转载请注明出处:http://blog.csdn.net/singwhatiwanna/article/details/17596225
前言
什么是AsyncTask,相信搞过android开发的朋友们都不陌生。AsyncTask内部封装了Thread和Handler,可以让我们在后台进行计算并且把计算的结果及时更新到UI上,而这些正是Thread+Handler所做的事情,没错,AsyncTask的作用就是简化Thread+Handler,让我们能够通过更少的代码来完成一样的功能,这里,我要说明的是:AsyncTask只是简化Thread+Handler而不是替代,实际上它也替代不了。同时,AsyncTask从最开始到现在已经经过了几次代码修改,任务的执行逻辑慢慢地发生了改变,并不是大家所想象的那样:AsyncTask是完全并行执行的就像多个线程一样,其实不是的,所以用AsyncTask的时候还是要注意,下面会一一说明。另外本文主要是分析AsyncTask的源代码以及使用时候的一些注意事项,如果你还不熟悉AsyncTask,请先阅读android之AsyncTask 来了解其基本用法。
这里先给出AsyncTask的一个例子:
- private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
- protected Long doInBackground(URL... urls) {
- int count = urls.length;
- long totalSize = 0;
- for (int i = 0; i < count; i++) {
- totalSize += Downloader.downloadFile(urls[i]);
- publishProgress((int) ((i / (float) count) * 100));
-
- if (isCancelled()) break;
- }
- return totalSize;
- }
-
- protected void onProgressUpdate(Integer... progress) {
- setProgressPercent(progress[0]);
- }
-
- protected void onPostExecute(Long result) {
- showDialog("Downloaded " + result + " bytes");
- }
- }
使用AsyncTask的规则
- AsyncTask的类必须在UI线程加载(从4.1开始系统会帮我们自动完成)
- AsyncTask对象必须在UI线程创建
- execute方法必须在UI线程调用
- 不要在你的程序中去直接调用onPreExecute(), onPostExecute, doInBackground, onProgressUpdate方法
- 一个AsyncTask对象只能执行一次,即只能调用一次execute方法,否则会报运行时异常
- AsyncTask不是被设计为处理耗时操作的,耗时上限为几秒钟,如果要做长耗时操作,强烈建议你使用Executor,ThreadPoolExecutor以及FutureTask
- 在1.6之前,AsyncTask是串行执行任务的,1.6的时候AsyncTask开始采用线程池里处理并行任务,但是从3.0开始,为了避免AsyncTask所带来的并发错误,AsyncTask又采用一个线程来串行执行任务
AsyncTask到底是串行还是并行?
给大家做一下实验,请看如下实验代码:代码很简单,就是点击按钮的时候同时执行5个AsyncTask,每个AsyncTask休眠3s,同时把每个AsyncTask执行结束的时间打印出来,这样我们就能观察出到底是串行执行还是并行执行。
- @Override
- public void onClick(View v) {
- if (v == mButton) {
- new MyAsyncTask("AsyncTask#1").execute("");
- new MyAsyncTask("AsyncTask#2").execute("");
- new MyAsyncTask("AsyncTask#3").execute("");
- new MyAsyncTask("AsyncTask#4").execute("");
- new MyAsyncTask("AsyncTask#5").execute("");
- }
-
- }
-
- private static class MyAsyncTask extends AsyncTask<String, Integer, String> {
-
- private String mName = "AsyncTask";
-
- public MyAsyncTask(String name) {
- super();
- mName = name;
- }
-
- @Override
- protected String doInBackground(String... params) {
- try {
- Thread.sleep(3000);
- } catch (InterruptedException e) {
- e.printStackTrace();
- }
- return mName;
- }
-
- @Override
- protected void onPostExecute(String result) {
- super.onPostExecute(result);
- SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
- Log.e(TAG, result + "execute finish at " + df.format(new Date()));
- }
- }
我找了2个手机,系统分别是4.1.1和2.3.3,按照我前面的描述,AsyncTask在4.1.1应该是串行的,在2.3.3应该是并行的,到底是不是这样呢?请看Log
Android 4.1.1上执行:从下面Log可以看出,5个AsyncTask共耗时15s且时间间隔为3s,很显然是串行执行的
Android 2.3.3上执行:从下面Log可以看出,5个AsyncTask的结束时间是一样的,很显然是并行执行
结论:从上面的两个Log可以看出,我前面的描述是完全正确的。下面请看源码,让我们去了解下其中的原理。
源码分析
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- package android.os;
-
- import java.util.ArrayDeque;
- import java.util.concurrent.BlockingQueue;
- import java.util.concurrent.Callable;
- import java.util.concurrent.CancellationException;
- import java.util.concurrent.Executor;
- import java.util.concurrent.ExecutionException;
- import java.util.concurrent.FutureTask;
- import java.util.concurrent.LinkedBlockingQueue;
- import java.util.concurrent.ThreadFactory;
- import java.util.concurrent.ThreadPoolExecutor;
- import java.util.concurrent.TimeUnit;
- import java.util.concurrent.TimeoutException;
- import java.util.concurrent.atomic.AtomicBoolean;
- import java.util.concurrent.atomic.AtomicInteger;
-
- public abstract class AsyncTask<Params, Progress, Result> {
- private static final String LOG_TAG = "AsyncTask";
-
-
- 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;
-
- 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>(128);
-
-
-
-
-
- public static final Executor THREAD_POOL_EXECUTOR
- = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
- TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
-
-
-
-
-
- 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);
- }
- }
- }
-
-
-
-
- public enum Status {
-
-
-
- PENDING,
-
-
-
- RUNNING,
-
-
-
- FINISHED,
- }
-
-
- public static void init() {
- sHandler.getLooper();
- }
-
-
- 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);
-
- 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() {
- }
-
-
-
-
-
- protected void onPostExecute(Result result) {
- }
-
-
-
-
-
-
-
- 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:
-
- 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;
- }
- }
- }
让你的AsyncTask在3.0以上的系统中并行起来
通过上面的源码分析,我已经给出了在3.0以上系统中让AsyncTask并行执行的方法,现在,让我们来试一试,代码还是之前采用的测试代码,我们要稍作修改,调用AsyncTask的executeOnExecutor方法而不是execute,请看:
- @TargetApi(Build.VERSION_CODES.HONEYCOMB)
- @Override
- public void onClick(View v) {
- if (v == mButton) {
- new MyAsyncTask("AsyncTask#1").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
- new MyAsyncTask("AsyncTask#2").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
- new MyAsyncTask("AsyncTask#3").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
- new MyAsyncTask("AsyncTask#4").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
- new MyAsyncTask("AsyncTask#5").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
- }
-
- }
-
- private static class MyAsyncTask extends AsyncTask<String, Integer, String> {
-
- private String mName = "AsyncTask";
-
- public MyAsyncTask(String name) {
- super();
- mName = name;
- }
-
- @Override
- protected String doInBackground(String... params) {
- try {
- Thread.sleep(3000);
- } catch (InterruptedException e) {
- e.printStackTrace();
- }
- return mName;
- }
-
- @Override
- protected void onPostExecute(String result) {
- super.onPostExecute(result);
- SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
- Log.e(TAG, result + "execute finish at " + df.format(new Date()));
- }
- }
下面是系统为4.1.1手机打印出的Log:很显然,我们的目的达到了,成功的让AsyncTask在4.1.1的手机上并行起来了,很高兴吧!希望这篇文章对你有用。