Android Handler源码分析

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Handler用了很久了,也看了很久,东西多了容易忘记一些,所以最近会记录一些源码相关的,防止忘记.
:Handler平时我们经常使用,因为大部分都是用来更新UI,其实Handler并不是只能用来更新UI,还有其他的功能,本篇文章主要是分析一下,便于忘记的时候随时来看.

主要分析包含

• Message
• Looper
• MessageQueue
• Handler

---

初始化消息队列

Android 开发是不允许在非主线更新UI,因为其他线程也可以更新就会导致线程不安全,如果要安全就需要加锁,加锁后就会导致更新效率低下,本来手机就需要在短时间做出快速的UI反应,所以才会有主线程去更新UI,并不是其他线程就不能更新.那么要在子线程中使用Handler,可以如下:

    public void run() {
        mTid = Process.myTid(); // 获取当前线程id.
        Looper.prepare(); //调用Looper的Prepare干什么的.?
        synchronized (this) {
            mLooper = Looper.myLooper(); // myLooper又是干什么的.?
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared(); // callBack
        Looper.loop(); // loop
        mTid = -1;
    } 

那我们先看看Looper.prepare方法:

    public static void prepare() {
        prepare(true);
    }

    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) { //sThreadLocal是一个用来线程共享数据的
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));//可以看到创建了一个新的Looper并设置到了sThreadLocal中.
    }

private static void prepare(boolean quitAllowed)方法中,有一个参数quitAllowed,该参数大致可以猜测出是否退出,默认是可以退出的.但是在主线程中的prepareMainLooper是false的.接下来我们再看看Looper初始化做了什么事情:

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);//初始化了一个MessageQueue,并存在了Handler的成员变量里面.
        mRun = true;
        mThread = Thread.currentThread();
    }

    MessageQueue(boolean quitAllowed) {
        mQuitAllowed = quitAllowed;
        nativeInit(); // 初始化了native的MessageQueue和Looper.
    } 

上面可以看到nativeInit方法,其实在Android系统中并不是只有java层用到了Looper,在FrameWork也用到了的.所以需要去nativeInit();

发送消息

通过Looper.prepare初始化好消息队列后就可以调用Looper.loop进入消息循环了，然后我们就可以向消息队列发送消息，消息循环就会取出消息进行处理，在看消息处理之前，先看一下消息是怎么被添加到消息队列的。

在Java层，Message类表示一个消息对象，要发送消息首先就要先获得一个消息对象，Message类的构造函数是public的，但是不建议直接new Message，Message内部保存了一个缓存的消息池，我们可以用obtain从缓存池获得一个消息，Message使用完后系统会调用recycle回收，如果自己new很多Message，每次使用完后系统放入缓存池，会占用很多内存的，如下所示：

   public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool; // 从Pool中获取一个Message
                sPool = m.next;// 获取下一个Message
                m.next = null; // 置空,这里的置空是将当前的与Pool中的其他Message断开,为什么.?因为pool中存放Message其实是链表的形式
                sPoolSize--; // 减一
                return m;
            }
        }
        return new Message();//如果没有,才会去创建一个新的消息
    }

    public void recycle() {
        clearForRecycle();

        synchronized (sPoolSync) {
            if (sPoolSize < MAX_POOL_SIZE) {
                next = sPool; // 将链串起来了.
                sPool = this;
                sPoolSize++;
            }
        }
    }

消息对象获取到了怎么发送呢，大家都知道是通过Handler的post、sendMessage等方法，其实这些方法最终都是调用的同一个方法sendMessageAtTime:

    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); // 往MessageQueue中添加一个消息.
    } 

那么我们看看enqueueMessage方法:

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis); // 真正的添加消息
    }

那我们看看queue.enqueueMessage方法:

final boolean enqueueMessage(Message msg, long when) {
        if (msg.isInUse()) {
            throw new AndroidRuntimeException(msg + " This message is already in use.");
        }
        if (msg.target == null) {
            throw new AndroidRuntimeException("Message must have a target.");
        }

        boolean needWake;
        synchronized (this) {
            if (mQuiting) {
                RuntimeException e = new RuntimeException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                return false;
            }

            msg.when = when;
            Message p = mMessages;
            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;
            }
        }
        if (needWake) {
            nativeWake(mPtr);
        }
        return true;
    }

在enqueueMessage中首先判断，如果当前的消息队列为空，或者新添加的消息的执行时间when是0，或者新添加的消息的执行时间比消息队列头的消息的执行时间还早，就把消息添加到消息队列头（消息队列按时间排序），否则就要找到合适的位置将当前消息添加到消息队列。

消息循环

消息队列初始化好了，也知道怎么发消息了，下面就是怎么处理消息了，看Handler.loop函数:

    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 (;;) {
        // next方法就是去从MessageQueue中取消息
            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.recycle();
        }
    }

loop每次从MessageQueue取出一个Message，调用msg.target.dispatchMessage(msg)，target就是发送message时跟message关联的handler，这样就调用到了熟悉的dispatchMessage，Message被处理后会被recycle。当queue.next返回null时会退出消息循环，接下来就看一下MessageQueue.next是怎么取出消息的，又会在什么时候返回null。其实Looper.loop()方法中简单理解就是两个死循环.

final Message next() {
        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;

        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }
            nativePollOnce(mPtr, nextPollTimeoutMillis);

            synchronized (this) {
                if (mQuiting) {
                    return null;
                }

                // 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);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                // 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;
        }
    }