public static void loop() {
//myLooper()方法作用是返回sThreadLocal存储的Looper实例，如果me为null，loop()则抛出异常
//也就是说loop方法的执行必须在prepare方法之后运行
//也就是说，消息循环必须要先在线程当中创建Looper实例
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
//获取looper实例中的mQueue（消息队列）
        final MessageQueue queue = me.mQueue;
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();
//进入消息循环
        for (;;) {
//next()方法用于取出消息队列里的消息
//如果取出的消息为空，则线程阻塞
            Message msg = queue.next(); // might block
            if (msg == null) {
          
                return;
            }
  
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }
//消息派发：把消息派发给msg的target属性，然后用dispatchMessage方法去处理
//Msg的target其实就是handler对象，下面会继续分析
            msg.target.dispatchMessage(msg);
            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }
            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);
            }
//释放消息占据的资源
            msg.recycle();
        }
}

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());
            }
        }
//通过Looper.myLooper()获取了当前线程保存的Looper实例，如果线程没有Looper实例那么会抛出异常
//这说明在一个没有创建Looper的线程中是无法创建一个Handler对象的
//所以说我们在子线程中创建一个Handler时首先需要创建Looper，并且开启消息循环才能够使用这个Handler。
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
//获取了这个Looper实例中保存的MessageQueue（消息队列）
//这样就保证了handler的实例与我们Looper实例中MessageQueue关联上了
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

   public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
//我们往下扒
   public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageDelayed(msg, delayMillis);
    }
 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) {
//直接获取MessageQueue
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
//调用了enqueueMessage方法
        return enqueueMessage(queue, msg, uptimeMillis);
    }
//调用sendMessage方法其实最后是调用了enqueueMessage方法
 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//为msg.target赋值为this，也就是把当前的handler作为msg的target属性
//如果大家还记得Looper的loop()方法会取出每个msg然后执行msg.target.dispatchMessage(msg)去处理消息，其实就是派发给相应的Handler
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
//最终调用queue的enqueueMessage的方法，也就是说handler发出的消息，最终会保存到消息队列中去。
        return queue.enqueueMessage(msg, uptimeMillis);
    }

showhandler.post(new Runnable() {
              @Override
              public void run() {
                  String line = "\n";
                  StringBuffer text = new StringBuffer(show.getText());
                          text.append(line).append("angelababy:Yes,I do");
                          show.setText(text);
              }

 public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }
  private static Message getPostMessage(Runnable r) {
//创建了一个Message对象
//创建Message对象可以new，也可以使用Message.obtain()方法；
//但是更建议使用obtain方法，因为Message内部维护了一个Message池用于Message的复用，避免使用new 重新分配内存。
        Message m = Message.obtain();
//将我们创建的Runable对象作为callback属性，赋值给了此message.
        m.callback = r;
//创建了一个Message对象
        return m;
    }
 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);
    }

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

//一个进程会默认生成一个主线程
public static void main(String[] args) {
 ......
//主线程生成时自动通过prepareMainLooper方法为主线程创建一个Looper
//prepare()方法是用于在子线程中创建一个Looper对象，在子线程中是可以退出消息循环的：调用消息队列的quit方法
//Looper生成时会自动生成与之配套的消息队列
Looper.prepareMainLooper(); 
ActivityThread thread = new ActivityThread(); 
thread.attach(false);
 ...... 
//loop()方法开启消息循环
//主线程的消息循环是不允许被退出的
Looper.loop(); 
throw new RuntimeException("Main thread loop unexpectedly exited");
}

boolean enqueueMessage(Message msg, long when) {
    ......
    synchronized (this) {
        ......
        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;
}

Message next() {
    ......
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }
// nativePollOnce方法在native层，若是nextPollTimeoutMillis为-1，这时候消息队列处于等待状态。 　　
        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());
            }
//按照我们设置的时间取出消息
            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 {
// 如果消息队列中没有消息，将nextPollTimeoutMillis设为-1，下次循环消息队列则处于等待状态
                nextPollTimeoutMillis = -1;
            }
           
//退出消息队列，返回null，这时候Looper中的消息循环也会终止。 
            if (mQuitting) {
                dispose();
                return null;
            }
            ......
        }
        .....
    }
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.carson_ho.handlerdemo.MainActivity">
    <TextView
        android:id="@+id/show"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="" />
</RelativeLayout>

package com.example.carson_ho.handlerdemo;
import android.os.Bundle;
import android.os.Handler;
import android.support.v7.app.AppCompatActivity;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
    public TextView show;
    public Handler showhandler;
    @Override
    //主线程创建时便自动创建Looper和对应的MessageQueue,之前执行Loop()进入消息循环
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        show = (TextView) findViewById(R.id.AA);
        StringBuffer text = new StringBuffer();
        text.append("Carson_Ho:Do you love me?");
        show.setText(text);
    //实例化Handler,这里并无指定Looper,即自动绑定当前线程(主线程)的Looper和MessageQueue
        showhandler = new Handler();
    //启动子线程
        new Thread_1().start();
        new Thread_2().start();
    }
    class Thread_1 extends Thread {
        @Override
        public void run() {
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            showhandler.post(new Runnable() {
                @Override
                public void run() {
                    String line = "\n";
                    StringBuffer text = new StringBuffer(show.getText());
                            text.append(line).append("angelababy:Yes,I do");
                            show.setText(text);
                }
                });
            }
        }
    class Thread_2 extends Thread {
        @Override
        public void run() {
            try {
                Thread.sleep(8000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            showhandler.post(new Runnable() {
                @Override
                public void run() {
                    String line = "\n";
                    StringBuffer text = new StringBuffer(show.getText());
                    text.append(line).append("黄晓明:what the fuck?");
                    show.setText(text);
                }
            });
        }
    }
}