AsyncTask实现代码原理

Android从1.5开始引入了AsyncTask这个类，可以帮助我们解决线程和界面刷新问题，主要是对Thread+Handler这样的封装，但在设计模式和代码维护方面都有不错的表现。早在2008年时Google推出了一个示例应用叫PhotoStream来演示UI在多线程网络慢速I/O下的刷新问题，里面的线程构造使用的正是AsyncTask的雏形，由于内部使用Java 1.5的并发库比普通初级Android开发者编写的Thread+Handler稳定很多，下面我们就android.os.AsyncTask的实现

public abstract class AsyncTask<Params, Progress, Result> {
private static final String LOG_TAG = "AsyncTask";

private static final int CORE_POOL_SIZE = 5; //线程池数量
private static final int MAXIMUM_POOL_SIZE = 128; //线程池中最大线程数
private static final int KEEP_ALIVE = 10;

private static final BlockingQueue<Runnable> sWorkQueue =
new LinkedBlockingQueue<Runnable>(10); //使用并发库的阻塞队列初始时保存10个Runnable对象

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 ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory); //创建线程池

private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
private static final int MESSAGE_POST_CANCEL = 0x3;

private static final InternalHandler sHandler = new InternalHandler(); //这个是对Handler的封装，内部处理Thread的状态。

private final WorkerRunnable<Params, Result> mWorker; //该类对Runnable做简单封装
private final FutureTask<Result> mFuture; //对于并发库而言FutureTask是最重要的，有兴趣的网友可以看下JDK源码

private volatile Status mStatus = Status.PENDING; //保存当前线程状态

public enum Status { //枚举类记录当前线程状态
PENDING,
RUNNING,
FINISHED,
}

public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() { //构造Runable对象
public Result call() throws Exception {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //设置线程优先级为后台，这里Android开发网提示大家低于标准线程优先级
return doInBackground(mParams);
}
};

mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
Message message;
Result result = null;

try {
result = get();
} catch (InterruptedException e) { //处理Thread中断异常
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occured while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));

message.sendToTarget();
return;
} catch (Throwable t) {
throw new RuntimeException("An error occured while executing "
+ "doInBackground()", t);
}

message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result)); //执行完后通过Handler通知结果
message.sendToTarget();
}
};
}

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) {
}

protected void onCancelled() {
}

public final boolean isCancelled() {
return mFuture.isCancelled();
}

public final boolean cancel(boolean mayInterruptIfRunning) {
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) {
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;
sExecutor.execute(mFuture);

return this;
}

protected final void publishProgress(Progress... values) { //通过Handler通知UI刷新
sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}

private void finish(Result result) {
if (isCancelled()) result = null;
onPostExecute(result);
mStatus = Status.FINISHED;
}

private static class InternalHandler extends Handler { //和我们的Handler没有什么不同
@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;
case MESSAGE_POST_CANCEL:
result.mTask.onCancelled();
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;
}
}
}

经过上面简单分析, 相信大家对Android AsyncTask会有更加深刻的理解，整个AsyncTask实现基于Thread+Handler，但对于Thread使用的是Java的并发包的FutureTask具体的可以参考JDK5以后的源码。