Message是Handler处理消息的载体，有些需要注意到的细节

1. Message表现为一个有序链表，链表中的数据Message以when字段升序排列

long when;

2.Message通过字段next链接下一个Message，形成链表结构

Message next;

3.callback执行优先级较高，参见Handler中dispatchMessage方法的执行顺序

    Runnable callback;

    public static Message obtain(Handler h, Runnable callback) {
        Message m = obtain();
        m.target = h;
        m.callback = callback;

        return m;
    }

Handler.java

    /**
     * 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);
        }
    }

4.FLAG_IN_USE值为1（二进制01），FLAG_ASYNCHRONOUS值为2（二进制10）

    static final int FLAG_IN_USE = 1 << 0;
    static final int FLAG_ASYNCHRONOUS = 1 << 1;
    int flags;

    public boolean isAsynchronous() {
        return (flags & FLAG_ASYNCHRONOUS) != 0;
    }
    public void setAsynchronous(boolean async) {
        if (async) {
            flags |= FLAG_ASYNCHRONOUS;
        } else {
            flags &= ~FLAG_ASYNCHRONOUS;
        }
    }
    /*package*/ boolean isInUse() {
        return ((flags & FLAG_IN_USE) == FLAG_IN_USE);
    }

    /*package*/ void markInUse() {
        flags |= FLAG_IN_USE;
    }

5.Message优先考虑从全局对象池复用对象，避免重复创建

    private static final Object sPoolSync = new Object();
    private static Message sPool;
    private static int sPoolSize = 0;

    private static final int MAX_POOL_SIZE = 50;

    private static boolean gCheckRecycle = true;

    /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

    /**
     * Recycles a Message that may be in-use.
     * Used internally by the MessageQueue and Looper when disposing of queued Messages.
     */
    void recycleUnchecked() {
        // Mark the message as in use while it remains in the recycled object pool.
        // Clear out all other details.
        flags = FLAG_IN_USE;
        what = 0;
        arg1 = 0;
        arg2 = 0;
        obj = null;
        replyTo = null;
        sendingUid = -1;
        when = 0;
        target = null;
        callback = null;
        data = null;

        synchronized (sPoolSync) {
            if (sPoolSize < MAX_POOL_SIZE) {
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }