Handler与Thread的关系

原文链接如下：http://erqi.github.io/2016/08/31/handler%E5%92%8CThread%E7%9A%84%E5%85%B3%E7%B3%BB/

Looper

Looper是Handler和Thread相关联的桥梁,也是APP开发中线程间通信的用的最多的一个.
然而使用率却是异常的低下,因为在向主线程交互的时Looper该做的工作已经做好,使用上只需要在主线程创建Handler对象,然后再需要的地方发送消息即可.

参考博客 Android 异步消息处理机制 让你深入理解 Looper、Handler、Message三者关系 洪洋

Looper的初始化创建

Looper和线程的关系是一一对应的,每一个线程有且只有一个Looper,可以从Looper的使用中看出.

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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));
}

在使用子线程中使用Hander时,需要有Looper对象,而Looper的构造函数是private修饰的,想要获得创建一个Looper对象就只能调用静态方法prepare.
在方法中我们可以看到调用了sThreadLocal.get()方法,返回的若非空就会抛出异常.这里就限定了一个线程只能有一个Looper.

Looper的构造函数

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

在构造函数中创建了一个MessageQueue(消息队列).

Looper的使用

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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();
    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
        Printer logging = me.mLogging;
        if (logging != null) {
            logging.println(">>>>> Dispatching to " + msg.target + " " +
                    msg.callback + ": " + msg.what);
        }
        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);
        }
        msg.recycleUnchecked();
    }
}

当操作完成就可以调用Looper.loop();从代码中可以看出在通过myLooper中拿到了当前线程的Looper对象,获取了对应的MessageQueue之后,该线程会执行一个死循环,在死循环中不停的读取MessageQueue队列中的Message并调用 msg.target.dispatchMessage(msg);去处理该消息.

Handler

在使用Handler时,我们通常都是实例化一个对象去发送消息,然后在handleMessage方法中去处理对应的消息,并作出对应的处理.

Hanlder的初始化

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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 that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

在初始化中我们看到了Handler将自己和Looper进行了关联,如若在子线程中未创建Looper就创建Hanlder那么便会抛出异常.并且在下面继续获取了Looper的消息队列.

Handler的使用

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Message message = Message.obtain();
message.what = TEXT;
message.obj = (int) (Math.random() * 1000);
mHandler.sendMessage(message);

Handler使用方式十分之简单,在需要使用的地方发送一个Message消息即可,在创建Handler时重写handleMessage方法中处理对应的Message消息即可.
多数人走到这一步就停下了,因为已经知道如何使用Hanlder和Looper了,能满足跨线程交互需求了,也就没有继续探索的愿望了,然后在面试的一个一问Handler为什么能切换到主线程去执行对应的方法就懵逼了(面试问懵逼好几个了….)

Handler跨线程的原因

既然发送消息是在子线程,在handleMessage中处理的时候已经到了主线程,那么线程切换的奥秘一定就在这两个方法调用过程中,我们就从sendMessage方法中一步一步往下看.

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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);
}

到这里就可以看出一点端倪了,mQueue出现了,也就是Handler在创建时获取的Looper的MessageQueue对象.
我们继续向下看

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private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

在这里我们又看到了一点意思的东西,msg.target = this在Looper.loop中我们看到msg.target.dispatchMessage(msg);这么一行代码,现在稍微一推敲就知道最终发生了什么,怎么切换线程的,但是我们还是继续往下看.

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boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }
    synchronized (this) {
        if (mQuitting) {
            IllegalStateException e = new IllegalStateException(
                    msg.target + " sending message to a Handler on a dead thread");
            Log.w(TAG, e.getMessage(), e);
            msg.recycle();
            return false;
        }
        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        boolean needWake;
        if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            // Inserted within the middle of the queue.  Usually we don't have to wake
            // up the event queue unless there is a barrier at the head of the queue
            // and the message is the earliest asynchronous message in the queue.
            needWake = mBlocked && p.target == null && msg.isAsynchronous();
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;
        }
        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

恩,到这里就是将Message加入到消息队列中去中去,hanlder中消息发送流程已经走完了.
在这里handler将Message加入到了消息队列中,而Looper一直线主线程中阻塞着,所以一收到消息就拿到了Message对象,然后在主线程中调用了Hanlder的处理方法.

Looper对消息的处理

从上面的分析已经了解到了子线程和主线程是如何切换的,我们继续看在主线程中Looper是如何处理发送过来的消息的.

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// Looper.loop();中代码
Message msg = queue.next(); // might block
// MessageQueue中next();代码
Message next() {
    // Return here if the message loop has already quit and been disposed.
    // This can happen if the application tries to restart a looper after quit
    // which is not supported.
    final long ptr = mPtr;
    if (ptr == 0) {
        return null;
    }
    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }
        nativePollOnce(ptr, nextPollTimeoutMillis);
        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {
                // Stalled by a barrier.  Find the next asynchronous message in the queue.
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    // Next message is not ready.  Set a timeout to wake up when it is ready.
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    // Got a message.
                    mBlocked = false;
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    return msg;
                }
            } else {
                // No more messages.
                nextPollTimeoutMillis = -1;
            }
            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }
            // If first time idle, then get the number of idlers to run.
            // Idle handles only run if the queue is empty or if the first message
            // in the queue (possibly a barrier) is due to be handled in the future.
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            if (pendingIdleHandlerCount <= 0) {
                // No idle handlers to run.  Loop and wait some more.
                mBlocked = true;
                continue;
            }
            if (mPendingIdleHandlers == null) {
                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
            }
            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
        }
        // Run the idle handlers.
        // We only ever reach this code block during the first iteration.
        for (int i = 0; i < pendingIdleHandlerCount; i++) {
            final IdleHandler idler = mPendingIdleHandlers[i];
            mPendingIdleHandlers[i] = null; // release the reference to the handler
            boolean keep = false;
            try {
                keep = idler.queueIdle();
            } catch (Throwable t) {
                Log.wtf(TAG, "IdleHandler threw exception", t);
            }
            if (!keep) {
                synchronized (this) {
                    mIdleHandlers.remove(idler);
                }
            }
        }
        // Reset the idle handler count to 0 so we do not run them again.
        pendingIdleHandlerCount = 0;
        // While calling an idle handler, a new message could have been delivered
        // so go back and look again for a pending message without waiting.
        nextPollTimeoutMillis = 0;
    }
}

瞧,我们发现了什么,又是一个死循环阻塞,原来主线程不是阻塞在Looper中,而是阻塞在MessageQueue对象中.
可以看到当消息队列有消息的时候立马返回消息,没有消息就阻塞(我就偷了个懒,不在继续深入MessageQueue的原理了…)
Looper收到返回的消息会调用msg.target.dispatchMessage(msg)去处理,也就是调用Handler去处理.
我们继续来到Hanlder的处理方法中.

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public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

msg.callback我们先放一放,就我们普通发送的Message通常是没有callback对象的,下面的mCallback我们来看一看,该对象出现在构造函数之中,通过构造函数来赋值,我们来看看这个对象的定义

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public interface Callback {
    public boolean handleMessage(Message msg);
}

可以看出就是一个接口,和自己重写handleMessage一样,只不过提供另一种方式来处理.(说实话,在成员对象创建上面还重写其方法,我感觉是看着挺难受的,这样抽取成对象,然后通过构造函数传递进去,看着舒服多了.)
回到正题中来,我们还剩下一个Messaged的callBack对象没整明白.
首先,我们来看一看这究竟是一个什么对象.

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/*package*/ Runnable callback;

可以看到就是一个任务而已,而handleCallback(msg)方法点进去再来看一看

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

只是单纯的是运行这个任务而已,但是这个任务是哪里来的呢?
答案在Handler,Handler不仅可以发消息给自己在主线程处理,也可以直接发送一个任务去主线程运行.

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public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

到这里也就看明白了,剩下就和普通消息一样的处理方式了.