Android消息机制分析

异步消息处理线程

对于普通的线程而言，执行完run()方法内的代码后线程就结束。而异步消息处理线程是指，线程启动后会进入一个无限循环体之中，每循环一次，从其内部的消息队列中取出一个消息，并回调相应的消息处理函数，执行完一个消息后继续循环。如果消息列表为空，线程会暂停，直到消息队列中有新的消息。

异步线程的实现思路

• 每个异步线程内部包含一个消息队列，队列中的消息一般采用排队机制，先到达的消息会先处理。
• 线程的执行体中使用while(true)进行无限循环，循环体中从消息队列中取出消息，并根据消息的来源，回调响应的消息处理函数。
• 其他外部线程可以向本线程的消息队列发送消息，消息队列内部的读写操作必须进行加锁，不能同时进行读/写操作。

Android 消息机制

Android消息机制主要指Handler的运行机制，Handler的运行机制需要底层的MessageQueue和Looper的支持。

ThreadLocal

ThreadLocal 是一个线程内部存储类，通过它可以在指定的线程中存储数据，同样只能在指定的线程里获取存储的数据，其他的线程无法获取。例如对于Handler来说，它需要获取当前线程的Looper，Looper的作用域就是线程中，并且不同的线程有不同的Looper。

存储数据

 public void set(T value) {
 Thread currentThread = Thread.currentThread();
 Values values = values(currentThread);
 if (values == null) {
 values = initializeValues(currentThread);
 }
 values.put(this, value);
 }
 
 
Values values(Thread current) {
 return current.localValues;
 }
 
 
Values initializeValues(Thread current) {
 return current.localValues = new Values();
 }
 

意思是先去获取当前线程的Values，null话会新建一个。这里这个Values是Thread的一个内部类，这个内部类里有一个数组

 /**
 * Map entries. Contains alternating keys (ThreadLocal) and values.
 * The length is always a power of 2.
 */
 private Object[] table;
 

注释说存储交替的key(ThreadLocal的引用)和values，这个数组的长度始终是2的幂。
然后是values.put

 void put(ThreadLocal key, Object value) {
 cleanUp();
 
 
        // Keep track of first tombstone. That's where we want to go back
        // and add an entry if necessary.
        int firstTombstone = -1;

        for (int index = key.hash & mask;; index = next(index)) {
            Object k = table[index];

            if (k == key.reference) {
                // Replace existing entry.
                table[index + 1] = value;
                return;
            }

            if (k == null) {
                if (firstTombstone == -1) {
                    // Fill in null slot.
                    table[index] = key.reference;
                    table[index + 1] = value;
                    size++;
                    return;
                }

                // Go back and replace first tombstone.
                table[firstTombstone] = key.reference;
                table[firstTombstone + 1] = value;
                tombstones--;
                size++;
                return;
            }

            // Remember first tombstone.
            if (firstTombstone == -1 && k == TOMBSTONE) {
                firstTombstone = index;
            }
        }
    }
 
 

这里所做的操作验证了刚才的注释，put时ThreadLocal reference的位置始终在value的前一个，这样就把这种键值对交替的存在Thread.values里的Object[] table这个数组里。这样一方面也说明了一个Thread里可以存多个ThreadLocal 和 Value的组合。

获取数据

 public T get() {
 // Optimized for the fast path.
 Thread currentThread = Thread.currentThread();
 Values values = values(currentThread);
 if (values != null) {
 Object[] table = values.table;
 int index = hash & values.mask;
 if (this.reference == table[index]) {
 return (T) table[index + 1];
 }
 } else {
 values = initializeValues(currentThread);
 }
 
 
    return (T) values.getAfterMiss(this);
}
 
 

和存相反，这里先通过当前ThreadLocal的引用获取当前ThreadLocal在table数组的位置，那么他的下一个位置就是之前存入的数据的位置，这样就可以获取数据了。

总结一下：一个Thread里的Values包含一个Obj数组，这个数组的存取是通过ThreadLocal这个类，存时用ThreadLocal的引用作为key，要存的数据做value，取同样，多个ThreadLocal存如同一个Thread的Values的obj数组，获取数据互不影响。验证了ThreadLocal的存取操作仅限于线程里。

MessageQueue

MessageQueue消息队列主要包含两个操作：插入和读取。

插入：

 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("MessageQueue", 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;
}
 
 

这个方法事件就是根据时间做入队的操作。

在hanlder里发送消息，最后都走了这个方法：

 public boolean sendMessageAtTime(Message msg, long uptimeMillis)
 {
 boolean sent = false;
 MessageQueue queue = mQueue;
 if (queue != null) {
 msg.target = this;
 sent = queue.enqueueMessage(msg, uptimeMillis);
 }
 else {
 RuntimeException e = new RuntimeException(
 this + " sendMessageAtTime() called with no mQueue");
 Log.w("Looper", e.getMessage(), e);
 }
 return sent;
 }
 

可以看见这个方法都是调用了MessageQueue的enqueueMessage方法，其中msg参数就是我们发送的Message对象，而uptimeMillis参数则表示发送消息的时间，它的值等于自系统开机到当前时间的毫秒数再加上延迟时间，如果你调用的不是sendMessageDelayed()方法，延迟时间就为0，然后将这两个参数都传递到MessageQueue的enqueueMessage()方法中。

然后是读取的next方法：

 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 (false) Log.v("MessageQueue", "Returning message: " + msg);
                    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("MessageQueue", "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;
    }
}
 
 

这个方法非常的长，还调用了ndk，大体的意思就是从消息队列里不停的读Message，当前时间大于Message的when就会返回这个Message，否则一直会阻塞，跳出阻塞的条件是：

 if (mQuitting) {
 dispose();
 return null;
 }
 
 

Looper

Looper作用是不停地从MessageQueue中查看是否有消息，有消息就会立即处理。下面看它的两个方法。
prepare：

 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));
 }
 

结合ThreadLocal的分析，这个prepare的操作实际就是在当前线程中存一个Looper。

loop：

 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();
    }
}
 
 

loop中有个死循环去调用MessgeQueue的next方法，当next返回的message为null时才会跳出死循环，结合对next分析我们知道，MessageQueue的msg为空时会一直阻塞，只有if（mQuitting）才会返回null，这时Looper也会跳出死循环，我们可以通过Loop的quit方法，这个quit方法会调用MessgeQueue的quit给mQuitting设置为true。如果这个有message，会走msg.target.dispatchMessage(msg)方法。
我们来看一下这个target是什么进入Message类：

 /package/ Handler target;
 

这个target就是hanlder。

Handler

 public Handler(Callback callback, boolean async) {
 if (FIND_POTENTIAL_LEAKS) {
 final Class 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 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);
 }
 public void dispatchMessage(Message msg) {
 if (msg.callback != null) {
 handleCallback(msg);
 } else {
 if (mCallback != null) {
 if (mCallback.handleMessage(msg)) {
 return;
 }
 }
 handleMessage(msg);
 }
 }
 

看了这两个方法，我脑子已经有一个画面了：

总结

通过Handler发送消息时，new Hanlder时会检查当前当前线程是否有Looper，没有就会报错
"Can't create handler inside thread that has not called Looper.prepare()");
Looper.prepare事件是利用ThreadLocal给当前线程存一个Looper，并通过Looper.Loop获取并启动他。handler发送的消息实际最后都会调用MessageQueue的enQueue的操作，这里会按时间排序，Looper启动后会不停的读取MessgeQueue.next的msg，时间达到的Msg就会被分发,调用handler.dispatchMessage方法，这里实际就是去调用在hanlderMessage()里所编写的业务代码，这样就成功的讲代码切换到Looper.pre里赋值的线程中执行

Tips

另外除了发送消息之外，我们还有以下几种方法可以在子线程中进行UI操作：

1. Handler的post()方法
2. View的post()方法
3. Activity的runOnUiThread()方法