Android异步消息处理机制掌握,从源码了解常使用的Handler

1、概述:

  大家都知道,在Android中,UI线程是不安全的,更新UI在UI线程中处理,其他耗时工作都不能在该线程执行,相信大家在面试的时候也知道Handler是面试官非常喜欢问的一个问题。同样我面试的也如此,每次面试前去复习不如写一遍博客记录下来更深刻。

2、Handler的简单使用:

private Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
        }
    };

上面这样使用可能会引起内存泄露的风险,因为当使用内部类(匿名内部类)来创建Handler的时候,Handler隐式持有一个外部类对象(Activity)的引用,而Handler常伴有一个耗时的后台线程操作,当在后台线程访问过程中,Activity不再使用了,那么它就在特定情况下会被GC回收,但由于这时线程尚未执行完,而该线程持有Handler的引用(不然它怎么发消息给Handler?),这个Handler又持有Activity的引用,就导致该Activity无法被回收(即内存泄露),直到网络请求结束(例如图片下载完毕)。(摘自:https://www.cnblogs.com/xujian2014/p/5025650.html)。今天这篇不是写该内容的解决方式,具体的解决方案就看这篇文章吧。

3、Looper消息循环:

  Handler的发送的消息发送到哪里,由谁来承载,由谁来发出呢。

  Handler发送的消息都发送到MessageQueue,而MessageQueue由Looper创建,进入一个无限循环不断从该MessageQueue读取消息并回调处理。

  根据上面的描述,我们先来看看Looper的源码,对于Looper主要有prepare()和loop()两个方法;

  我们经常会在当前线程调用Looper.prepare()和Looper.loop(),首先来看看prepare()方法:

public static void prepare() {
    prepare(true);
}
private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed)); }

  我们实际会调用的是Looper的有参构造函数,sThreadLocal是一个ThreadLocal对象,可以在一个线程中存储变量,我们可以看到sThreadLocal.set(new Looper(quitAllowed))函数往该对象中存储了一个Looper对象,并在前面判断该get()方法是否为空,不为空则抛出异常,该异常相信大家都见到过,在同一个线程内调用多次prepare()方法,这就说明了Looper.prepare()方法不能被调用两次,同时也保证了一个线程中仅有一个Looper对象。

  下面我们看Looper的构造方法:

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

  构造方法创建了一个MessageQueue(消息队列),同时变量mThread指向当前线程;

  好的,上面我们看完了Looper.prepare()的源码,我们再来看看Looper.loop()的源码。

    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn‘t called on this thread.");
        }
        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();

        // Allow overriding a threshold with a system prop. e.g.
        // adb shell ‘setprop log.looper.1000.main.slow 1 && stop && start‘
        final int thresholdOverride =
                SystemProperties.getInt("log.looper."
                        + Process.myUid() + "."
                        + Thread.currentThread().getName()
                        + ".slow", 0);

        boolean slowDeliveryDetected = false;

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            // This must be in a local variable, in case a UI event sets the logger
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
            long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
            if (thresholdOverride > 0) {
                slowDispatchThresholdMs = thresholdOverride;
                slowDeliveryThresholdMs = thresholdOverride;
            }
            final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
            final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

            final boolean needStartTime = logSlowDelivery || logSlowDispatch;
            final boolean needEndTime = logSlowDispatch;

            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }

            final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
            final long dispatchEnd;
            try {
                msg.target.dispatchMessage(msg);
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (logSlowDelivery) {
                if (slowDeliveryDetected) {
                    if ((dispatchStart - msg.when) <= 10) {
                        Slog.w(TAG, "Drained");
                        slowDeliveryDetected = false;
                    }
                } else {
                    if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                            msg)) {
                        // Once we write a slow delivery log, suppress until the queue drains.
                        slowDeliveryDetected = true;
                    }
                }
            }
            if (logSlowDispatch) {
                showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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);
            }

            msg.recycleUnchecked();
        }
    }

  看源码,我们可以看到Looper对象是通过myLooper()方法获取的,该方法也就是上面prepare()方法中说过的sThreadLocal对象调用get()方法获取到存入进去的Looper对象。绕不绕口绕不绕口我就问你,我都差点没明白,看下面,就是如此简单。

    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    } 
1、MessageQueue queue = me.mQueue获取到该消息队列;
2、在循环体for(;;)中进入我们前面所说的无限循环;
3、Message msg = queue.next()获取消息,没有消息则返回;

4、msg.target.dispatchMessage(msg)把消息给msg.target处理,msg.tartget其实就是发送该msg的Handler对象,解析到Handler的时候就会看到;

总结:Looper两个静态方法,一个是prepare(),一个是loop(),prepare()方法主要是绑定当前线程,创建一个Looper对象,且当前线程唯一,同时也创建了一个MessageQueue(消息队列);loop()方法主要是
循环消息队列中的Message,并交给msg的target属性处理。

4、Handler解析:
 
通常我们使用Handler的时候都会new一个Handler对象,我们来看看Handler的构造函数;
    public Handler() {
        this(null, false);
    }
    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 " + Thread.currentThread()
                        + " that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }
  同样,调用的依然是有参构造函数,在if(mLooper == null)里抛出的异常就是我们经常能遇到的,现在看本质这个异常其实就是当前线程Looper对象为null,我们通常解决就是在该线程中调用Looper.prepare()
和Looper.loop()方法;mQueue = mLooper.mQueue获取到Looper对象中的消息队列,那么我们的Handler就跟MessageQueue消息队列关联上了。
  OK,Handler我们知道是如何构造的了,我们看看我们常用的sendMessage(Message msg)方法是如何工作的,ctrl+左键进去看看源码:
    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); }
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); }
  很明显,是调用的这些方法。sendMessageAtTime(Message msg, long uptimeMillis)方法判断了MessageQueue(消息队列)对象是否null;重点来了,enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis)
中的msg.target = this,刚刚我们提到的,将msg给msg.target属性处理,我们这里能看到,msg.target指向了this,也就是我们的Handler,所以前面说的,msg.target属性其实就是当前消息的Handler对象,那么前面的流程就是looper
循环处理MessageQueue(消息队列),获取到消息就给发送该消息的Handler对象的dispatchMessage()方法处理;
  so,我们来看看dispatchMessage()方法是如何处理的:
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

   该方法很简单,就是判断msg的callback是否为null,不为null,则执行该回调,否则执行handleMessage(msg)方法,我们使用sendMessage()方法的时候,是没有给到callback的,那么我们就是调用handleMessage()方法,看看该方法实现了什么:

    /**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }

  handleMessage(Message msg)是一个空方法,没错,该方法是由我们自己实现的,上面我们创建的Handler对象实现了handlerMessage(Message msg)方法,就是调用的我们自己实现的这个方法。

  什么时候会调用handleCallback(Message msg)方法呢,其实handler.post(new Runnable{})就是回调的我们这个方法,看看 post(Runnable run) 方法的源码吧:

    public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }

        public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

  我们可以看到,post方法其实还是调用了我们前面所看到的sendMessageDelayed()方法,不同的是,我们处理了一下传进来的Runnable对象,我们看看getPostMessage(r)方法:

    private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

  这里我们创建了一个Message对象,该对象的callback属性指向我们传进来的Runnable对象,so,前面我们看到的判空部分,handleCallback(Message msg) 那么就会执行,

    private static void handleCallback(Message message) {
        message.callback.run();
    }

  所以,Runnable其实不是一个新的线程在工作,只是作为一个回调实现;

 

总结:

整个异步消息处理流程其实并不复杂,明白他们之间的关系就很容易理解,我来画个图更好的理解下这整个流程:

Android异步消息处理机制掌握,从源码了解常使用的Handler

 Ok,大概就是这样理解的,网上大佬们的解析也很多,借鉴了很多大佬的语言描述,感激不尽。这里只是简单记录我对知识的理解,如有错误请指出。


 

 

Android异步消息处理机制掌握,从源码了解常使用的Handler

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