Handler消息机制详解

Handler机制是Android开发中最常见的机制，可以说贯穿整个Android，在探究Handler机制原理之前，我们先来捋一下用法

1.handler.post(Runnable)

2.handler.postdelayed(Runnable,int)

3.sendMessage(Message)

4.sendDelayMessage(Message,int)

从形式上就可以看出，第一种用法和第二种用法其实是一样的，只不过一个立即发送一个延迟发送而已，同理三四也是一样

我们先探究handler.post，post具体用法是

handler = new Handler();

hander.post(new runnable(){run(//具体逻辑){}});

首先我们找到Handler源码

    public Handler() {
        this(null, false);
    }

    public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class 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;
        mAsynchronous = async;
    }

代码不多，一个一个解析，当我们handler = new Handler();时，实际上传入了两个参数进入Handler，一个是null，另一个是false

callback是一个接口，里面只有一个方法

    public interface Callback {
        public boolean handleMessage(Message msg);
    }

这里传入的是null，暂且放下不谈。这个函数主要过程是初始化Handler里的变量mLooper，mQueue，mCallback以及mAsynchronous

首先mLooper = Looper.myLooper();从这里拿到一个looper，为什么这里会拿到一个looper对象？从这里就要引出ActivityThread.main方法

一个app启动开始，最先调用的方法是ActivityThread.main方法，如同java里的main方法一样，这个方法是程序的入口，那么这个方法到底干了什么

    public static void main(String[] args) {
        SamplingProfilerIntegration.start();

        // 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();

        // Set the reporter for event logging in libcore
        EventLogger.setReporter(new EventLoggingReporter());

        Security.addProvider(new AndroidKeyStoreProvider());

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

        Process.setArgV0("");

        Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

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

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

        Looper.loop();

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

多余的东西我们不看，我们只看Looper.prepareMainLooper();以及Looper.loop();

    /**
     * Initialize the current thread as a looper, marking it as an
     * application's main looper. The main looper for your application
     * is created by the Android environment, so you should never need
     * to call this function yourself.  See also: {@link #prepare()}
     */
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

这里先调用了prepare（false）

    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));
    }

这里先通过sThreadLocal.get()去查找looper，如果发现线程中以及有looper了就抛出异常，Only one Looper may be created per thread每一个线程中只能有一个loop

这里我们是程序第一次启动，当然没有looper，于是接下来调用new Looper创建一个looper

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

looper里包含了一个mQueue和mThread，将当前线程传入，这里我们是主线程传入

然后将looper放入sThreadLocal中，ThreadLocal是一个数据内部储存类，它的神奇之处就在于，使用ThreadLocal储存的对象在不同线程访问会得出不同的对象

打个比方，我们用static进行变量储存的时候，就像我们玩游戏时的公会银行，谁都可以取，谁也可以存，不管谁取出来都是一样的东西，因此在我们不同线程取出来static变量其实是一样的。而使用ThreadLocal进行储存的话，就像我们现实中的银行，一人一个账户，我不可以取你存进去的东西。这里有什么好处呢，这里就可以保证了我(Thread)取出来的东西(looper)一定是我的。其实说了这么说，ThreadLocal里面的实现也很简单，只不过将当前的Thread作为key传进去，本质上还是键值对的方式进行存储。

好，说了这么多让我们回到主线，prepare方法我们已经执行完了，在里面我们新建了looper并存储在ThreadLocal中

程序继续走下去，走到sMainLooper = myLooper();

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

这里设置了主线程的looper。

综上分析，mLooper = Looper.myLooper();拿到的是主线程的looper。这里就要注意一点，如果我们是在子线程中调用Handler，我们必须给它传入一个looper，因为在子线程中是没有looper的，那我们在子线程中就应该通过Looper.prepare（）拿到一个新的looper，将这个looper传入Handler的构造函数就好了

OK，我们回到handler的构造函数中，我们已经拿到了一个looper对象，接下来我们就可以从Looper中取出mQueue，然后的初始化就没有什么好分析了。

我们来看看handler.post函数为什么明明在子线程中，而它的run方法却是在主线程中

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

这里我们进入getPostMessage看看

    private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

Message.obtain返回一个新的Message对象

    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();
    }

然后将Runnable塞进m.callback里

然后我们接着调用sendMessageDelayed(getPostMessage(r), 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);
    }

最后将mQueue，msg，uptimeMillis一起传入

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

这里的target是一个Handler变量，这里它传入了主线程的Handler

接着走下去

    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("MessageQueue", 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;
    }

这里我们可以看到一个死循环，我们不必拘泥于代码本身，这段代码的意思将manager插入mQueue

代码走到这里仿佛走死了，说好的调用切换线程呢？

不急，还记得我们ActivityThread.main函数吗？里面我们还有一个方法没讲呢，那就是looper.loop（）;

这就是Handler之所以能切换线程的关键所在，让我们走进方法

    /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the 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
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(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();
        }
    }

进入queue.next()方法

    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 (false) Log.v("MessageQueue", "Returning message: " + msg);
                        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("MessageQueue", "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;
        }
    }

这里的next方法是一个无线循环的方法，如果消息队形中没有消息，那么next方法会一直阻塞在这里，当有新消息时，next方法会返回这条消息并将消息删除

其实当初看书看到这里还是有疑问的，作者本身爱较真，奈何能力实在有限，等到很长时间才反应过来。作者是这么理解的，我的方法虽然在子线程调用，但是我将子线程中执行的方法打包成一个Message，然后发送到一个共同的Messagequeue，提醒主线程告诉它我已经发消息了，主线程收到我的提醒以后就去Messagequeue中拿出Message，把Message中的方法调用出来执行，这才是Handler消息机制的原理。很多Handler详解都没有告诉作者原来还有提醒这一步，导致作者原来对Handler机制十分模糊。

提醒主线程这一步是在native层实现的，翻了翻源码，发现只有在enqueueMessage方法中if (needWake) nativeWake(mPtr);比较可疑，只能靠一些边角东西推理了

private long mPtr; // used by native code

而在next方法中有这么一个方法

nativePollOnce(ptr, nextPollTimeoutMillis);

作者猜测这个方法应该是阻塞这个方法并接收nativeWake的了。

扯了这么多，还是回到Looper.loop方法中，我们已经取出Message了，接下来就调用msg.target.dispatchMessage(msg);

    
    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

这就是消息处理的方法，不同的消息发送处理方式是有先后之分的，一开始我们调用的方法中是将runnable放入Message中，所以msg.callback != null

这个方法实现就更简单了

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

直接就调用runnable.run方法，简单粗暴我喜欢，到这里通过handler.post(Runnable)的流程已经梳理完了

接下来我们来看看sendMessage(Message)，其实两种方式原理都是一样的，只不过一开始走的路不太一样

如果我们要用sendMessage方法，我们需要重写Handler.handleMessage方法，让我们进入源码

    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);
    }

剩下的源码都和post是一模一样的，只有结尾是不一样的，那我再贴一遍好了

    
    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

因为我们没有传入Runnable，所以msg.callback == null，而mcallback是Handler初始化时传入的参数，我们调用的不是这个构造方法，所以mcallback也是空，最后调用我们重写的handleMessage方法。

其实Handler使用方法还不止四种，作者只是举出了几个常见的，例如还有view.post方法，或者new一个实现了callback接口的对象，将callback传入Handler一样能异步调用，这里就不再多说了。当然，最常见的还是直接在子线程中调用runOnUIThread方法，该方法里的逻辑代码全部运行在主线程中。

最后再说一点其他的，毕竟文章写了是详解

1.假如我们需要消息回传怎么办呢，我不满足子线程光发送消息给主线程，我还想主线程发消息给子线程？

我们回看当初子线程给主线程发送消息的机制，其实是子线程拿到了主线程的looper，从Looper中取出了mqueque，然后两个线程通过mqueue进行通信

所以如果主线程要想发消息给子线程，就一定要拿到子线程的looper。子线程的looper怎么创建在上文已经说了，最后还要调用Looper.loop方法，不然光主线程做好发消息的准备了，子线程还没做好接收消息的准备。

2.Handler中还有其他构造方法，比如说传入looper对象进行构造，当主线程给子线程Thread进行通信时，调用Thread.loop可能拿不到子线程的loop，因为这个时候子线程的loop可能还没创建，这时候又该怎么办？

换个构造方法！！！！好吧，你不想换怎么办，那就使用HandlerThread，HandlerThread是一个扩展了Thread的类，专门用于Handler通信，在HandlerThread的run方法里自带创建looper和loop.loop

public class HandlerThread extends Thread

HandlerThread中有一个getloop方法，我们来看一下

    public Looper getLooper() {
        if (!isAlive()) {
            return null;
        }
        
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }

如果Thread还没有开始，那么就会调用wait方法，那么何时被唤醒呢

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }

在HandlerThread的notifyAll中，调用这个方法后线程就不再处于wait状态，于是主线程的Handler就可以获取到子线程的loop

3.非UI线程真的不能更新ui吗，平常我们都是用Handler来更新UI的，可是真的如此吗？

并不是，我们之所以不让子线程更新UI，是因为有一个并发的问题，Android为了解决这个问题，就在View类里更新UI的方法进行了判断，判断该线程是不是主线程，可是View的实现类ViewRootImp是在OnResume中初始化的额，于是，当我们在oncrate中创建线程并更新UI的时候就有可能在OnResume方法之前完成，因此凡事都不是绝对的

详解到这里就结束了，要是解决了你的疑惑，别忘了顶一个，这是对作者的肯定