Android Handler:全面,详细解读

有时我们需要在子线程中进行耗时的I/O操作,可能是读取文件或者访问网络等,当耗时操作完成后可能需要UI上做一些改变,由于android的开发规范限制,我们不能在子线程中访问UI控件,否则会触发异常,这个时候通过Handler就可以将更新UI的操作切换到主线程中。

概述

Handler的运行需要底层的MessageQueue和Looper的支撑。MessageQueue的中文翻译是消息队列,它的内部存储了一组消息,以队列的形式对外提供插入和删除工作。虽然叫消息队列,但内部存储结构并不是真正的队列,而是采用单链表的数据结构来存储消息列表。Looper的中文翻译为循环,由于MessageQueue只是一个消息的存储器,它不处理消息,而Looper填补了这功能。Looper中还有一个特殊的概念,那就是ThreadLocal,ThreadLocal并不是线程,它的作用是可以在每个线程中存储数据。我们知道Handler创建的时候会采用当前线程的Looper来构造消息循环系统,那么Handler内部如何获取当前线程Looper呢,这就要用到ThreadLocal了,ThreadLocal可以在不同线程中互不干扰地存储并提供数据,通过ThreadLocal可以轻松获得每个线程的Looper。当然需要注意的是,线程默认没有Looper,如果需要使用Handler就必须为线程创建Looper。我们的UI线程,它就是ActivityThread,ActivityThread被创建时就会初始化Looper,这也是在主线程中默认可以使用Handler的原因。如果需要在子线程中使用Handler,需如下操作: 创建Looper

new Thread(new Runnable() {  
            public void run() {  
                Looper.prepare();  
                Handler handler = new Handler(){  
                    @Override  
                    public void handleMessage(Message msg) {  
                        Toast.makeText(getApplicationContext(), "handler msg", Toast.LENGTH_LONG).show();  
                    }  
                };  
                handler.sendEmptyMessage(1);  
                Looper.loop();  
            };  
        }).start(); 

Handler创建完毕后,这个时候其内部的Looper以及MessageQueue就可以和Handler一起协同工作了,然后通过Handler的Handler的send方法发送一个消息到Looper中去处理,也可以通过post方法将一个Runnable投递到Looper中。其实post方法最终也是通过send方法调用的,它会调用MessageQueue的enqueueMessage方法将这个消息放入消息队列中,然后Looper发现有新消息到来时,就会处理这个消息,最终消息中的Runnable或者Handler的handleMessage方法就会调用。注意Looper是运行在创建Handler所在的线程中的,这样一来Handler的业务逻辑就被切换到创建Handler所在的线程中去执行了,这个过程可以用下图表示。

Handler的工作过程.png
相关概念

关于 Handler 异步通信机制中的相关概念如下:

ThreadLocal,Message、Message Queue、Looper,接下来结合源码分析它们的工作原理。

ThreadLocal的工作原理

ThreadLocal是一个线程内部的数据存储类,通过它可以在指定的线程中存储数据,数据存储后,只有在指定的线程中可以获取存储的数据,对于其他线程则无法获取到数据。 在日常开发中用到ThreadLocal的地方较少, 一般来说,当某些数据是以线程为作用域并且不同线程具有不同数据副本时,可以考虑采用ThreadLocal。比如对于Handler来说,它需要获取当前线程的Looper,很显然Looper的作用域就是线程并且不同线程具有不同的Looper,这个时候通过ThreadLocal就可以轻松实现Looper在线程中存取。下面通过实际的例子来演示ThreadLocal的真正含义。首先定义一个ThreadLocal对象,选择Boolean类型,如下所示。

 private ThreadLocal mBooleanThreadLocal=new ThreadLocal();

然后分别在主线程,子线程1和子线程2中设置和访问它的值,代码如下:

  mBooleanThreadLocal.set(true);
        Log.d("ThreadLocal","mainThread="+mBooleanThreadLocal.get());
        
        new Thread("Thread1"){
            @Override
            public void run() {
                mBooleanThreadLocal.set(false);
                Log.d("ThreadLocal","thread1="+mBooleanThreadLocal.get());
            }
        }.start();

        new Thread("Thread2"){
            @Override
            public void run() {
                Log.d("ThreadLocal","thread2="+mBooleanThreadLocal.get());
            }
        }.start();

上述代码中,子线程设置为true,子线程1中设置false,子线程2中不设置值,日志如下:

ThreadLocal: mainThread=true
ThreadLocal: thread1=false
ThreadLocal: thread2=null

从上面日志看,不同线程中访问的同一个ThreadLocal对象,获取值却不一样。不同线程访问同一个ThreadLocal的get方法,ThreadLocal内部会从各自线程中取出一个数组,然后再从数组中根据当前ThreadLocal的索引去查找对应的value值。下面我们来看看set和get方法,首先看ThreadLocal的set方法,如下:

 public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

从上面的set方法中,首先通过getMap方法获取当前线程中的ThreadLocalMap数据,如果为空就对其初始化。我们再看看 map.set方法

   private void set(ThreadLocal key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal k = e.get();

                if (k == key) {
                    e.value = value;
                    return;
                }

                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }

            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

我们再来看看get方法

 public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

ThreadLocal的get方法,同样是取出当前线程的ThreadLocalMap对象,如果这个对象为null就返回初始值,初始值由ThreadLocal的initialValue方法来描述,默认为null。

MessageQueue的工作原理

主要包含插入和读取操作,对应的方法分别为enqueueMessage和next。enqueueMessage源码如下:

 boolean enqueueMessage(Message msg, long when) {
        synchronized (this) {
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w(TAG, 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;
    }

从enqueueMessage的实现来看,主要是单链表的插入操作,下面看一下next的源码:

 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 (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        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(TAG, "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;
        }
    }

可以发现next方法是一个无限循环的方法,如果消息队列中没有消息,那么next方法一直阻塞在这里,当有新消息时,next方法会返回这条消息。

Looper的工作原理

Looper会不停地从MessageQueue中查看是否有新消息,如果有新消息就立即处理,否则就一直阻塞在那里,首先看下它的构造方法,在构造方法中会创建一个MessageQueue,然后将当前线程的对象保存起来。

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

我们知道Handler的工作需要Looper,没有Looper的线程会报错,上面也讲述了如何为线程创建Looper,通过Looper.prepaer()即可为当前线程创建一个Looper,通过Looper.loop()来开启消息循环,如下:

new Thread("Thread1"){
            @Override
            public void run() {
                Looper.prepare();
                Handler handler=new Handler();
                Looper.loop();
            }
        }.start();

Looper除了prepare方法外,还提供了prepareMainLooper()方法,这个方法主要是给主线程创建Looper使用的,其本质也是通过prepare方法。由于主线程的Looper比较特殊,所以Looper提供了一个getMainLooper()方法,通过它可以在任何地方获取主线程的Looper。Looper最重要的一个方法是loop方法,只有调用了loop后,消息循环系统才会真正起作用,如下:

 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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            final long end;
            try {
                msg.target.dispatchMessage(msg);
                end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (slowDispatchThresholdMs > 0) {
                final long time = end - start;
                if (time > slowDispatchThresholdMs) {
                    Slog.w(TAG, "Dispatch took " + time + "ms on "
                            + Thread.currentThread().getName() + ", h=" +
                            msg.target + " cb=" + msg.callback + " msg=" + msg.what);
                }
            }

            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();
        }
    }

Looper的loop方法是一个死循环,会调用MessageQueue的next方法来获取新消息,而next是一个阻塞操作,当没有消息时,next方法会一直阻塞在那里,这也导致loop方法会一直阻塞。如果MessageQueue的next方法返回了新消息,Looper会处理这条消息: msg.target.dispatchMessage(msg),这里的 msg.target是发送这条消息的Handler对象,这样Handler发送的消息最终又交给它的dispatchMessage方法来处理了,最终回调复写的handleMessage(Message msg)。

Handler的工作原理

Handler的工作主要包含消息的发送和接收过程。消息的发送可以通过send,post的一系列方法实现。post的方法最终也是通过send来实现,发送一条消息的典型过程如下:

    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);
    }

可以发现,Handler发送消息就是向消息队列插入一条消息,MessageQueue的next方法就会返回这条消息给Looper,Looper收到消息后就开始处理,最终消息由Looper交由Handler处理,即上面说的dispatchMessage方法被调用,代码如下:

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

Handler处理消息的过程如下:
首先检查Message的callback是否为mCallbacknull,不为null就通过handleCallback来处理消息,Message的callback是一个Runnable对象,实际是Handler的post方法传递的Runnable参数,handleCallback逻辑如下:

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

其次,检查mCallback是否为null,不为null就调用mCallback的handleMessage方法处理消息,mCallback是个接口,定义如下:

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

通过Callback可以采用如下方式来创建Handler对象:Handler handler=new Handler(callback)。那么Callback的意义是什么呢,源码的注释说明了,可以用来创建一个Handler的实例但并不需要派生Handler的子类。日常开发中,创建Handler,最常见是派生一个Handler的子类并重写handlerMessage方法来处理具体消息,而Callback给我们提供了另一种使用Handler的方式。
最后,调用Handler的handleMessage方法来处理消息。

Handler还有一个构造方法,就是通过一个特定的Looper来构造Handler,实现如下:

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

下面看下Handler的默认构造方法

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

this(null, false)实现如下,很明显,如果当前线程没有创建Looper,就会抛出 "Can't create handler inside thread that has not called Looper.prepare()"这个异常。

 public Handler(Callback callback, boolean async) {
        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;
    }

以上就是Handler相关内容

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