欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页

Android Handler:全面,详细解读

程序员文章站 2022-05-14 07:49:56
...

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。

Android Handler:全面,详细解读

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<Boolean> mBooleanThreadLocal=new ThreadLocal<Boolean>();

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