Android消息机制（基于源码解析)

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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：

  1. 首先加上锁机制会让UI访问的逻辑变得复杂；
  2. 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

  1. 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
  2. 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

  1. Message（消息）
  2. MessageQueue（消息队列）
  3. Looper（消息循环）
  4. 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

消息机制交互过程

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<Looper> sThreadLocal = new ThreadLocal<Looper>();
    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：

  1. prepare()其核心就是将looper对象定义为ThreadLocal
  2. 一个Thread只能有一个Looper对象
  3. 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) <var>uptimeMillis</var>.
 * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
 * 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/

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