Handler的主要作用是将一个任务切换到某个指定的线程去执行。今天从JVM的角度看看它是如何运作起来的。

老规矩先上图

类图

Handler.png

在线程1执行的代码

现在假设一个情景：
有两个线程，线程1和线程2，第一步：在线程1中调用Looper.prepare()。第二步创建一个Handler对象handler。第三步调用Looper.loop()。
在线程2中调用handler.sendMessage()发送消息，那最终这个消息会在线程1中被handler.handleMassage()处理。

第一步：我们看下Looper.prepare方法：

    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));    //只有在线程1可以获取该Looper对象
    }

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

创建了一个Looper对象，并将它设置到ThreadLocal对象之中，那么只能在线程1通过ThreadLocal.get获取到这个Looper对象。对ThreadLocal不熟悉的可以看我的ThreadLocal源码分析。

第二步：创建Handler（在线程1执行）

   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();   //获取线程1的Looper对象
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;     //获取线程1的MessageQueue
        mCallback = callback;
        mAsynchronous = async;
    }

我们在线程1创建了Handler，创建过程中保存了线程1的MessageQueue和Looper的引用。

第三步：调用Looper.run开启事件循环

    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 (;;) {              //无限循环
            Message msg = queue.next(); // might block，从消息队列获取下一个message，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);   //msg.target就是Handler对象，所以这里调用Handler.dispatchMessage处理这个message

            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();
        }
    }

开启无限循环，从消息队列获取消息，调用Handler.dispatchMessage处理消息。
到目前为止都是在线程1执行的，我们看创建了哪些对象。

Handler内存分布.png

这个Handler是所有线程都可访问的，Looper和MessageQueue只能被线程1访问。

在线程2执行的方法

假设这时线程切换到了线程2，线程2中我们调用了handler.sendMessage发送了一个Message。

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;              //message的target指向这个handler
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);   //加入线程1中创建的MessageQueue中
    }

最关键的操作时将message加入线程1中创建的MessageQueue中。

再次切换到线程1

假设线程再次切换到线程1，Looper的loop会从MessageQueue获取刚加入的Message，并调用它target引用的handler的dispatchMessage方法处理当前的message。到这里就达到，在线程2发送消息，在线程1处理消息的目的。是不是很巧妙？

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