Android Handler消息机制分析

目录

• Handler是什么？
• Handler 的基本使用
• 用法一：通过 send 方法
• 用法二：通过 post 方法
• Handler 类
• MessageQueue 类
• Looper 类
• Handler 的消息接收过程

Handler是什么？

Handler 是一个可以实现多线程间切换的类，通过 Handler 可以轻松地将一个任务切换到 Handler 所在的线程中去执行。我们最常用的使用的场景就是更新 UI 了，比如我们在子线程中访问网络，拿到数据后我们 UI 要做一些改变，如果此时我们直接访问 UI 控件，就会触发异常了。这个时候我们往往会通过 Handler 将更新 UI 的操作切换到主线程中。

Handler 的基本使用

用法一：通过 send 方法

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    private MyHandler mMyHandler = new MyHandler();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Thread(new Runnable() {
            @Override
            public void run() {
                Message message = Message.obtain(mMyHandler,0,"通过 send 方法");
                mMyHandler.sendMessage(message);
            }
        }).start();
    }

    private static class MyHandler extends Handler{
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what){
                case 0:
                    Toast.makeText(MainActivity.this,msg.obj.toString(),Toast.LENGTH_SHORT).show();
                    break;
            }
        }
    }
}

用法二：通过 post 方法

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    private Handler mMyHandler = new Handler();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Thread(new Runnable() {
            @Override
            public void run() {
                mMyHandler.post(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(MainActivity.this,"通过post方法",Toast.LENGTH_SHORT).show();
                    }
                });
            }
        }).start();
    }
}

其实，通过 post 方法最后通过 send 方法来完成的。这个我们稍后会分析。讲到 Handler，我们不得不提起 MessageQueue 类 和 Looper 类。 Handler 通过 send 方法 发送一个消息，会调用 MessageQueue 的 enqueueMessage 方法 将这个消息插入到 MessageQueue 中，然后 Looper 发现有消息来临时，通过一系列的方法调用后，Handler 如果是通过 post 方法就会执行 post 方法里面的 Runnable ，如果是通过 send 方法就会执行 Handler 的 handleMessage 。这么说感觉有点云里雾里的，让我们仔细的来看下 Handler 类、MessageQueue 类和 Looper 类。

Handler 类

我们先来看下 Handler 类的结构

Handler 类结构.png

Handler 的工作主要包括消息的发送和接收过程。一般来说，消息的发送和消息的接收是位于不同的线程。我们首先来看 post 方法。

/**
 * Causes the Runnable r to be added to the message queue.
 * The runnable will be run on the thread to which this handler is
 * attached.
 *
 * @param r The Runnable that will be executed.
 *
 * @return Returns true if the Runnable was successfully placed in to the
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.
 */
public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

这里调用了 sendMessageDelayed 方法

/**
 * Enqueue a message into the message queue after all pending messages
 * before (current time + delayMillis). You will receive it in
 * {@link #handleMessage}, in the thread attached to this handler.
 *
 * @return Returns true if the message was successfully placed in to the
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.  Note that a
 *         result of true does not mean the message will be processed -- if
 *         the looper is quit before the delivery time of the message
 *         occurs then the message will be dropped.
 */
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

而 sendMessageDelayed 又调用了 sendMessageAtTime（） 方法

/**
 * Enqueue a message into the message queue after all pending messages
 * before the absolute time (in milliseconds) <var>uptimeMillis</var>.
 * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
 * Time spent in deep sleep will add an additional delay to execution.
 * You will receive it in {@link #handleMessage}, in the thread attached
 * to this handler.
 *
 * @param uptimeMillis The absolute time at which the message should be
 *         delivered, using the
 *         {@link android.os.SystemClock#uptimeMillis} time-base.
 *
 * @return Returns true if the message was successfully placed in to the
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.  Note that a
 *         result of true does not mean the message will be processed -- if
 *         the looper is quit before the delivery time of the message
 *         occurs then the message will be dropped.
 */
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);
}

千呼万唤始出来，在 sendMessageAtTime 这个方法我们终于看到了 MessageQueue 类，这里的逻辑主要向 MessageQueue 中插入了一条消息（Message）。咦？我们不是通过 post 方法传进来的 Runnable 么？什么时候变成 Message 了？其实刚才我们忽略了一个方法。

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

没错，就是 getPostMessage 方法

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

从这里看到，系统通过调用 Message.obtain() 创建一个 Message，并把我们通过 post 方法传进来的 Runnable 赋值给 Message 的 callback。这里的 callback 需要留意，这个在我们之后的分析会用到。接下里我们看 Handler 的 send 方法。

/**
 * Pushes a message onto the end of the message queue after all pending messages
 * before the current time. It will be received in {@link #handleMessage},
 * in the thread attached to this handler.
 *
 * @return Returns true if the message was successfully placed in to the
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.
 */
public final boolean sendMessage(Message msg)
{
    return sendMessageDelayed(msg, 0);
}

是不是很熟悉？post 方法也是调用这个 sendMessageDelayed 方法，这也是为什么我们之前说 post 方法 也是通过 send 方法来执行的。到此为止，我们已经弄懂 Handler 的消息发送过程。总结的来说，通过 post 方法系统会把 我们传进来的 Runnable 转变成 Message，然后就和 send 方法一样，通过一系列的方法调用之后把 Message 插入到 MessageQueue 当中。至于 Handler 的消息接收过程，我们暂且放一下，先来看 MessageQueue 类。

MessageQueue 类

前面说到，Handler 发送消息的过程就是往 MessageQueue 中插入 一个 Message，即调用 MessageQueue 的 enqueueMessage 方法。首先，我们来看下 MessageQueue 的类结构

MessageQueue类结构.png

我们看到 MessageQueue 是比较简单的。其实，MessageQueue 主要包含两个操作：插入和读取。

插入方法:enqueueMessage

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;
}

读取方法:next

需要注意的是：读取操作本身会伴随着删除操作

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;
    }
}

Looper 类

首先，我们也来看下 Looper 的类结构

Looper类结构.png

关于 Looper ，我们首先要明确一点，Looper 是线程相关的，即每个线程的 Looper 是不一样的，但是线程默认是没有 Looper 的。可能会有点绕，要理清这里面的逻辑的关系，我们首先要了解 ThreadLocal，关于 ThreadLocal 网上的资料挺多的。简单地来说，ThreadLocal 是一个线程内部的数据存储类，比如有有一个 int 类型的 x，在线程 A 的值是 1，在线程 B 的值可以是 0,1，2,..,在线程 C 的值可以是 0,1,2... 我们来看下 Looper 相关的源码

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

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));
}

/**
 * Return the Looper object associated with the current thread.  Returns
 * null if the calling thread is not associated with a Looper.
 */
public static Looper myLooper() {
    return sThreadLocal.get();
}

我们为什么要明确 Looper 是线程相关的呢？因为 Handler 创建的时候会采用当前线程的 Looper 来构造消息循环系统的。Handler 创建的时候要先创建 Looper，这时候疑问就来了？我们平常创建 Handler 的时候直接就创建了啊，没有创建什么 Looper 啊。这是因为我们通常是在主线程 ActivityThread 中创建 Handler。我们看到 Loop 类中有个 prepareMainLooper 方法。

/**
 * 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();
    }
}

主线程在创建时，就会调用这个方法创建 Looper。但是如果我们在子线程（如下代码）直接创建 Handler 就会抛出异常

        new Thread(new Runnable() {
            @Override
            public void run() {
                //Looper.prepare();
                Handler handler = new Handler();
               // Looper.loop();
            }
        }).start();

这时只要我们把注释去掉就不会报异常了。通过源码我们知道 Looper.prepare() 主要是为当前线程一个 Looper 对象。

 /** Initialize the current thread as a looper.
  * This gives you a chance to create handlers that then reference
  * this looper, before actually starting the loop. Be sure to call
  * {@link #loop()} after calling this method, and end it by calling
  * {@link #quit()}.
  */
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));
}

那么，Looper.loop（）方法是干什么的呢？其实，Looper 最重要的一个方法就是 loop 方法了。只有调用 loop 后，消息系统才会真正地起作用。我们来看 loop 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();
    }
}

我们可以看到 loop 方法是一个死循环，在这个死循环方法里面会调用 MessageQueue 的 next 方法来获取新消息。但是如果 next 方法返回了 null，loop 就退出循环。这种情况发生在 Loop 的 quit 方法被调用时，Looper 会 调用 MessageQueue 的 quit 方法来通知消息队列退出，当消息队列被标记退出状态时，它的 next 方法就会返回 null。由于 next 是一个阻塞方法，所以 loop 也会一直阻塞在那里，如果有消息到来， msg.target.dispatchMessage(msg)。这个 msg.target 就是发送这个消息的 Handler 对象啦。这样 Handler 发送的消息最终又交给自己的 dispatchMessage 方法来处理了。因为 Handler 的 dispatchMessage 方法是创建 Handler 时使用的 Looper 中执行的，这样就成功地完成线程切换了。

Handler 的消息接收过程

经过跋山涉水，通过 Handler 发送的消息最终又会回到自己的 diapatchMessage 中来，那就让我们来看下 diapatchMessage 方法。

/**
 * 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);
    }
}

首先，检查 Messgae 的 callback 是否为 null，不为 null 就调用 handleCallback 方法，这个 Message 的 callback 就是我们之前post的。其次，检查 mCallback 是否为 null ，不为 null 就调用 mCallback 的 handleMessage 方法来处理消息。如果我们是通过继承 Handler 来实现逻辑的话，此时的mCallback 是为空的，即会调用 handleMessage(msg)，也就是我们重写的 handleMessage 方法。至此，完成了完美的闭环。

有的同学可能会疑问 mCallback 是什么？什么时候会为空？

 /**
 * Callback interface you can use when instantiating a Handler to avoid
 * having to implement your own subclass of Handler.
 *
 * @param msg A {@link android.os.Message Message} object
 * @return True if no further handling is desired
 */
public interface Callback {
    public boolean handleMessage(Message msg);
}

 /**
 * Constructor associates this handler with the {@link Looper} for the
 * current thread and takes a callback interface in which you can handle
 * messages.
 *
 * If this thread does not have a looper, this handler won't be able to receive messages
 * so an exception is thrown.
 *
 * @param callback The callback interface in which to handle messages, or null.
 */
public Handler(Callback callback) {
    this(callback, false);
}

通过源码可以看出，我们也可以采用 Handler handler = new Handler(callback) 来创建 Handler，这时dispatchMessage 里面就会走 mCallback 不为空的逻辑。

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