空消息对象，以节省资源。
如果你的message只需要携带简单的int信息，请优先使用Message.arg1和Message.arg2来传递信息，这比用Bundle更省内存
用message.what来标识信息，以便用不同方式处理message

4.3 Looper

/*
* This class contains the code required to set up and manage an event loop
* based on MessageQueue. APIs that affect the state of the queue should be
* defined on MessageQueue or Handler rather than on Looper itself. For example,
* idle handlers and sync barriers are defined on the queue whereas preparing the
* thread, looping, and quitting are defined on the looper.
*/

这个类是基于消息队列用来设置和管理事件循环的代码，对队列的改变应该在MessageQueue或Handler上定义，而不是在Looper本身定义，例如空间处理和同步障碍应该在队列上定义，然而准备线程、循环和退出在Looper本身定义，

4.4 Handler

/**
* A Handler allows you to send and process {@link Message} and Runnable
* objects associated with a thread’s {@link MessageQueue}. Each Handler
* instance is associated with a single thread and that thread’s message
* queue. When you create a new Handler, it is bound to the thread /
* message queue of the thread that is creating it – from that point on,
* it will deliver messages and runnables to that message queue and execute
* them as they come out of the message queue.
*
*

There are two main uses for a Handler: (1) to schedule messages and
* runnables to be executed as some point in the future; and (2) to enqueue
* an action to be performed on a different thread than your own.
*
*

Scheduling messages is accomplished with the
* {@link #post}, {@link #postAtTime(Runnable, long)},
* {@link #postDelayed}, {@link #sendEmptyMessage},
* {@link #sendMessage}, {@link #sendMessageAtTime}, and
* {@link #sendMessageDelayed} methods. The post versions allow
* you to enqueue Runnable objects to be called by the message queue when
* they are received; the sendMessage versions allow you to enqueue
* a {@link Message} object containing a bundle of data that will be
* processed by the Handler’s {@link #handleMessage} method (requiring that
* you implement a subclass of Handler).
*
*

When posting or sending to a Handler, you can either
* allow the item to be processed as soon as the message queue is ready
* to do so, or specify a delay before it gets processed or absolute time for
* it to be processed. The latter two allow you to implement timeouts,
* ticks, and other timing-based behavior.
*
*

When a
* process is created for your application, its main thread is dedicated to
* running a message queue that takes care of managing the top-level
* application objects (activities, broadcast receivers, etc) and any windows
* they create. You can create your own threads, and communicate back with
* the main application thread through a Handler. This is done by calling
* the same post or sendMessage methods as before, but from
* your new thread. The given Runnable or Message will then be scheduled
* in the Handler’s message queue and processed when appropriate.
*/

这个有点长，简单概括下：每一个Handler实例关联了一个单一的ghread和这个thread的messagequeue，当Handler的实例被创建的时候它就被绑定到了创建它的thread。它用来调度message和runnables在未来某个时间点的执行，还可以排列其他线程里执行的操作。

Handler主要用的来管理某个线程(也可能是进程)的消息队列。
比如处理主线程的消息队列，包含消息的发送和接收过程。
消息的发送可以通过Post的一系列方法和Sende的一系列方法来实现。
而post的一系列方法最终是通过send的一系列方法来实现的。这样就可以将一些耗时任务放到其他线程之中，待任务完成之后就往主线程的消息队列中添加一个消息，这样Handler的Callback，即handleMessage就会被调用。但是Handler并不是线程安全的，因此建议将Handler作为一个静态内部类。
所以Handler只是处理消息，耗时任务放在其他线程。

4.5 四元素的交互过程

具体工作过程

消息队列的创建

消息循环

消息的发送最基本的两个API

带一个Runnable参数，会被转换为一个Message参数

带一个Message参数，用来描述消息的内容

Handler.sendMessage

Handler.post

消息的处理

基于消息的异步任务接口

android.os.HandlerThread

适合用来处于不需要更新UI的后台任务
android.os.AyncTask

适合用来处于需要更新UI的后台任务

4.5.1 Looper与消息队列的创建

首先确认当前线程是否具有Looper（主线程默认具有Looper）如果没有则创建
我们知道Android上一个应用的入口，应该是ActivityThread。和普通的Java类一样，入口是一个main方法。创建主线程源码示例：

public static void main(String[] args) {    //~省略部分无关代码~    //创建Looper和MessageQueue对象，用于处理主线程的消息    Looper.prepareMainLooper();    //创建ActivityThread对象    ActivityThread thread = new ActivityThread();    //建立Binder通道 (创建新线程)    thread.attach(false);    if (sMainThreadHandler == null) {        sMainThreadHandler = thread.getHandler();    }    if (false) {        Looper.myLooper().setMessageLogging(new                LogPrinter(Log.DEBUG, "ActivityThread"));    }    // End of event ActivityThreadMain.    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);    //消息循环运行    Looper.loop();    throw new RuntimeException("Main thread loop unexpectedly exited");}

我们可以看到主方法首先通过Looper.prepareMainLooper()初始化了我们主线程（UI）的Looper并且启动它。然后就可以处理子线程和其他组件发来的消息了

如果不是主线程的两线程进行通信，可以通过以下方式来创建

class LooperThread extends Thread {    public Handler mHandler;    public void run() {        //将当前线程初始化为Looper线程        Looper.prepare();        // ...其他处理，如实例化handler        mHandler = new Handler() {            public void handleMessage(Message msg) {                // process incoming messages here            }        };        // 开始循环处理消息队列        Looper.loop();    }}

Looper源码如下：

public final class Looper {    private static final String TAG = "Looper";    // sThreadLocal.get() will return null unless you've called prepare().    static final ThreadLocal sThreadLocal = new ThreadLocal();    private static Looper sMainLooper;  // guarded by Looper.class    //Looper内的消息队列    final MessageQueue mQueue;    // 当前线程    final Thread mThread;    private Printer mLogging;    private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed);        mThread = Thread.currentThread();    }     /** Initialize the current thread as a looper.      * This gives you a chance to create handlers that then reference      * this looper, before actually starting the loop. Be sure to call      * {@link #loop()} after calling this method, and end it by calling      * {@link #quit()}.      */    public static void prepare() {        prepare(true);    }    private static void prepare(boolean quitAllowed) {        //试图在有Looper的线程中再次创建Looper将抛出异常        if (sThreadLocal.get() != null) {            throw new RuntimeException("Only one Looper may be created per thread");        }        sThreadLocal.set(new Looper(quitAllowed));    }    /**     * Initialize the current thread as a looper, marking it as an     * application's main looper. The main looper for your application     * is created by the Android environment, so you should never need     * to call this function yourself.  See also: {@link #prepare()}     */    public static void prepareMainLooper() {        prepare(false);        synchronized (Looper.class) {            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }            sMainLooper = myLooper();        }    }    //~省略部分无关代码~}

从中我们可以看到以下几点：

prepare()其核心就是将looper对象定义为ThreadLocal
一个Thread只能有一个Looper对象
prepare()方法会调用Looper的构造方法，初始化一个消息队列，并且指定当前线程

ThreadLocal并不是一个Thread，而是Thread的局部变量。
当使用ThreadLocal维护变量时，ThreadLocal为每个使用该变量的线程提供独立的变量副本。
所以每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本。
从线程的角度看，目标变量就象是线程的本地变量，这也是类名中“Local”所要表达的意思。

在调用Looper.loop()方法之前，确保已经调用了prepare(boolean quitAllowed)方法，并且我们可以调用quite方法结束循环

接下来再看看Looper.loop()

/** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */public static void loop() {    //得到当前线程Looper    final Looper me = myLooper();    if (me == null) {        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");    }    //得到当前looper的MessageQueue    final MessageQueue queue = me.mQueue;    // Make sure the identity of this thread is that of the local process,    // and keep track of what that identity token actually is.    Binder.clearCallingIdentity();    final long ident = Binder.clearCallingIdentity();    //开始循环    for (;;) {        Message msg = queue.next(); // might block        if (msg == null) {            // No message indicates that the message queue is quitting.            //没有消息表示消息队列正在退出            return;        }        // This must be in a local variable, in case a UI event sets the logger        Printer logging = me.mLogging;        if (logging != null) {            logging.println(">>>>> Dispatching to " + msg.target + " " +                    msg.callback + ": " + msg.what);        }        //将真正的处理工作交给message的target，即handler        msg.target.dispatchMessage(msg); if (logging != null) { logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } //回收消息资源 msg.recycleUnchecked(); }}

通过这段代码可知，调用loop方法后，Looper线程就开始真正工作了，它不断从自己的MessageQueue中取出队头的消息(或者说是任务)执行。

除了prepare()和loop()方法，Looper类还有一些比较有用的方法，比如

Looper.myLooper()得到当前线程looper对象
getThread()得到looper对象所属线程
quit()方法结束looper循环这里需要注意的一点是，quit（）方法其实调用的是MessageWueue的quite（boolean safe）方法。

void quit(boolean safe) {    if (!mQuitAllowed) {        throw new IllegalStateException("Main thread not allowed to quit.");    }    synchronized (this) {        if (mQuitting) {            return;        }        mQuitting = true;        if (safe) {            removeAllFutureMessagesLocked();        } else {            removeAllMessagesLocked();        }        // We can assume mPtr != 0 because mQuitting was previously false.        nativeWake(mPtr);    }}

我们看到其实主线程是不能调用这个方法退出消息队列的。至于mQuitAllowed参数是在Looper初始化的时候初始化的，主线程初始化调用的是Looper.prepareMainLooper()方法，这个方法把参数设置为false。

4.5.2 消息的发送和处理（Handler）

从MessageQueue的注释中，我们知道添加消息到消息队列是通过Handler来操作的。我们通过源码来看下具体是怎么实现的

public class Handler {    //~省略部分无关代码~    final MessageQueue mQueue;    final Looper mLooper;    public Handler() {        this(null, false);    }    public Handler(Looper looper) {        this(looper, null, false);    }    public Handler(boolean async) {        this(null, async);    }    public Handler(Callback callback, boolean async) {        if (FIND_POTENTIAL_LEAKS) {            final Class<? extends Handler> klass = getClass();            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&                    (klass.getModifiers() & Modifier.STATIC) == 0) {                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +                    klass.getCanonicalName());            }        }        mLooper = Looper.myLooper();        if (mLooper == null) {            throw new RuntimeException(                "Can't create handler inside thread that has not called Looper.prepare()");        }        mQueue = mLooper.mQueue;        mCallback = callback;        mAsynchronous = async;    }    public Handler(Looper looper, Callback callback, boolean async) {        mLooper = looper;        mQueue = looper.mQueue;        mCallback = callback;        mAsynchronous = async;    }    //~省略部分无关代码~}

先看构造方法，其实里边的重点是初始化了两个变量，把关联looper的MessageQueue作为自己的MessageQueue，因此它的消息将发送到关联looper的MessageQueue上。

有了handler之后，我们就可以使用Handler提供的post和send系列方法向MessageQueue上发送消息了。其实post发出的Runnable对象最后都被封装成message对象

接下来我们看一下handler是如何发送消息的

/** * Causes the Runnable r to be added to the message queue. * The runnable will be run on the thread to which this handler is  * attached.  *   * @param r The Runnable that will be executed. *  * @return Returns true if the Runnable was successfully placed in to the  *         message queue.  Returns false on failure, usually because the *         looper processing the message queue is exiting. */public final boolean post(Runnable r){   return  sendMessageDelayed(getPostMessage(r), 0);}/** * Enqueue a message into the message queue after all pending messages * before (current time + delayMillis). You will receive it in * {@link #handleMessage}, in the thread attached to this handler. *   * @return Returns true if the message was successfully placed in to the  *         message queue.  Returns false on failure, usually because the *         looper processing the message queue is exiting.  Note that a *         result of true does not mean the message will be processed -- if *         the looper is quit before the delivery time of the message *         occurs then the message will be dropped. */public final boolean sendMessageDelayed(Message msg, long delayMillis){    if (delayMillis < 0) {        delayMillis = 0;    }    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}/** * Enqueue a message into the message queue after all pending messages * before the absolute time (in milliseconds) uptimeMillis. * The time-base is {@link android.os.SystemClock#uptimeMillis}. * Time spent in deep sleep will add an additional delay to execution. * You will receive it in {@link #handleMessage}, in the thread attached * to this handler. *  * @param uptimeMillis The absolute time at which the message should be *         delivered, using the *         {@link android.os.SystemClock#uptimeMillis} time-base. *          * @return Returns true if the message was successfully placed in to the  *         message queue.  Returns false on failure, usually because the *         looper processing the message queue is exiting.  Note that a *         result of true does not mean the message will be processed -- if *         the looper is quit before the delivery time of the message *         occurs then the message will be dropped. */public boolean sendMessageAtTime(Message msg, long uptimeMillis) {    MessageQueue queue = mQueue;    if (queue == null) {        RuntimeException e = new RuntimeException(                this + " sendMessageAtTime() called with no mQueue");        Log.w("Looper", e.getMessage(), e);        return false;    }    return enqueueMessage(queue, msg, uptimeMillis);}private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {    msg.target = this;    if (mAsynchronous) {        msg.setAsynchronous(true);    }    return queue.enqueueMessage(msg, uptimeMillis);}

这里我们只列出了一种调用关系，其他调用关系大同小异，我们来分析一下

调用getPostMessage(r)，把runnable对象添加到一个Message对象中。
sendMessageDelayed(getPostMessage(r), 0)，基本没做什么操作，又继续调用sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis)方法，在这个方法里拿到创建这个Handler对象的线程持有的MessageQueue。
调用enqueueMessage(queue, msg, uptimeMillis)方法，给msg对象的target变量赋值为当前的Handler对象，然后放入到MessageQueue。

那发送消息说完了，那我们的消息是怎样被处理的呢？

我们看到message.target为该handler对象，这确保了looper执行到该message时能找到处理它的handler，即loop()方法中的关键代码。

/** * Callback interface you can use when instantiating a Handler to avoid * having to implement your own subclass of Handler. * * @param msg A {@link android.os.Message Message} object * @return True if no further handling is desired */public interface Callback {    public boolean handleMessage(Message msg);}/** * Subclasses must implement this to receive messages. */public void handleMessage(Message msg) {}/** * Handle system messages here. */public void dispatchMessage(Message msg) {    if (msg.callback != null) {        handleCallback(msg);    } else {        if (mCallback != null) {            if (mCallback.handleMessage(msg)) {                return;            }        }        handleMessage(msg);    }}private static void handleCallback(Message message) {    message.callback.run();}

我们看到这里最终又调用到了我们重写的handleMessage(Message msg)方法来做处理子线程发来的消息或者调用handleCallback(Message message)去执行我们子线程中定义并传过来的操作

为什么主线程不会因为Looper.loop()里的死循环卡死或者不能处理其他事务

这里涉及到的东西比较多，概括的理解是这样的

handler机制是使用pipe来实现的，主线程没有消息处理时会阻塞在管道的读端。
binder线程会往主线程消息队列里添加消息，然后往管道写端写一个字节，这样就能唤醒主线程从管道读端返回，也就是说queue.next()会调用返回。

主线程大多数时候都是处于休眠状态，并不会消耗大量CPU资源。

既然是死循环又如何去处理其他事务呢？答案是通过创建新线程的方式。
我们看到main方法里调用了thread.attach(false)，这里便会创建一个Binder线程（具体是指ApplicationThread，Binder的服务端，用于接收系统服务AMS发送来的事件），该Binder线程通过Handler将Message发送给主线程。

参考资料：

http://android.jobbole.com/84957/