线程管理（一）AsyncTask与Thread的差别

AsycTask，做Android的应该都是熟悉的不能再熟悉了，当我们在进行耗时操作的时候，就可以使用这个了
但是有的人就在想，Thread不是也可以开一个子线程进行耗时操作吗？那这两个有什么样的区别呢？
那么一起来看下AsycTask源码啦

public abstract class AsyncTask<Params, Progress, Result> {    private static final String LOG_TAG = "AsyncTask";    //获取cpu可用处理器的数量，线程的数量为cpu*2+1比较好    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();    private static final int CORE_POOL_SIZE = CPU_COUNT + 1;    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;    private static final int KEEP_ALIVE = 1;    //这里有个创建线程的方法，说明AsyncTask里面是含有Thread的    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());        }    };    //创建一个Linked的队列，先进先出    private static final BlockingQueue<Runnable> sPoolWorkQueue =            new LinkedBlockingQueue<Runnable>(128);    //创建线程池啦    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 volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;    private static InternalHandler sHandler;    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();    //创建一个空数组双端队列容纳16个元素的初始容量    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();            }         }        //从队列中取出最先的并执行        protected synchronized void scheduleNext() {            if ((mActive = mTasks.poll()) != null) {                THREAD_POOL_EXECUTOR.execute(mActive);            }        }    }    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,    }    //从这里开始使用Handler机制，用来跟主线程进行交互    private static Handler getHandler() {        synchronized (AsyncTask.class) {            if (sHandler == null) {                sHandler = new InternalHandler();            }            return sHandler;        }    }    public static void setDefaultExecutor(Executor exec) {        sDefaultExecutor = exec;    }    public AsyncTask() {        mWorker = new WorkerRunnable<Params, Result>() {            public Result call() throws Exception {                mTaskInvoked.set(true);                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);                        //我们实现的doInBackground方法，就是在这里调用的                                        return postResult(doInBackground(mParams));            }        };        //使用来管理线程的        mFuture = new FutureTask<Result>(mWorker) {            //这里就是耗时操作完成了            @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);        }    }    //这里就是向主线程发送消息    private Result postResult(Result result) {        @SuppressWarnings("unchecked")        Message message = getHandler().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);    }    /**     * 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();    }    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;    }    //我们调用的new AsycTask（。。）.execute就是调用线程池来执行异步任务    public static void execute(Runnable runnable) {        sDefaultExecutor.execute(runnable);    }    protected final void publishProgress(Progress... values) {        if (!isCancelled()) {            getHandler().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 {        public InternalHandler() {            super(Looper.getMainLooper());        }        @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;        }    }}

简单的捋一下，就是通过线程池创建线程执行异步任务，然后通过handler机制，发信息给主线程，让主线程知道现在是出于什么样的状态

区别：1。AsycTask跟Thread的区别就是AsycTask它能管理的线程数量是有限的！
MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
2。AsycTask是使用了Handler机制能跟主线程进行交互，而Thread只是异步操作

那么，如果我们只是进行异步操作不跟主线程进行交互，还需要使用AscyTask吗？
如果我们需要用到很多的异步操作，能使用AsycTask吗？
当然就是不能拉，下一节介绍，我们自己建立一个线程管理器