典型实例

  class LooperThread extends Thread {
        public Handler mHandler;
  
        public void run() {
        //第一步
            Looper.prepare();
       //第二步
            mHandler = new Handler() {
                public void handleMessage(Message msg) {
                    // process incoming messages here
                }
            };
        //第三步
            Looper.loop();
       }
    }

结构图： Handler.jpg

第一步: Looper.prepare()

无参默认调用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是一个ThreadLocal<Looper>对象
if判断保证一个线程只执行一次prepare(true)
1.1 ThreadLocal：
线程本地存储区（Thread Local Storage，简称为TLS）每个线程都有自己的私有的本地存储区域，不同线程之间彼此不能访问对方的TLS区域。

public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

一个ThreadLocal对应一个Looper，Looper循环器只会循环一个线程的消息而不会循环到其他线程的，保证线程安全。

public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

 private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

1.2 Looper的构造方法

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

1.3 loop()
见第三步

1.4 myLooper()

 public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

1.5 quit()

public void quit() {
        mQueue.quit(false);
    }

public void quitSafely() {
        mQueue.quit(true);
    }

1.6 MessageQueue.quit()

 void quit(boolean safe) {
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }

        synchronized (this) {
            if (mQuitting) {
                return;
            }
            mQuitting = true;

            if (safe) {
                removeAllFutureMessagesLocked();
            } else {
                removeAllMessagesLocked();
            }

            // We can assume mPtr != 0 because mQuitting was previously false.
            nativeWake(mPtr);
        }
    }

第二步： new Handler()

无参构造

 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;//回调方法
        mAsynchronous = async;//设置消息是否为异步处理方式

    }

对于Handler的无参构造方法，默认采用当前线程TLS中的Looper对象，并且callback回调方法为null，且消息为同步处理方式。

2.1.1 post

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

 public final boolean postAtTime(Runnable r, long uptimeMillis)
    {
        return sendMessageAtTime(getPostMessage(r), uptimeMillis);
    }

public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
    {
        return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
    }

 public final boolean postDelayed(Runnable r, long delayMillis)
    {
        return sendMessageDelayed(getPostMessage(r), delayMillis);
    }

2.1.2 sendMessageDelayed

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

2.1.3 sendEmptyMessageAtTime

 public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageAtTime(msg, uptimeMillis);
    }

2.1.4 sendMessageAtTime

 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);
    }

2.1.5 sendMessageAtFrontOfQueue

public final boolean sendMessageAtFrontOfQueue(Message msg) {
        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, 0);
    }

2.1.7 enqueueMessage

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

2.1.8 dispatchMessage 消息的分发

 public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);//当Message存在回调方法，回调msg.callback.run()方法；
        } else {
            if (mCallback != null) {
            //当Handler存在Callback成员变量时，回调方法handleMessage()；
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
          //Handler自身的回调方法handleMessage()
            handleMessage(msg);
        }
    }

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

2.1.9 removeMessages

public final void removeMessages(int what) {
        mQueue.removeMessages(this, what, null);
    }

public final void removeMessages(int what, Object object) {
        mQueue.removeMessages(this, what, object);
    }

2.2 MessageQueue.enqueueMessage(msg, uptimeMillis)

 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) {//正在退出回收msg
                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 (;;) {
                  //循环找到最后一个msg
                    prev = p;
                    p = p.next;
                  //如果已经找到了msg或者最后一个msg的触发时间比队列的第一个msg的时间小
                    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;
    }

第三步：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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            final long end;
            try {
                msg.target.dispatchMessage(msg);
                end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (slowDispatchThresholdMs > 0) {
                final long time = end - start;
                if (time > slowDispatchThresholdMs) {
                    Slog.w(TAG, "Dispatch took " + time + "ms on "
                            + Thread.currentThread().getName() + ", h=" +
                            msg.target + " cb=" + msg.callback + " msg=" + msg.what);
                }
            }

            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();
        }
    }

无限循环在MessageQueue队列取出msg，直到为空退出
取到msg调用Handler(即msg.target)的dispatchMessage()进行处理
处理完成调用recycleUnchecked()回收消息。
3.1MessageQueue.next()

 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 {
                      //找到最后一个msg
                        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) {
                          //把msg的上一个msg指向下一个
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        //在把msg自己的下一个指向空
                        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;
                }
          ....省略好多代码.....
        }
    }

3.2Message.recycle

  public void recycle() {
        if (isInUse()) {
            if (gCheckRecycle) {
                throw new IllegalStateException("This message cannot be recycled because it "
                        + "is still in use.");
            }
            return;
        }
        recycleUnchecked();
    }

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++;
            }
        }
    }

3.3Message.obtain()

 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();
    }