AsyncTask用于在Android中异步处理耗时操作，用法上非常简单。那么，我们不仅要会用，还要知道AsyncTask内部是怎么实现的。首先我们需要知道的是AsyncTask的内部包含哪些变量。
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE_SECONDS = 30;
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);
/**
* An {@link Executor} that can be used to execute tasks in parallel.
*/
public static final Executor THREAD_POOL_EXECUTOR;
static {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
sPoolWorkQueue, sThreadFactory);
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
}
AsyncTask内部维护着一个线程池，首先我们定义三个变量，分别是当前电脑CPU核数+1，CPU核数 * 2+1，初始化一个线程池的时候，分别用作该线程池的核心线程以及最大线程数。接着初始化一个ThreadFactory，内部维护一个AtomicInteger，用于计算开辟的线程数，以及实现newThread（Runnable r）方法，初始化一个线程。接下来就是初始化一个队列和线程池。
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
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);
}
}
}
AsyncTask内部维护着一个静态的SerialExecutor，同时这是AsyncTask的默认线程池。需要注意的是，SerialExecutor是串行处理线程的。
在SerialExecutor内部有一个双端队列，调用execute方法后，会将传入的Runnable进行封装并传至队尾。在mActive为null或者执行完一个Runnable后调用scheduleNext（）从队列中返回一个Runnalbe传入ThreadPoolExecute进行处理。
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;
}
}
}
在InternalHandler中得到主线程的Looper，并接受从AsyncTask后台线程中发送过来的数据。我们可以看一下AsyncTaskResult的内部实现。
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
在实例化AsyncTask时，构造器中会初始化一个WorkRunnable和一个FutureTask，并将WorkRunnable的对象传递给mTask。
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
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 occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
在mWork中会调用doInBackground方法，在后台线程中执行任务并返回一个Result，接着调用postResult（result）将消息发送至主线程中的消息队列。
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
mFuture重写了done（）方法，done（）方法无论如何都会被执行，所以不论是否取消还是执行完毕，都会调用postResultIfNotInvoked（）方法，需要注意的是，如果传入了get（）会一直阻塞直到得到返回结果。
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
那么我们在主线程调用了AsyncTask.execute（）后怎么执行呢？在execure（）方法中会调用executeOnExecutor（....，...）方法。传入的参数为默认线程池以及刚传入的数据。在该方法中会先判断线程的状态，只有处于未运行状态才会执行，否则抛出异常。接着讲传入的params赋值给mWork并调用线程池执行mFuture。
@MainThread
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;
}
在执行过程中调用publishProgress（Progress ....values）方法会将即使数据传递至主线程中的InternalHanlder中。
@WorkerThread
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
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作者：qq_38256015
来源：CSDN
原文：blog.csdn.net/qq_38256015…
版权声明：本文为博主原创文章，转载请附上博文链接！ |
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