大家都知道 handler 主要是用在子线程中去更新UI线程，就如下面的代码一样：

public class MainActivity extends AppCompatActivity {
    private TextView mTv;
    private Handler mHandler = new Handler(){
        @Override
        public void handleMessage(@NonNull Message msg) {
            mTv.setText((String)msg.obj);
            super.handleMessage(msg);
        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTv = findViewById(R.id.tv);
        new Thread(new Runnable() {
            @Override
            public void run() {
                Message message1 = Message.obtain();
                mHandler.sendMessage(message1 );
                Message message2 = Message.obtain();
                mHandler.sendMessageDelayed(message2,2000);
                Message message3 = Message.obtain();
                mHandler.sendMessageDelayed(message3,500);
            }
        }).start();
    }
}

如果直接在run方法中执行 setText()操作的话，肯定是报错的。会报：

android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

这个错误。这句话的意思就是只能在原始线程中才能更新UI也就是主线程中，那么为什么通过Handler就行更新呢？

我们先看 Message.obtain() 的源码：

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();
    }

这是一个消息复用机制，一开始的时候 sPool == null，所以会执行 new Message();这样 Message.obtain()就得到一个Message对象了。
下一步我们 sendMessage()

 public final boolean sendMessage(@NonNull Message msg) {
        return sendMessageDelayed(msg, 0);
    }

再看sendMessageDelayed其中delayMillis==0

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

再看sendMessageAtTime

  public boolean sendMessageAtTime(@NonNull 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);
    }

到这里我们就发现有一个

MessageQueue queue = mQueue;

我们先不考虑它是怎么来的，继续往下看 enqueueMessage(queue, msg, uptimeMillis)

    private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
            long uptimeMillis) {
        msg.target = this;
        msg.workSourceUid = ThreadLocalWorkSource.getUid();

        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

这里有一个重要的信息：

msg.target = this;

this其实就是

Handler

对吧？
然后看 queue.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) {
                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;
    }

我们扣出主要的代码：

msg.when = when;
        Message p = mMessages;
        if (p == null || when == 0 || when < p.when) {
            msg.next = p;
            mMessages = msg;
        } else {
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
            }
            msg.next = p;
            prev.next = msg;
        }

当执行 mHandler.sendMessage(message1);我们看一下执行的过程，一开始的时候
mMassages = null
Message p = null
msg = message1
when = 0
message1.when = 0
条件 if (p == null || when == 0 || when < p.when) 成立
message1.next = null
mMessage = message1
最后是：
mMessage = message1
message1.next=null
结束

下一个执行 mHandler.sendMessageDelayed(message2,2000);
mMessage = message1
p = mMessage
p = message1
when = 2000
p.when = message1.when
p.when =0
条件 if (p == null || when == 0 || when < p.when) 不成立，则进入else语句的for死循环
prev = p
prev = message1
p = p.next
p = message1.next
p = null
所以 if (p == null || when < p.when) 条件成立，跳出死循环
msg.next = p
message2.next = p
message2.next = null
prev.next = message1.next
message1.next = message2
最后是：
mMessage = message1
message1.next = message2
message2.next = null
结束

下一个执行：mHandler.sendMessageDelayed(message3,500);
mMessage = message1
p = mMessage
p = message1
when = 500
p.when = message1.when
p.when = 0
when = 500
所以条件 if (p == null || when == 0 || when < p.when) 不成立 进入 else语句中的for死循环
prev = p
prev = message1
p = p.next
p = message1.next
p = message2
p.when = 2000
条件if (p == null || when < p.when)成立 跳出循环
msg.next = message3.next
message3.next= p
message3.next = message2
prev.next = message1.next
message1.next = message3
最后是：
mMessage = message1
message1.next = message3
message2.next = null
message3.next = message2

根据以上分析得到是Handler存储的消息队列是链式存储，采用这样的存储方式更加快速的插入消息和删除消息。

到这里为止，我们并没有发现有回调 handleMessage() 这个方法，不着急。至少到这里我们明白当执行了sendMessage()之后，message只是被存储到了队列中，然后就没有然后了。

在这里我们需要注意的是 不管你是sendMessage()还是sendMessageDelayed()其实都是调用 sendMessageAtTime()。

好了，现在消息过去了，怎么回调到handleMessage()这个方法呢？
到这里我们就要关注这个了

Looper

Looper是什么呢？它是一个消息处理器，看他的源码，不难会发现里面的消息循环，下面我们一点一点来。

当activity被创建的时候，ActivityThread中的main方法就会执行，我们看代码：

 public static void main(String[] args) {
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");

        // Install selective syscall interception
        AndroidOs.install();

        // CloseGuard defaults to true and can be quite spammy.  We
        // disable it here, but selectively enable it later (via
        // StrictMode) on debug builds, but using DropBox, not logs.
        CloseGuard.setEnabled(false);

        Environment.initForCurrentUser();

        // Make sure TrustedCertificateStore looks in the right place for CA certificates
        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
        TrustedCertificateStore.setDefaultUserDirectory(configDir);

        Process.setArgV0("<pre-initialized>");

        Looper.prepareMainLooper();

        // Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
        // It will be in the format "seq=114"
        long startSeq = 0;
        if (args != null) {
            for (int i = args.length - 1; i >= 0; --i) {
                if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
                    startSeq = Long.parseLong(
                            args[i].substring(PROC_START_SEQ_IDENT.length()));
                }
            }
        }
        ActivityThread thread = new ActivityThread();
        thread.attach(false, startSeq);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

在这里我们只需要关注两行代码：

Looper.prepareMainLooper();

Looper.loop();

先看prepareMainLooper

   public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

再看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));
    }

其实这个就是我们在子线程调用的Loop.prepare() 。到这里我们知道 Looper.prepareMainLooper()和Loop.prepare()其实是一样的。

sThreadLocal 是什么呢？它是一个静态变量

  static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

再看ThreadLocal 中的 set()方法

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

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

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

看到没有，这里是创建一个ThreadLocal，key是当前线程，value是Looper。

 if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }

这里就保证了一个线程只能有一个Looper，否则会报错。
当去创建一个Looper的时候就创建了一个消息队列，有且只有一个MessageQueue

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

下面我们接着看 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();

        // Allow overriding a threshold with a system prop. e.g.
        // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
        final int thresholdOverride =
                SystemProperties.getInt("log.looper."
                        + Process.myUid() + "."
                        + Thread.currentThread().getName()
                        + ".slow", 0);

        boolean slowDeliveryDetected = false;

        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);
            }
            // Make sure the observer won't change while processing a transaction.
            final Observer observer = sObserver;

            final long traceTag = me.mTraceTag;
            long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
            long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
            if (thresholdOverride > 0) {
                slowDispatchThresholdMs = thresholdOverride;
                slowDeliveryThresholdMs = thresholdOverride;
            }
            final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
            final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

            final boolean needStartTime = logSlowDelivery || logSlowDispatch;
            final boolean needEndTime = logSlowDispatch;

            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }

            final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
            final long dispatchEnd;
            Object token = null;
            if (observer != null) {
                token = observer.messageDispatchStarting();
            }
            long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
            try {
                msg.target.dispatchMessage(msg);
                if (observer != null) {
                    observer.messageDispatched(token, msg);
                }
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } catch (Exception exception) {
                if (observer != null) {
                    observer.dispatchingThrewException(token, msg, exception);
                }
                throw exception;
            } finally {
                ThreadLocalWorkSource.restore(origWorkSource);
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (logSlowDelivery) {
                if (slowDeliveryDetected) {
                    if ((dispatchStart - msg.when) <= 10) {
                        Slog.w(TAG, "Drained");
                        slowDeliveryDetected = false;
                    }
                } else {
                    if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                            msg)) {
                        // Once we write a slow delivery log, suppress until the queue drains.
                        slowDeliveryDetected = true;
                    }
                }
            }
            if (logSlowDispatch) {
                showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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();
        }
    }

这里我们抠出重点来讲就行

//得到Looper
 final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
//得到looper中的消息队列
        final MessageQueue queue = me.mQueue;
for (;;) {
            Message msg = queue.next(); // might block
          
            try {
                msg.target.dispatchMessage(msg);
              
            } catch (Exception exception) {
               
            }
        
            msg.recycleUnchecked();
        }

我们只看三个代码一个：Message msg = queue.next();还有一个就是 msg.recycleUnchecked();另一个就是msg.target.dispatchMessage(msg);
下面看Message msg = queue.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 {
                        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;
        }
    }

这里就是不断的循环消息队列得到Message，并且判断message的target是不是空，也就是handler是不是空。

我们接着看 msg.target.dispatchMessage(msg);

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

不容易呀，到这里终于看到handleMessage()了。
最后看 消息回收：msg.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 = UID_NONE;
        workSourceUid = UID_NONE;
        when = 0;
        target = null;
        callback = null;
        data = null;

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

到这里为止，Handle的源码分析已经全部结束。
总结：当activity创建的时候，activityThread执行main方法，执行里面的Loop.prepareMainLooper()，创建Looper和消息队列，然后执行Loop.loop()，开始循环不断的去扫描消息队列。当创建一个Hanlder的时候，就调用mLooper = Looper.myLooper();mQueue = mLooper.mQueue;得到消息队列，执行sendMessage()就往这个队列中插入消息，loop()不断的循环就得到消息，回调到handleMessage()中。

完毕。。。

该吃午饭了，分析的不容易，希望看到同学们给个点赞。谢谢