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要想說清楚Handler消息機制原理，先得搞清楚消息機制中的幾個類，以及它們的關系

public class Handler
public final class Looper
public final class MessageQueue
public final class Message implements Parcelable

先說說Looper類，它有一個靜態變量sThreadLocal和myLooper()的靜態函數

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }
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));
    }

sThreadLocal對象為線程局部對象，簡單來說通過它的get函數每個不同的線程都會獲取到唯一的對象為T類型的變量，這里面T的類型為Looper類型，所以每個Thread里面通過sThreadLocal的get函數都能獲取到唯一的Looper變量。ThreadLocal的原理是什么，就是根據ThreadLocalMap進行保存值，而ThreadLocalMap對象又保存在Thread中。
言規正傳，通過以上的分析我們知道了Looper調用myLooper()就是獲取到當前Thread的Looper對象（在第一次調用myLooper()之前需要先調用prepare()）。
Looper類里面有一個類型為MessageQueue的成員變量mQueue

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

在講MessageQueue之前，先說說Message，Message也是我們常用的，經常通過obtain獲取Message

public final class Message implements Parcelable {
    
    public int what;
    public int arg1; 
    public int arg2;
    public Object obj;

    public Messenger replyTo;
    public int sendingUid = -1;
    /*package*/ long when;
    /*package*/ Bundle data;
    /*package*/ Handler target;
    /*package*/ Runnable callback;
    
    // sometimes we store linked lists of these things
    /*package*/ Message next;
}

Message中有一個重要的指向下一個Message的next引用。這就很像數據結構中的單鏈表的節點。實際上Message存儲在MessageQueue中就是以單鏈表的形式保存的，同時也是有序的。以Message中的long when的大小進行從小到大排序。
同時還要一個Handler target對象，表示該消息最終由哪個Handler處理。
現在來說MessageQueue，MessageQueue中有兩個重要的函數enqueueMessage()和next()，enqueueMessage是向消息隊列中插入一條消息，以when的大小進行從小到大的有序插入。

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(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.
//如果mMessages消息為空，或者when為0或者when小于mMessages的when，插入在表頭
                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;
//否則找到一個時間大于when的Message的前面插入
                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;
    }

很簡單，以when為大小，有序插入到mMessages為表頭的鏈表中。
next()函數就更簡單了，取出mMessages表頭中的消息，如果當前時間小于表頭Message時間，則先等待msg.when-currentTime時間，然后取出表頭消息。

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;
                }
            nextPollTimeoutMillis = 0;
        }
    }

如其名MessageQueue就是一個消息隊列的數據結構，里面有插入，查詢，刪除等操作。
那么又是誰在使用MessageQueue呢，當時是Handler了。我們經常用到的Handler里的sendMessage最終就是調用MessageQueue的enqueueMessage函數，向隊列中插入一條消息。

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;//注意這句，將發送消息的Handler賦值給Message對象，在Looper循環的時候就能找到對應的Handler處理消息
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

Handler中的mQueue對象是一個MessageQueue類型，它是誰賦值的呢

final Looper mLooper;
final MessageQueue mQueue;
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;
    }

可以看到它引用的是Looper里面的mQueue。
這里我們可以總結下，一個Thread里面只有一個Looper，主線程的Looper就是sMainLooper。每個Looper里面有一個MessageQueue。一個Thread里面Handler可以有很多個實例對象，可以new很多個。但是它們最終指向的Looper對象只會有一個，MessageQueue也只會有一個。所有通過Handler發送的消息，最終都是插入到當前Thread對應的Looper中的MessageQueue中，且以時間進行排序。
那么最后來看看Looper的消息循環

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 (;;) {
//調用MessageQueue的next()函數獲取到Message
            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;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
//然后將msg丟給msg里面的Handler對象進行處理。
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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里面就很簡單了，調用MessageQueue的next()函數獲取到Message，然后將msg丟給msg里面的Handler對象進行處理。這樣消息就最終回到了Handler處理，我們來看看Handler的dispatchMessage

public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            //如果Handler的mCallback不為空先調用mCallback處理，返回true，直接return
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

private static void handleCallback(Message message) {
        message.callback.run();
    }
    /**
     * Subclasses must implement this to receive messages.
     *這就是new Handler的時候實現的handleMessage處理消息
     */
    public void handleMessage(Message msg) {
    }

如果msg的callback不為空，則調用callback處理。否則判斷Handler的mCallback是否能處理，否則調用handleMessage處理消息。
以上就是Handler消息機制，還是比較簡單的。