AsyncTask原理

AsyncTask的基本原理是创建一个线程池，通过线程池执行一个Runnable对象（FutureTask），然后通过Handler通知UI线程。

1、线程池的创建。创建线程池，并返回一个执行器对象（Executor）

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1. private static final int CORE_POOL_SIZE = 5;
2. private static final int MAXIMUM_POOL_SIZE = 128;
3. private static final int KEEP_ALIVE = 10;
5. private static final BlockingQueue<Runnable> sWorkQueue =
6. new LinkedBlockingQueue<Runnable>(10);
8. private static final ThreadFactory sThreadFactory = new ThreadFactory() {
9. private final AtomicInteger mCount = new AtomicInteger(1);
11. public Thread newThread(Runnable r) {
12. return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
13. }
14. };
15. private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
16. MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);

2、将被执行的Runnable对象（FutureTask）的创建。在构造方法中创建FutureTask。

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1. public AsyncTask() {
2. mWorker = new WorkerRunnable<Params, Result>() {
3. public Result call() throws Exception {
4. Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
5. return doInBackground(mParams);
6. }
7. };
9. mFuture = new FutureTask<Result>(mWorker) {
10. @Override
11. protected void done() {
12. Message message;
13. Result result = null;
15. try {
16. result = get();
17. } catch (InterruptedException e) {
18. android.util.Log.w(LOG_TAG, e);
19. } catch (ExecutionException e) {
20. throw new RuntimeException("An error occured while executing doInBackground()",
21. e.getCause());
22. } catch (CancellationException e) {
23. <span style="color:#ff0000;"> </span>message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
24. new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));
25. message.sendToTarget();
26. return;
27. } catch (Throwable t) {
28. throw new RuntimeException("An error occured while executing "
29. + "doInBackground()", t);
30. }
32. message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
33. new AsyncTaskResult<Result>(AsyncTask.this, result));
34. message.sendToTarget();
35. }
36. };
37. }

其中WorkRunnable是一个Callable对象：

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1. private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
2. Params[] mParams;
3. }

3、执行，execute方法。

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1. public final AsyncTask<Params, Progress, Result> execute(Params... params) {
2. if (mStatus != Status.PENDING) {
3. switch (mStatus) {
4. case RUNNING:
5. throw new IllegalStateException("Cannot execute task:"
6. + " the task is already running.");
7. case FINISHED:
8. throw new IllegalStateException("Cannot execute task:"
9. + " the task has already been executed "
10. + "(a task can be executed only once)");
11. }
12. }
14. mStatus = Status.RUNNING;
16. onPreExecute();
18. mWorker.mParams = params;
19. sExecutor.execute(mFuture);
21. return this;
22. }

4、使用Handler通知UI线程，在上面的第二步中最后两行代码。