【Android开发】线程间通讯机制（提高篇）——深入浅出实现原理

前言：

这一篇博文主要是和大家讲解一下线程间通讯机制的内部实现原理，即Handler、Message、MessageQueue、Looper、HandlerThread、AsyncTask类的实现以及之间的关系。如果还没有接触过Handler+Message+Runnable、HandlerThread、AsyncTask的朋友可以先看看基础篇：

【Android开发】线程间通讯机制（基础篇）——Handler、Runnable、HandlerThread、AsyncTask的使用

有时候，如果你能带着问题或者目标去探索新知识的话，这样的学习效率就高很多。所以我们先从最基础的实现方式（Handler+Message+Runnable）说起。

一、Handler+Message+Runnable内部解析

问题：我们在使用Handler类的时候，都知道有sendMessage（Message）等发送消息的功能和post（Runnable）发送任务的功能，然后还有能够处理接受到的Message的功能。这时候我就会提出这样的问题：

1、有发送、接受Message的功能，是不是sendMessage方法是直接调用handleMessage的重写方法里呢？

2、不是有按时间计划发送Message和Runnable吗？如果问题1成立的话，handleMessage可能会同时接受多个Message，但是此方法不是线程安全的（没有synchronized修饰），这样会出现问题了。

    

解决问题：如果对API有任何疑惑，最根本的方法就是查看源代码。

在看源代码之前，需要了解几个类：

Handler：负责发送Message和Runnable到MessageQueue中，然后依次处理MessageQueue里面的队列。

MessageQueue：消息队列。负责存放一个线程的Message和Runnable的集合。

Message：消息实体类。

Looper：消息循环器。负责把MessageQueue中的Message或者Runnable循环取出来，然后分发到Handler中。

四者的关系：一个线程可以有多个Handler实例，一个线程对应一个Looper，一个Looper也只对应一个MessageQueue，一个MessageQueue对应多个Message和Runnable。所以就形成了一对多的对应关系，一方：线程、Looper、MessageQueue；多方：Handler、Message。同时可以看出另一个一对一关系：一个Message实例对应一个Handler实例。

一个Handler实例都会与一个线程和消息队列捆绑在一起，当实例化Handler的时候，就已经完成这样的工作。源码如下：

Handler类

/**     * Default constructor associates this handler with the {@link Looper} for the     * current thread.     *     * If this thread does not have a looper, this handler won't be able to receive messages     * so an exception is thrown.     */    public Handler() {        this(null, false);    }

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

  可以从mLooper = Looper.myLooper()

mQueue = mLooper.mQueue;看出，实例化Handler就会绑定一个Looper实例，并且一个Looper实例包涵一个MessageQueue实例。

问题来了，为什么说一个线程对应一个Looper实例？我们通过Looper.myLooper()找原因：

Looper类

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

/**     * 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();    }

ThreadLocal类

Implements a thread-local storage, that is, a variable for which each thread has its own value. All threads sharethe sameThreadLocal object, but each sees a different value when accessing it, and changes made by onethread do not affect the other threads. The implementation supportsnull values.

——实现一个线程本地的存储，就是说每个线程都会有自己的内存空间来存放线程自己的值。所有线程都共享一个ThreadLocal对象，但是不同的线程都会对应不同的value，而且单独修改不影响其他线程的value，并且支持null值。

所以说，每个线程都会存放一个独立的Looper实例，通过ThreadLocal.get()方法，就会获得当前线程的Looper的实例。

好了，接下来就要研究一下Handler发送Runnable，究竟怎么发送？

Handler类：

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

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

可以看出，其实传入的Runnable对象都是封装到Message类中，看下Message是存放什么信息：

Message类：

public final class Message implements Parcelable {      public int what;      public int arg1;      public int arg2;      public Object obj;      public Messenger replyTo;      long when;      Bundle data;      Handler target;           Runnable callback;       Message next;      private static Object mPoolSync = new Object();      private static Message mPool;      private static int mPoolSize = 0;      private static final int MAX_POOL_SIZE = 10; 

When: 向Handler发送Message生成的时间
Data: 在Bundler 对象上绑定要线程中传递的数据
Next: 当前Message 对一下个Message 的引用
Handler: 处理当前Message 的Handler对象.
mPool: 通过字面理解可能叫他Message池,但是通过分析应该叫有下一个Message引用的Message链更加适合.
其中Message.obtain(),通过源码分析就是获取断掉Message链关系的第一个Message.

       对于源码的解读，可以明确两点：

        1）Message.obtain()是通过从全局Message pool中读取一个Message，回收的时候也是将该Message 放入到pool中。

        2）Message中实现了Parcelable接口

所以接下来看下Handler如何发送Message：

Handler类

 /**     * 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>     * 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);    }

 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {        msg.target = this;        if (mAsynchronous) {            msg.setAsynchronous(true);        }        return queue.enqueueMessage(msg, uptimeMillis);    }

其实无论是按时间计划发送Message或者Runnable，最终是调用了sendMessageAtTime方法，里面核心执行的是enqueueMessage方法，就是调用了MessageQueue中的enqueueMessage方法，就是把消息Message加入到消息队列中。

这时候问题又来了，如果发送消息只是把消息加入到消息队列中，那谁来把消息分发到Handler中呢？

不妨我们看看Looper类：

/**     * 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.recycle();        }    }

里面loop方法找到调用Handler的dispatchMessage的方法，我们再看看Handler的dispatchMessage：

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

dispatchMessage最终是回调了handleMessage。换句话说，Loop的loop()方法就是取得当前线程中的MessageQueue实例，然后不断循环消息分发到对应的Handler实例上。就是只要调用Looper.loop()方法，就可以执行消息分发。

小结：Handler、Message、MessageQueue、Looper的关系原理图：

整个机制实现原理流程：当应用程序运行的时候，会创建一个主线程（UI线程）ActivityThread,这个类里面有个main方法，就是java程序运行的最开始的入口

public static void main(String[] args) {        SamplingProfilerIntegration.start();        // CloseGuard defaults to true and can be quite spammy.  We        // disable it here, but selectively enable it later (via        // StrictMode) on debug builds, but using DropBox, not logs.        CloseGuard.setEnabled(false);        Process.setArgV0("<pre-initialized>");        Looper.prepareMainLooper();        if (sMainThreadHandler == null) {            sMainThreadHandler = new Handler();        }        ActivityThread thread = new ActivityThread();        thread.attach(false);        if (false) {            Looper.myLooper().setMessageLogging(new                    LogPrinter(Log.DEBUG, "ActivityThread"));        }        Looper.loop();        throw new RuntimeException("Main thread loop unexpectedly exited");    }

UI线程就开始就已经调用了loop消息分发，所以当在UI线程实例的Handler对象发送消息或者任务时，会把Message加入到MessageQueue消息队列中，然后分发到Handler的handleMessage方法里。

二、HandlerThread

其实上述就是线程间通讯机制的实现，而HandlerThread和AsyncTask只是对通讯机制进行进一步的封装，要理解也很简单：

HandlerThread类：

public class HandlerThread extends Thread {    int mPriority;    int mTid = -1;    Looper mLooper;    public HandlerThread(String name) {        super(name);        mPriority = Process.THREAD_PRIORITY_DEFAULT;    }        /**     * Constructs a HandlerThread.     * @param name     * @param priority The priority to run the thread at. The value supplied must be from      * {@link android.os.Process} and not from java.lang.Thread.     */    public HandlerThread(String name, int priority) {        super(name);        mPriority = priority;    }        /**     * Call back method that can be explicitly overridden if needed to execute some     * setup before Looper loops.     */    protected void onLooperPrepared() {    }    public void run() {        mTid = Process.myTid();        Looper.prepare();        synchronized (this) {            mLooper = Looper.myLooper();            notifyAll();        }        Process.setThreadPriority(mPriority);        onLooperPrepared();        Looper.loop();        mTid = -1;    }        /**     * This method returns the Looper associated with this thread. If this thread not been started     * or for any reason is isAlive() returns false, this method will return null. If this thread      * has been started, this method will block until the looper has been initialized.       * @return The looper.     */    public Looper getLooper() {        if (!isAlive()) {            return null;        }                // If the thread has been started, wait until the looper has been created.        synchronized (this) {            while (isAlive() && mLooper == null) {                try {                    wait();                } catch (InterruptedException e) {                }            }        }        return mLooper;    }        /**     * Ask the currently running looper to quit.  If the thread has not     * been started or has finished (that is if {@link #getLooper} returns     * null), then false is returned.  Otherwise the looper is asked to     * quit and true is returned.     */    public boolean quit() {        Looper looper = getLooper();        if (looper != null) {            looper.quit();            return true;        }        return false;    }        /**     * Returns the identifier of this thread. See Process.myTid().     */    public int getThreadId() {        return mTid;    }}

可以看得出,HandlerThread继承了Thread，从run()方法可以看出，HandlerThread要嗲用start()方法，才能实例化HandlerThread的Looper对象，和消息分发功能。

所以使用HandlerThread，必须先运行HandlerThread,才能取出对应的Looper对象，然后使用Handler（Looper）构造方法实例Handler，这样Handler的handleMessage方法就是子线程执行了。

三、AsyncTask

AsyncTask现在是android应用开发最常用的工具类，这个类面向调用者是轻量型的，但是对于系统性能来说是重量型的。这个类很强大，使用者很方便就能使用，只需要在对应的方法实现特定的功能即可。就是因为AsyncTask的强大封装，所以说不是轻量型的，先看下源代码吧：

public abstract class AsyncTask<Params, Progress, Result> {    private static final String LOG_TAG = "AsyncTask";    private static final int CORE_POOL_SIZE = 5;    private static final int MAXIMUM_POOL_SIZE = 128;    private static final int KEEP_ALIVE = 1;    private static final ThreadFactory sThreadFactory = new ThreadFactory() {        private final AtomicInteger mCount = new AtomicInteger(1);        public Thread newThread(Runnable r) {            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());        }    };    private static final BlockingQueue<Runnable> sPoolWorkQueue =            new LinkedBlockingQueue<Runnable>(10);    /**     * An {@link Executor} that can be used to execute tasks in parallel.     */    public static final Executor THREAD_POOL_EXECUTOR            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);    /**     * An {@link Executor} that executes tasks one at a time in serial     * order.  This serialization is global to a particular process.     */    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();    private static final int MESSAGE_POST_RESULT = 0x1;    private static final int MESSAGE_POST_PROGRESS = 0x2;    private static final InternalHandler sHandler = new InternalHandler();    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;    private final WorkerRunnable<Params, Result> mWorker;    private final FutureTask<Result> mFuture;    private volatile Status mStatus = Status.PENDING;        private final AtomicBoolean mCancelled = new AtomicBoolean();    private final AtomicBoolean mTaskInvoked = new AtomicBoolean();    private static class SerialExecutor implements Executor {        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();        Runnable mActive;        public synchronized void execute(final Runnable r) {            mTasks.offer(new Runnable() {                public void run() {                    try {                        r.run();                    } finally {                        scheduleNext();                    }                }            });            if (mActive == null) {                scheduleNext();            }        }        protected synchronized void scheduleNext() {            if ((mActive = mTasks.poll()) != null) {                THREAD_POOL_EXECUTOR.execute(mActive);            }        }    }    /**     * Indicates the current status of the task. Each status will be set only once     * during the lifetime of a task.     */    public enum Status {        /**         * Indicates that the task has not been executed yet.         */        PENDING,        /**         * Indicates that the task is running.         */        RUNNING,        /**         * Indicates that {@link AsyncTask#onPostExecute} has finished.         */        FINISHED,    }    /** @hide Used to force static handler to be created. */    public static void init() {        sHandler.getLooper();    }    /** @hide */    public static void setDefaultExecutor(Executor exec) {        sDefaultExecutor = exec;    }    /**     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.     */    public AsyncTask() {        mWorker = new WorkerRunnable<Params, Result>() {            public Result call() throws Exception {                mTaskInvoked.set(true);                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);                //noinspection unchecked                return postResult(doInBackground(mParams));            }        };        mFuture = new FutureTask<Result>(mWorker) {            @Override            protected void done() {                try {                    postResultIfNotInvoked(get());                } catch (InterruptedException e) {                    android.util.Log.w(LOG_TAG, e);                } catch (ExecutionException e) {                    throw new RuntimeException("An error occured while executing doInBackground()",                            e.getCause());                } catch (CancellationException e) {                    postResultIfNotInvoked(null);                }            }        };    }    private void postResultIfNotInvoked(Result result) {        final boolean wasTaskInvoked = mTaskInvoked.get();        if (!wasTaskInvoked) {            postResult(result);        }    }    private Result postResult(Result result) {        @SuppressWarnings("unchecked")        Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,                new AsyncTaskResult<Result>(this, result));        message.sendToTarget();        return result;    }        public final Status getStatus() {        return mStatus;    }        protected abstract Result doInBackground(Params... params);       protected void onPreExecute() {    }        @SuppressWarnings({"UnusedDeclaration"})    protected void onPostExecute(Result result) {    }        @SuppressWarnings({"UnusedDeclaration"})    protected void onProgressUpdate(Progress... values) {    }       @SuppressWarnings({"UnusedParameters"})    protected void onCancelled(Result result) {        onCancelled();    }                protected void onCancelled() {    }        public final boolean isCancelled() {        return mCancelled.get();    }        public final boolean cancel(boolean mayInterruptIfRunning) {        mCancelled.set(true);        return mFuture.cancel(mayInterruptIfRunning);    }        public final Result get() throws InterruptedException, ExecutionException {        return mFuture.get();    }        public final Result get(long timeout, TimeUnit unit) throws InterruptedException,            ExecutionException, TimeoutException {        return mFuture.get(timeout, unit);    }        public final AsyncTask<Params, Progress, Result> execute(Params... params) {        return executeOnExecutor(sDefaultExecutor, params);    }       public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,            Params... params) {        if (mStatus != Status.PENDING) {            switch (mStatus) {                case RUNNING:                    throw new IllegalStateException("Cannot execute task:"                            + " the task is already running.");                case FINISHED:                    throw new IllegalStateException("Cannot execute task:"                            + " the task has already been executed "                            + "(a task can be executed only once)");            }        }        mStatus = Status.RUNNING;        onPreExecute();        mWorker.mParams = params;        exec.execute(mFuture);        return this;    }        public static void execute(Runnable runnable) {        sDefaultExecutor.execute(runnable);    }        protected final void publishProgress(Progress... values) {        if (!isCancelled()) {            sHandler.obtainMessage(MESSAGE_POST_PROGRESS,                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();        }    }    private void finish(Result result) {        if (isCancelled()) {            onCancelled(result);        } else {            onPostExecute(result);        }        mStatus = Status.FINISHED;    }    private static class InternalHandler extends Handler {        @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})        @Override        public void handleMessage(Message msg) {            AsyncTaskResult result = (AsyncTaskResult) msg.obj;            switch (msg.what) {                case MESSAGE_POST_RESULT:                    // There is only one result                    result.mTask.finish(result.mData[0]);                    break;                case MESSAGE_POST_PROGRESS:                    result.mTask.onProgressUpdate(result.mData);                    break;            }        }    }    private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {        Params[] mParams;    }    @SuppressWarnings({"RawUseOfParameterizedType"})    private static class AsyncTaskResult<Data> {        final AsyncTask mTask;        final Data[] mData;        AsyncTaskResult(AsyncTask task, Data... data) {            mTask = task;            mData = data;        }    }}

要理解这个工具类，主要是理解这几个成员对象：

private static final InternalHandler sHandler = new InternalHandler();

    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

    private final WorkerRunnable<Params, Result> mWorker;

    private final FutureTask<Result> mFuture;

分析：sHandler

消息的发送者和处理者

 sDefualtExecutor

线程执行者。实际上就是一个线程池。

 mWorker

WorkerRunnable实现了Callable接口,就是有返回值的线程任务。

 mFuture

FutureTask是对Callable执行的一个管理类，能够获得线程执行返回的结果，和取消执行等操作。我们再深入一下FutureTask，其中的done()方法是回调方法：

   /**     * Removes and signals all waiting threads, invokes done(), and     * nulls out callable.     */    private void finishCompletion() {        // assert state > COMPLETING;        for (WaitNode q; (q = waiters) != null;) {            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {                for (;;) {                    Thread t = q.thread;                    if (t != null) {                        q.thread = null;                        LockSupport.unpark(t);                    }                    WaitNode next = q.next;                    if (next == null)                        break;                    q.next = null; // unlink to help gc                    q = next;                }                break;            }        }        done();        callable = null;        // to reduce footprint    }

只要线程移除或者挂起（取消）的时候，就会调用done()方法，然后在AsyncTask类中的mTask实现了done()方法，最后回调onCancelled()方法。

具体的流程原理是这样的：

1、当第一次AsyncTask在UI线程实例化，其实是实例化Handler，同时UI线程的Looper和MessageQueue绑定在sHandler对象中，之后再去实例话AsyncTask不会在初始化Handler，因为sHandler是类变量。

2、当执行execute方法的时候，实际上是调用线程池的execute方法运行线程

3、callable线程执行体就是调用了doInBackground(mParams)方法，然后以返回结果result当参数，又调用postResult(Result result)，实际上就是利用sHandler来发送result到UI线程的MessageQueue中，最后sHandler接受到result后，回调onPostExecute方法。

4、如果主动调用publishProgress(Progress... values)方法，就会利用sHandler把value发送到UI线程的MessageQueue中，然后sHandler接收到value后，回调onProgressUpdate(Progress... values)方法。

注意：sHandler和mDefaultExecutor是类变量

  mWorker和mFuture是实例变量

所以，无论进程中生成多少个AysncTask对象，sHandler和mDefaultExecutor都是同一个，只是任务不同而已。

四、总结

由于我放上去的源代码删除了一些注释，如果还不能了解清楚的话，可以自行去源代码上观看。线程间通讯机制的核心就是Handler+Message+Looper+MessageQueue，只要理解这个四者的实现原理，再多的封装好的工具类也难理解。所以，必须记住一点：android应用开发多线程是必不可少的，所以我们必须遵循UI线程模式开发，就是所有耗时不能在UI线程执行，操作UI必须在UI线程中执行。