Android Handler 消息机制(解惑篇)

Android Handler 消息机制(解惑篇)

Android中的消息处理机制概述

大家对于Android中的消息处理机制的用法一定都比较熟悉,至于工作原理估计不少人有研究。就像我们自己写的类我们用起来比较熟悉一样,如果我们熟悉了消息处理机制的具体实现,那么我们用起来肯定也会事半功倍。

博主之前只是稍有涉猎,对其中一些地方也还心存疑虑,比如既然Looper.loop()里是一个死循环,那它会不会很消耗CPU呢?死循环阻塞了线程,那我们其他的事务是如何被处理的呢?Android的UI线程是在哪里被初始化的呢?等等。索性今天就把他们放到一起,说道说道。

Android中线程的分类

  • 带有消息队列,用来执行循环性任务(例如主线程、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

带有消息队列线程概述

四要素

  • Message(消息)

  • MessageQueue(消息队列)

  • Looper(消息循环)

  • Handler(消息发送和处理)

四要素的交互过程

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

    • Handler.sendMessage

      • 带一个Message参数,用来描述消息的内容
    • Handler.post

      • 带一个Runnable参数,会被转换为一个Message参数
  • 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

带有消息队列线程的具体实现

ThreadLocal

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。

Looper

用于在指定线程中运行一个消息循环,一旦有新任务则执行,执行完继续等待下一个任务,即变成Looper线程。Looper类的注释里有这样一个例子:

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();
    }
}

其实核心代码就两行,我们先来看下Looper.prepare()方法的具体实现

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:

  • 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
  • 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

说到初始化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。

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。

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。

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

/**

  • 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.
  • */

注释比较简单,这里就不过多翻译了,主要内容是: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。

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

  1. 调用getPostMessage(r),把runnable对象添加到一个Message对象中。
  2. 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。
  3. 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。

思考

为什么要有Handler机制

这个问题可以这么考虑

  1. 我们如何在子线程更新UI?——使用Handler机制传递消息到主线程(UI线程)
  2. 为什么我们不在子线程更新UI呢?——因为Android是单线程模型
  3. 为什么要做成单线程模型呢?——多线程并发访问UI可能会导致UI控件处于不可预期的状态。如果加锁,虽然能解决,但是缺点也很明显:1.锁机制让UI访问逻辑变得复杂;2.加锁导致效率低下。

Handler机制与命令模式

我在之前分享过Android源码中的命令模式,我们仔细分下一下不难看出Handler机制其实是一个非典型的命令模式

  • 接收者:Handler,执行消息处理操作。

  • 调用者:Looper,调用消息的的处理方法。

  • 命令角色:Message,消息类。

  • 客户端:Thread,创建消息并绑定Handler(接受者)。

Android主线程是如何管理子线程消息的

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。

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");
}

我们可以看到其实我们在这里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。

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

  1. 为什么不会卡死

    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。

  2. 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。

    ActivityThread对应的Handler是一个内部类H,里边包含了启动Activity、处理Activity生命周期等方法。

参考资料

http://www.cnblogs.com/codingmyworld/archive/2011/09/12/2174255.html

http://czpsailer.iteye.com/blog/655942

https://www.zhihu.com/question/34652589

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