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最近无意间看到了关于AsyncTask的一篇分析文章AsyncTask源码分析，记得很早之前还看过郭神博客里面分析了AsyncTask源码。去查看AsyncTask源码会发现里面使用了FutureTask在它自己的构造函数里面，我的sdk是android-23里面查看的。
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*/
public AsyncTask() {
// 这里注意它实现了Callable
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
Result result = doInBackground(mParams);
Binder.flushPendingCommands();
return postResult(result);
}
};
// 这里使用了FutureTask
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);
}
}
};
}其实这里把Callable和FutureTask2个比较重要的家伙弄懂，就能知道AsyncTask大致是怎么实现的了。进入源码去看发现它们都是java提供的用来实现java并发编程的。
先具体看下Callable的代码:
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}从代码注释很明显可以看出，是返回一个结果，那也表明我们可以在里面进行一些耗时操作之后（比如网络通信或者是数据库操作等等），然后在返回计算好之后的结果。
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
// 这里在call里面doInBackground
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
Result result = doInBackground(mParams);
Binder.flushPendingCommands();
return postResult(result);
}
};可以看到WorkerRunnable也是实现了Callable，然后里面调用了doInBackground。
然后我们再去看下FutureTask代码:
public class FutureTask<V> implements RunnableFuture<V> public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}public interface Future<V> {
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when {@code cancel} is called,
* this task should never run. If the task has already started,
* then the {@code mayInterruptIfRunning} parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return {@code true}. Subsequent calls to {@link #isCancelled}
* will always return {@code true} if this method returned {@code true}.
*
* @param mayInterruptIfRunning {@code true} if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return {@code false} if the task could not be cancelled,
* typically because it has already completed normally;
* {@code true} otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns {@code true} if this task was cancelled before it completed
* normally.
*
* @return {@code true} if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
boolean isDone();
/**
* 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
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @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
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}代码不多，算是比较简单的吧。
为了深刻理解它具体的运用，那我们自己write simple(丧心病狂) code。
private static class RealData implements Callable<String> {
@Override
public String call() throws Exception {
//这里是真实的业务逻辑，耗时很长
StringBuffer stringBuffer = new StringBuffer();
for (int i = 0; i < 10; i++) {
stringBuffer.append(i);
//模拟一段耗时操作
SystemClock.sleep(1 * 1000);
}
return stringBuffer.toString();
}
}
private void testCallable() {
FutureTask<String> futureTask = new FutureTask<String>(new RealData()) {
@Override
protected void done() {
//FutureTask执行完的回调
try {
// FutureTask的get()是一个同步方法,会有时间等待,最好避免在主线程执行
String str = get();
System.out.println();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
doSomething();
}
};
//自定义ExecutorService，我会在后面的文章中总结。
ExecutorService executor = Executors.newFixedThreadPool(1);
//在这里执行RealData的call内容
executor.submit(futureTask);
System.out.println();
}
private void doSomething() {
System.out.println();
/*执行回调结果*/
}好了简单的示例代码写好了,在call里面处理耗时并且返回结果,然后在done里面通过get拿到获取的结果，就可以利用Handler通知主线程更新UI了。还记得AsyncTask构造函数里面的代码么:
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
Result result = doInBackground(mParams);
Binder.flushPendingCommands();
return postResult(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);
}
}
};
}
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}它也是在done里面利用Handler发送Message去更新UI的。好了简单滴记录一下，今天就先这样吧！！！
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原文地址：http://blog.csdn.net/t1623183652/article/details/51352958
