Handler

Handler的声明：

    Handler handler = new Handler(new Handler.Callback() {
        @Override
        public boolean handleMessage(Message msg) {
            return false;
        }
    });

其中构造方法可以不传参数，可以像上面一样传入Handler.Callback,也可以传入Looper

使用：
```
handler.post(new Runnable() {
    @Override
    public void run() {
        Log.d(TAG, "run: currentThread:"+Thread.currentThread());
    }
});
```
这里打印的线程是主线程

还可以使用hander.sendMessage等一系列方法，这种方式会回调创建匿名类中的handleMessage方法中。

不管是post还是sendMessage，最终都会调用到Handler的sendMessageAtTime方法：

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

可以发现是往queue消息队列中加入了一条消息！！！

那么当有消息来的时候，Looper会调用Handler的dispatchMessage方法：
```
public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
```
其中msg.callback是我们post方法传入的runnable对象。

mCallback是创建Handler的时候传入的Callback对象。

所以我们发送消息时候会回调我们实现的Callback中的handleMessage方法。

最后的handleMessage方法是一个空方法。

MessageQueue

MessageQueue是消息队列，但是内部实现为单链表，主要是通过next方法取消息，并将消息移出队列。next方法中是一个无限的循环，如果没有消息，将会阻塞。

enqueueMessage方法用于插入消息到队列之中。

Looper

looper是消息循环，不断的从MessageQueue中查看是否有新消息，如果有就立刻处理，如果没有就阻塞。

在子线程中创建一个Looper：

        new Thread("Thread #2"){
            @Override
            public void run() {
                Looper.prepare(); #为当前线程创建一个Looper
                Handler handler = new Handler();
                Looper.loop(); #开启消息循环  会阻塞线程
            }
        }.start();

Looper的prepare方法会转到另一个重载方法，将Looper对象加入sThreadLocal：

    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声明如下，泛型是Looper：

// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

然后new Handler()最终会转到这个构造方法：

public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
        ...
        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;
    }

其中的Looper.myLooper()的方法实现如下：

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

所以，如果在子线程中没有调用Looper.prepare()，那么sThreadLocal里面保存的就会为空，那么会抛出运行时异常：java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

接下来看看loop方法：

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

            msg.target.dispatchMessage(msg);//分发消息

            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();
        }
    }

可以看到loop方法里面有一个无限的循环，一直向queue消息队列获取消息，如果没有消息，那么会阻塞，有了消息之后会调用msg.target.dispatchMessage(msg);分发消息，msg.target就是Handler对象，这个在一开始就分析了Handler分发消息的过程。