[Android] Handler源码解析 (Java层)

*** 之前写过一篇文章,概述了Android应用程序消息处理机制。本文在此文基础上,在源码级别上展开进行概述***

简单用例

Handler的使用方法如下所示:

Handler myHandler = new Handler() {  
          public void handleMessage(Message msg) {   
               switch (msg.what) {   
                    ...  
               }
          }   
     };

class myThread implements Runnable {   
          public void run() {  
               while (!Thread.currentThread().isInterrupted()) {    

                    Message message = Message.obtain();   
                    message.what = TestHandler.GUIUPDATEIDENTIFIER;
                    TestHandler.this.myHandler.sendMessage(message);   
                    message.recycle();
                    try {   
                         Thread.sleep(100);    
                    } catch (InterruptedException e) {   
                         Thread.currentThread().interrupt();   
                    }   
               }   
          }   
     }

或者:

mHandler=new Handler();
mHandler.post(new Runnable(){

    void run(){
       ...
     }
});

又或者:

class LooperThread extends Thread {
    public Handler mHandler;

    public void run() {
        Looper.prepare();

        mHandler = new Handler() {
            public void handleMessage(Message msg) {
                // process incoming messages here
            }
        };

        Looper.loop();
    }
}

源码解析

首先看其构造函数:

new Handler()
...
public Handler() {
    this(null, false);
}
...
public Handler(Looper looper, Callback callback) {
    this(looper, callback, false);
}
...
public Handler(Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) { // 默认为false,若为true则会检测当前handler是否是静态类
        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());
        }
    }
    // 1. 获得当前线程的looper
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
    //2. 获得looper上的message queue
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

由此引入了两个关键对象Looper和MessageQueue。

Looper

先看 mLooper = Looper.myLooper(); 这一句发生了什么:

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

可以看到,该方法返回一个sThreadLocal对象中保存的Looper。关于ThreadLocal类,请参考这里,本文不展开。
如果尚未在当前线程上运行过Looper.prepare()的话,myLooper会返回null。接下来看看Looper.prepare()的实现:

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));
}

可以看到该方法只是简单地新建了一个Looper对象,并将其保存在sThreadLocal中。接下来看一下Looper的构造函数。

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

调用完Looper.prepare()后,需调用Looper.loop()才能使消息循环运作起来,其源码如下所示:

public static void loop() {
    final Looper me = myLooper(); //1. 取出looper对象
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue; //2. 取出looper绑定的message queue

    // 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();
    // loop time.
    long tm = 0;
    ...
    for (;;) {
        Message msg = queue.next(); // 3. 堵塞式在message queue中取数据
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }

        ...
        msg.target.dispatchMessage(msg); 4. 分发message到指定的target handler

        ...

        // 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(); // 5. 回收message对象
        ...
    }
}

可以简单地将Looper.loop()理解成一个不断检测message queue是否有数据,若有即取出并执行回调的死循环。 接下来看一下Message类:

public final class Message implements Parcelable {

    public int what;

    public int arg1;

    public int arg2;

    public Object obj;

    ...

    /*package*/ int flags;

    /*package*/ long when;

    /*package*/ Bundle data;

    /*package*/ Handler target;

    /*package*/ Runnable callback;

    /*package*/ Message next;

    private static final Object sPoolSync = new Object();
    private static Message sPool;
    private static int sPoolSize = 0;
}

what、arg1、arg2这些属性本文不作介绍,我们把目光集中在next、sPoolSync、sPool、sPoolSize这四个静态属性上。

当我们调用Message.obtain()时,返回了一个Message对象。Message对象使用完毕后,调用recycle()方法将其回收。其中obtain方法的代码如下所示:

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();
}

可以看到,obtain方法被调用时,首先检测sPool对象是否为空,若否则将其当做新的message对象返回,并“指向”message对象的next属性,sPoolSize自减。可以看出message对象通过next属性串成了一个链表,sPool为“头指针”。再来看看recycle方法的实现:

public void recycle() {
    if (isInUse()) {
        if (gCheckRecycle) {
            throw new IllegalStateException("This message cannot be recycled because it "
                    + "is still in use.");
        }
        return;
    }
    recycleUnchecked();
}

/**
 * Recycles a Message that may be in-use.
 * Used internally by the MessageQueue and Looper when disposing of queued Messages.
 */
void recycleUnchecked() {
    // Mark the message as in use while it remains in the recycled object pool.
    // Clear out all other details.
    flags = FLAG_IN_USE;
    what = 0;
    arg1 = 0;
    arg2 = 0;
    obj = null;
    replyTo = null;
    sendingUid = -1;
    when = 0;
    target = null;
    callback = null;
    data = null;

    synchronized (sPoolSync) {
        if (sPoolSize < MAX_POOL_SIZE) {
            next = sPool;
            sPool = this;
            sPoolSize++;
        }
    }
}

如果message对象不是处于正在被使用的状态,则会被回收。其属性全部恢复到原始状态后,放在了链表的头部。sPool对象“指向”它,sPoolSize自增。

综上可以看出,通过obtain和recycle方法可以重用message对象。通过操作next、sPoolSync、sPool、sPoolSize这四个属性,实现了一个类似栈的对象池。

msg.target为handler类型,即向handler成员的dispatchMessage方法传入msg参数,其实现如下所示:

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

这里可以看到回调了各种接口。

到目前为止,我们知道了如何处理在消息队列里面的msg对象,但仍不知道msg对象是如何放到消息队列里面的。通常来说,我们通过Handler的sendMessage(msg)方法来发送消息,其源码如下所示:

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

可知sendMessage最终会调用queue.enqueueMessage(msg, uptimeMillis)将msg对象保存至message queue中,uptimeMillis表示msg执行回调的时刻。 我们来看一下MessageQueue类的enqueueMessage方法:

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;
        }

        // 1.设置当前msg的状态
        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        boolean needWake;

        // 2.检测当前头指针是否为空(队列为空)或者没有设置when 或者设置的when比头指针的when要前
        if (p == null || when == 0 || when < p.when) {

            // 3. 插入队列头部,并且唤醒线程处理msg
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {

            //4. 几种情况要唤醒线程处理消息:1)队列是堵塞的 2)barrier,头部结点无target 3)当前msg是堵塞的
            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;
                }
            }

            // 5. 将当前msg插入第一个比其when值大的结点前。
            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;
}

结合注释,我们可以了解到msg push到queue中时,queue的状态的变化和处理队列的逻辑。

前文中Looper对象的loop方法中:

for (;;) {
    ...
    Message msg = queue.next(); // 3. 堵塞式在message queue中取数据
    if (msg == null) {
        // No message indicates that the message queue is quitting.
        return;
    }
    ...
    msg.target.dispatchMessage(msg); 4. 分发message到指定的target handler

    ...
}

可以看出,message queue的next方法被调用时,可能会发生堵塞。我们来看一看message queue的next方法:

Message next() {
    // 1. 判断当前loop是否已经使用过,下文会解释这个mPtr
    final long ptr = mPtr;
    if (ptr == 0) {
        return null;
    }

    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;

    // 2. 进入死循环,直到获取到合法的msg对象为止。
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands(); // 这个是什么?
        }

        // 3. 进入等待,nextPollTimeoutMillis为等待超时值
        nativePollOnce(ptr, nextPollTimeoutMillis);

        synchronized (this) {
            // 4. 获取下一个msg
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {

                // 当前节点为barrier,所以要找到第一个asynchronous节点
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {

                    // 当前队列里最早的节点比当前时间还要晚,所以要进入堵塞状态,超时值为nextPollTimeoutMillis
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    // 删除当前节点,并返回
                    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 {
                // 头结点指向null
                nextPollTimeoutMillis = -1;
            }

            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }

            // 5. 如果当前状态为idle(就绪),则进入idle handle的代码块
            //    进入idle的情况有:队列为空;队列头元素blocking;
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            if (pendingIdleHandlerCount <= 0) {
                // 6. 本轮唤醒(next被调用)时没处理任何东西,故再次进入等待。
                mBlocked = true;
                continue;
            }

            if (mPendingIdleHandlers == null) {
                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
            }
            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
        }

        // 在一次next调用中,这个代码块只会执行一次
        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 {
                // 如果返回true,则idler被保留,下次next的idle时会被调用。
                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;
    }
}

代码执行流程见注释。其中IdleHandler是一个接口:

/**
 * Callback interface for discovering when a thread is going to block
 * waiting for more messages.
 */
public static interface IdleHandler {
    /**
     * Called when the message queue has run out of messages and will now
     * wait for more.  Return true to keep your idle handler active, false
     * to have it removed.  This may be called if there are still messages
     * pending in the queue, but they are all scheduled to be dispatched
     * after the current time.
     */
    boolean queueIdle();
}

IdleHandler提供了一个在MessageQueue进入idle时的一个hook point。更多时与barrier机制一起使用,使message queue遇到barrier时产生一个回调。

总结

前面涉及到的几个主要的类Handler、Looper、MessageQueue和Message的关系如下所述:
1. Handler负责将Looper绑定到线程,初始化Looper和提供对外API。
2. Looper负责消息循环和操作MessageQueue对象。
3. MessageQueue实现了一个堵塞队列。
4. Message是一次业务中所有参数的载体。

框架图如下所示:

            +------------------+
            |      Handler     |
            +----^--------+----+
                 |        |
        dispatch |        | send
                 |        |
                 |        v
                +----+ <---+
                |          |
                |  Looper  |
                |          |
                |          |
                +---> +----+
                  ^      |
             next |      | enqueue
                  |      |
         +--------+------v----------+
         |       MessageQueue       |
         +--------+------+----------+
                  |      |
  nativePollOnce  |      |   nativeWake
                  |      |
+-----------------v------v---------------------+ Native Layer

最后,留意到MessageQueue中有4个native方法:

// 初始化和销毁
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
// 等待和唤醒
private native static void nativePollOnce(long ptr, int timeoutMillis);
private native static void nativeWake(long ptr);
// 判断native层的状态
private native static boolean nativeIsIdling(long ptr);

将会在后续文章中进行介绍。

“[Android] Handler源码解析 (Java层)”的一个回复

发表评论

您的电子邮箱地址不会被公开。

此站点使用Akismet来减少垃圾评论。了解我们如何处理您的评论数据