我答同行问（续三）

    本片文章出自http://blog.csdn.net/andywuchuanlong，转载请说明出处，谢谢！

    我答同行问序列目录http://blog.csdn.net/andywuchuanlong/article/details/44194043

    4、网络请求的时候，我们都需要开启线程，那么是使用asyncTask还是使用Thread+Handler模式呢？

    网络请求是每个app都需要进行的，很多人会使用asyncTask，也有人喜欢Thread+Handler，下面我按照我的想法讲解一下他们两者的区别。

    1、asyncTask

     asyncTask本质上也是一个线程池，2.3之前默认最多同时执行5个任务，是并行执行任务的。而3.0之后默认是串行执行，也就是说只有一个任务在执行，下面主要讲解下3.0之后的asyncTask。

     AsyncTask中维护者一个静态并发线程池，可以用来并行执行任务，尽管从3,0开始，AsyncTask默认是串行执行任务
但是我们仍然能构造出并行的AsyncTask。可能有人觉得奇怪了，明明维护者一个并发线程池，怎么说是串行执行的呢，主要还是AsyncTask维护了一个静态串行任务执行器其内部实现了串行控制，当任务开始执行的时候，这个task会被加入到任务队列中，然后从队列中循环的取出一个个的任务交给并发线程池去执行，这个任务执行器就是3.0之后默认的执行器，如果我们要改为并行执行把这个默认的执行器替换掉就可以了。任务执行完毕之后，会通过AsyncTask内部的一个handler发送消息到主线程，这里也就说明了为什么AsyncTask只能在ui线程中创建，因为AsyncTask内部维护了一个Handler，而消息时有Looper来循环的，子线程中默认是没有Looper的。  

      

public abstract class AsyncTask<Params, Progress, Result> {    private static final String LOG_TAG = "AsyncTask";    //获取当前的cpu核心数      private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();  //线程池核心容量      private static final int CORE_POOL_SIZE = CPU_COUNT + 1;  //线程池最大容量      private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;    private static final int KEEP_ALIVE = 1;  //ThreadFactory 线程工厂，通过工厂方法newThread来获取新线程      private static final ThreadFactory sThreadFactory = new ThreadFactory() {      //原子整数，可以在超高并发下正常工作         private final AtomicInteger mCount = new AtomicInteger(1);        public Thread newThread(Runnable r) {            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());        }    };    //静态阻塞式队列，用来存放待执行的任务，初始容量：128个    private static final BlockingQueue<Runnable> sPoolWorkQueue =            new LinkedBlockingQueue<Runnable>(128);    /**     * 静态并发线程池，可以用来并行执行任务，尽管从3.0开始，AsyncTask默认是串行执行任务     * 但是我们仍然能构造出并行的AsyncTask     */      public static final Executor THREAD_POOL_EXECUTOR            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);    /**     * 静态串行任务执行器，其内部实现了串行控制，     * 循环的取出一个个任务交给上述的并发线程池去执行     */      public static final Executor SERIAL_EXECUTOR = new SerialExecutor();  //消息类型：发送结果      private static final int MESSAGE_POST_RESULT = 0x1;    //消息类型：更新进度      private static final int MESSAGE_POST_PROGRESS = 0x2;    /**静态Handler，用来发送上述两种通知，采用UI线程的Looper来处理消息     * 这就是为什么AsyncTask必须在UI线程调用，因为子线程     * 默认没有Looper无法创建下面的Handler，程序会直接Crash     */      private static final InternalHandler sHandler = new InternalHandler();  //默认任务执行器，被赋值为串行任务执行器，就是它，AsyncTask变成串行的了      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();    /**串行执行器的实现，我们要好好看看，它是怎么把并行转为串行的     *目前我们需要知道，asyncTask.execute(Params ...)实际上会调用     *SerialExecutor的execute方法，这一点后面再说明。也就是说：当你的asyncTask执行的时候，     *首先你的task会被加入到任务队列，然后排队，一个个执行     */      private static class SerialExecutor implements Executor {     //线性双向队列，用来存储所有的AsyncTask任务        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();        //当前正在执行的AsyncTask任务         Runnable mActive;        public synchronized void execute(final Runnable r) {         //将新的AsyncTask任务加入到双向队列中             mTasks.offer(new Runnable() {                public void run() {                    try {                     //执行AsyncTask任务                          r.run();                    } finally {                    //当前AsyncTask任务执行完毕后，进行下一轮执行，如果还有未执行任务的话                          //这一点很明显体现了AsyncTask是串行执行任务的，总是一个任务执行完毕才会执行下一个任务                          scheduleNext();                    }                }            });            if (mActive == null) {            //如果当前没有任务在执行，直接进入执行逻辑                  scheduleNext();            }        }        protected synchronized void scheduleNext() {            if ((mActive = mTasks.poll()) != null) {             //从任务队列中取出队列头部的任务，如果有就交给并发线程池去执行                  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.     */    /**     * 任务的三种状态     */      public enum Status {        /** 任务等待执行         * Indicates that the task has not been executed yet.         */        PENDING,        /**任务正在执行         * Indicates that the task is running.         */        RUNNING,        /**任务已经执行结束         * Indicates that {@link AsyncTask#onPostExecute} has finished.         */        FINISHED,    }    /** @hide Used to force static handler to be created.      * 隐藏API：在UI线程中调用，用来初始化Handler      * */    public static void init() {//        sHandler.getLooper();    }    /** @hide  为AsyncTask设置默认执行器*/    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;    }    /**     * Returns the current status of this task.     *     * @return The current status.     */    public final Status getStatus() {        return mStatus;    }    /**     * Override this method to perform a computation on a background thread. The     * specified parameters are the parameters passed to {@link #execute}     * by the caller of this task.     *     * This method can call {@link #publishProgress} to publish updates     * on the UI thread.     *     * @param params The parameters of the task.     *     * @return A result, defined by the subclass of this task.     *     * @see #onPreExecute()     * @see #onPostExecute     * @see #publishProgress     */    protected abstract Result doInBackground(Params... params);    /**     * Runs on the UI thread before {@link #doInBackground}.     *     * @see #onPostExecute     * @see #doInBackground     */    protected void onPreExecute() {    }    /**     * <p>Runs on the UI thread after {@link #doInBackground}. The     * specified result is the value returned by {@link #doInBackground}.</p>     *      * <p>This method won't be invoked if the task was cancelled.</p>     *     * @param result The result of the operation computed by {@link #doInBackground}.     *     * @see #onPreExecute     * @see #doInBackground     * @see #onCancelled(Object)      */    @SuppressWarnings({"UnusedDeclaration"})    protected void onPostExecute(Result result) {    }    /**     * Runs on the UI thread after {@link #publishProgress} is invoked.     * The specified values are the values passed to {@link #publishProgress}.     *     * @param values The values indicating progress.     *     * @see #publishProgress     * @see #doInBackground     */    @SuppressWarnings({"UnusedDeclaration"})    protected void onProgressUpdate(Progress... values) {    }    /**     * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and     * {@link #doInBackground(Object[])} has finished.</p>     *      * <p>The default implementation simply invokes {@link #onCancelled()} and     * ignores the result. If you write your own implementation, do not call     * <code>super.onCancelled(result)</code>.</p>     *     * @param result The result, if any, computed in     *               {@link #doInBackground(Object[])}, can be null     *      * @see #cancel(boolean)     * @see #isCancelled()     */    @SuppressWarnings({"UnusedParameters"})    protected void onCancelled(Result result) {        onCancelled();    }            /**     * <p>Applications should preferably override {@link #onCancelled(Object)}.     * This method is invoked by the default implementation of     * {@link #onCancelled(Object)}.</p>     *      * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and     * {@link #doInBackground(Object[])} has finished.</p>     *     * @see #onCancelled(Object)      * @see #cancel(boolean)     * @see #isCancelled()     */    protected void onCancelled() {    }    /**     * Returns <tt>true</tt> if this task was cancelled before it completed     * normally. If you are calling {@link #cancel(boolean)} on the task,     * the value returned by this method should be checked periodically from     * {@link #doInBackground(Object[])} to end the task as soon as possible.     *     * @return <tt>true</tt> if task was cancelled before it completed     *     * @see #cancel(boolean)     */    public final boolean isCancelled() {        return mCancelled.get();    }    /**     * <p>Attempts to cancel execution of this task.  This attempt will     * fail if the task has already completed, already been cancelled,     * or could not be cancelled for some other reason. If successful,     * and this task has not started when <tt>cancel</tt> is called,     * this task should never run. If the task has already started,     * then the <tt>mayInterruptIfRunning</tt> parameter determines     * whether the thread executing this task should be interrupted in     * an attempt to stop the task.</p>     *      * <p>Calling this method will result in {@link #onCancelled(Object)} being     * invoked on the UI thread after {@link #doInBackground(Object[])}     * returns. Calling this method guarantees that {@link #onPostExecute(Object)}     * is never invoked. After invoking this method, you should check the     * value returned by {@link #isCancelled()} periodically from     * {@link #doInBackground(Object[])} to finish the task as early as     * possible.</p>     *     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this     *        task should be interrupted; otherwise, in-progress tasks are allowed     *        to complete.     *     * @return <tt>false</tt> if the task could not be cancelled,     *         typically because it has already completed normally;     *         <tt>true</tt> otherwise     *     * @see #isCancelled()     * @see #onCancelled(Object)     */    public final boolean cancel(boolean mayInterruptIfRunning) {        mCancelled.set(true);        return mFuture.cancel(mayInterruptIfRunning);    }    /**     * Waits if necessary for the computation to complete, and then     * retrieves its result.     *     * @return The computed result.     *     * @throws CancellationException If the computation was cancelled.     * @throws ExecutionException If the computation threw an exception.     * @throws InterruptedException If the current thread was interrupted     *         while waiting.     */    public final Result get() throws InterruptedException, ExecutionException {        return mFuture.get();    }    /**     * Waits if necessary for at most the given time for the computation     * to complete, and then retrieves its result.     *     * @param timeout Time to wait before cancelling the operation.     * @param unit The time unit for the timeout.     *     * @return The computed result.     *     * @throws CancellationException If the computation was cancelled.     * @throws ExecutionException If the computation threw an exception.     * @throws InterruptedException If the current thread was interrupted     *         while waiting.     * @throws TimeoutException If the wait timed out.     */    public final Result get(long timeout, TimeUnit unit) throws InterruptedException,            ExecutionException, TimeoutException {        return mFuture.get(timeout, unit);    }    /**     * Executes the task with the specified parameters. The task returns     * itself (this) so that the caller can keep a reference to it.     *      * <p>Note: this function schedules the task on a queue for a single background     * thread or pool of threads depending on the platform version.  When first     * introduced, AsyncTasks were executed serially on a single background thread.     * Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed     * to a pool of threads allowing multiple tasks to operate in parallel. Starting     * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being     * executed on a single thread to avoid common application errors caused     * by parallel execution.  If you truly want parallel execution, you can use     * the {@link #executeOnExecutor} version of this method     * with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings     * on its use.     *     * <p>This method must be invoked on the UI thread.     *     * @param params The parameters of the task.     *     * @return This instance of AsyncTask.     *     * @throws IllegalStateException If {@link #getStatus()} returns either     *         {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.     *     * @see #executeOnExecutor(java.util.concurrent.Executor, Object[])     * @see #execute(Runnable)     */ /**     * 这个方法如何执行和系统版本有关，在AsyncTask的使用规则里已经说明，如果你真的想使用并行AsyncTask，     * 也是可以的，只要稍作修改     * 必须在UI线程调用此方法     */      public final AsyncTask<Params, Progress, Result> execute(Params... params) {      //串行执行          return executeOnExecutor(sDefaultExecutor, params);        //如果我们想并行执行，这样改就行了，当然这个方法我们没法改          //return executeOnExecutor(THREAD_POOL_EXECUTOR, params);     }    /**     * Executes the task with the specified parameters. The task returns     * itself (this) so that the caller can keep a reference to it.     *      * <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to     * allow multiple tasks to run in parallel on a pool of threads managed by     * AsyncTask, however you can also use your own {@link Executor} for custom     * behavior.     *      * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from     * a thread pool is generally <em>not</em> what one wants, because the order     * of their operation is not defined.  For example, if these tasks are used     * to modify any state in common (such as writing a file due to a button click),     * there are no guarantees on the order of the modifications.     * Without careful work it is possible in rare cases for the newer version     * of the data to be over-written by an older one, leading to obscure data     * loss and stability issues.  Such changes are best     * executed in serial; to guarantee such work is serialized regardless of     * platform version you can use this function with {@link #SERIAL_EXECUTOR}.     *     * <p>This method must be invoked on the UI thread.     *     * @param exec The executor to use.  {@link #THREAD_POOL_EXECUTOR} is available as a     *              convenient process-wide thread pool for tasks that are loosely coupled.     * @param params The parameters of the task.     *     * @return This instance of AsyncTask.     *     * @throws IllegalStateException If {@link #getStatus()} returns either     *         {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.     *     * @see #execute(Object[])     */    /**     * 通过这个方法我们可以自定义AsyncTask的执行方式，串行or并行，甚至可以采用自己的Executor     * 为了实现并行，我们可以在外部这么用AsyncTask：     * asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, Params... params);     * 必须在UI线程调用此方法     */      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;    }    /**     * Convenience version of {@link #execute(Object...)} for use with     * a simple Runnable object. See {@link #execute(Object[])} for more     * information on the order of execution.     *     * @see #execute(Object[])     * @see #executeOnExecutor(java.util.concurrent.Executor, Object[])     *//**     * 这是AsyncTask提供的一个静态方法，方便我们直接执行一个runnable     */      public static void execute(Runnable runnable) {        sDefaultExecutor.execute(runnable);    }    /**     * This method can be invoked from {@link #doInBackground} to     * publish updates on the UI thread while the background computation is     * still running. Each call to this method will trigger the execution of     * {@link #onProgressUpdate} on the UI thread.     *     * {@link #onProgressUpdate} will note be called if the task has been     * canceled.     *     * @param values The progress values to update the UI with.     *     * @see #onProgressUpdate     * @see #doInBackground     */    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;    }  //AsyncTask内部Handler，用来发送后台计算进度更新消息和计算完成消息      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;        }    }}

      2、Thread+Handler模式

      每个需要请求服务器的地方都创建一个线程，等到线程执行完毕就通过handler发送消息到主线程。这样的线程运行是不受控制的。

     总结一下：

     AsyncTask是封装好的线程池，比起Thread+Handler的方式，AsyncTask在操作UI线程上更方便，因为onPreExecute()、onPostExecute()及更新UI方法onProgressUpdate()均运行在主线程中，这样就不用Handler发消息处理了。
     AsyncTask真正的缺点来自于3.0之前全局线程池只有5个工作线程，也就是说，一个APP如果运用AsyncTask技术来执行线程，那么同一时间最多只能有5个线程同时运行，其他线程将被阻塞，但是在3.0之后可以自定义执行器，多少个线程同时运行不受到限制。不运用AsyncTask执行的线程，也就是自己new出来的线程不受此限制。

     所以在3.0之前AsyncTask不要用于多线程取网络数据，因为很可能这样会产生阻塞，从而降低效率。3.0之后我建议最好是使用AsyncTask。