blocks|key|264607|text|AsyncTask实例只能使用一次。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|264608|取而代之的是，将你的任务命名为new+MyAsyncTask().execute("");|264609|在AsyncTask应用编程接口文档中：|264610|线程化规则|264611|要让这个类正常工作，必须遵循一些线程规则：|264612|必须在UI+thread.|264613|execute(Params...)上创建|unordered-list-item|264614|任务实例必须在UI线程上调用。|264615|不调用onPreExecute()、onPostExecute(结果)、doInBackground(参数...)、onProgressUpdate(进度...)manually.|264616|The任务只能执行一次(如果第二次执行为attempted.)|BOLD|264617|，则会引发异常|264618|264619|entityMap^0|0|9|0|F|U|0|0|0|0|0|0|0|0|0|V|0|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@$9|15|A|16|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|17|8|@$9|18|A|19|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|1A|8|@]|D|@]|E|$]]|$1|J|3|K|5|6|7|1B|8|@]|D|@]|E|$]]|$1|L|3|M|5|6|7|1C|8|@]|D|@]|E|$]]|$1|N|3|O|5|6|7|1D|8|@]|D|@]|E|$]]|$1|P|3|Q|5|R|7|1E|8|@]|D|@]|E|$]]|$1|S|3|T|5|R|7|1F|8|@]|D|@]|E|$]]|$1|U|3|V|5|R|7|1G|8|@]|D|@]|E|$]]|$1|W|3|X|5|R|7|1H|8|@$9|1I|A|1J|B|Y]]|D|@]|E|$]]|$1|Z|3|10|5|6|7|1K|8|@]|D|@]|E|$]]|$1|11|3|-4|5|6|7|1L|8|@]|D|@]|E|$]]|$1|12|3|-4|5|6|7|1M|8|@]|D|@]|E|$]]]|13|$]]

<code>AsyncTask</code> instances can only be used one time.
Instead, just call your task like <code>new MyAsyncTask().execute(&quot;&quot;);</code>
From the AsyncTask API docs:
<h3>Threading rules</h3>
There are a few threading rules that must be followed for this class to work properly:
<ul>
<li>The task instance must be created on the UI thread.</li>
<li>execute(Params...) must be invoked on the UI thread.</li>
<li>Do not call onPreExecute(), onPostExecute(Result), doInBackground(Params...), onProgressUpdate(Progress...) manually.</li>
<li>The task can be executed only once (an exception will be thrown if a second execution is attempted.)</li>
</ul>

blocks|key|264629|text|Steve+Prentice的回答很好地解释了ASyncTask“即发即忘”实例的原因--然而，当你执行一个ASyncTask的次数受到限制时，你可以在线程运行时自由地做你喜欢的事情……|type|unstyled|depth|inlineStyleRanges|entityRanges|data|264630|将可执行代码放在doInBackground()内的循环中，并使用并发锁来触发每次执行。您可以使用publishProgress()/onProgressUpdate()检索结果。|264631|示例：|offset|length|style|BOLD|264632|class+GetDataFromServerTask+extends+AsyncTask<Input,+Result,+Void>+{

++++private+final+ReentrantLock+lock+=+new+ReentrantLock();
++++private+final+Condition+tryAgain+=+lock.newCondition();
++++private+volatile+boolean+finished+=+false;

++++@Override
++++protected+Void+doInBackground(Input...+params)+{

++++++++lock.lockInterruptibly();

++++++++do+{+
++++++++++++//+This+is+the+bulk+of+our+task,+request+the+data,+and+put+in+"result"
++++++++++++Result+result+=+....

++++++++++++//+Return+it+to+the+activity+thread+using+publishProgress()
++++++++++++publishProgress(result);

++++++++++++//+At+the+end,+we+acquire+a+lock+that+will+delay
++++++++++++//+the+next+execution+until+runAgain()+is+called..
++++++++++++tryAgain.await();

++++++++}+while(!finished);

++++++++lock.unlock();
++++}

++++@Override
++++protected+void+onProgressUpdate(Result...+result)+
++++{
++++++++//+Treat+this+like+onPostExecute(),+do+something+with+result

++++++++//+This+is+an+example...
++++++++if+(result+!=+whatWeWant+&&+userWantsToTryAgain())+{
++++++++++++runAgain();
++++++++}
++++}

++++public+void+runAgain()+{
++++++++//+Call+this+to+request+data+from+the+server+again
++++++++tryAgain.signal();
++++}

++++public+void+terminateTask()+{
++++++++//+The+task+will+only+finish+when+we+call+this+method
++++++++finished+=+true;
++++++++lock.unlock();
++++}

++++@Override
++++protected+void+onCancelled()+{
++++++++//+Make+sure+we+clean+up+if+the+task+is+killed
++++++++terminateTask();
++++}
}|code-block|syntax|javascript|264633|当然，这比ASyncTask的传统用法稍微复杂一些，并且您放弃了使用publishProgress()来进行实际的进度报告。但是如果内存是您关心的问题，那么这种方法将确保在运行时堆中只有一个ASyncTask。|264634|entityMap^0|0|0|0|3|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|U|8|@$F|V|G|W|H|I]]|9|@]|A|$]]|$1|J|3|K|5|L|7|X|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

The reasons for fire-and-forget instances of ASyncTask are detailed pretty well in Steve Prentice's answer - However, whilst you are restricted on how many times you execute an ASyncTask, you are free to do what you like whilst the thread is running...

Put your executable code inside a loop within doInBackground() and use a concurrent lock to trigger each execution. You can retrieve the results using publishProgress()/onProgressUpdate().

Example:

<pre><code>class GetDataFromServerTask extends AsyncTask&lt;Input, Result, Void&gt; {

 private final ReentrantLock lock = new ReentrantLock();
 private final Condition tryAgain = lock.newCondition();
 private volatile boolean finished = false;

 @Override
 protected Void doInBackground(Input... params) {

 lock.lockInterruptibly();

 do { 
 // This is the bulk of our task, request the data, and put in "result"
 Result result = ....

 // Return it to the activity thread using publishProgress()
 publishProgress(result);

 // At the end, we acquire a lock that will delay
 // the next execution until runAgain() is called..
 tryAgain.await();

 } while(!finished);

 lock.unlock();
 }

 @Override
 protected void onProgressUpdate(Result... result) 
 {
 // Treat this like onPostExecute(), do something with result

 // This is an example...
 if (result != whatWeWant &amp;&amp; userWantsToTryAgain()) {
 runAgain();
 }
 }

 public void runAgain() {
 // Call this to request data from the server again
 tryAgain.signal();
 }

 public void terminateTask() {
 // The task will only finish when we call this method
 finished = true;
 lock.unlock();
 }

 @Override
 protected void onCancelled() {
 // Make sure we clean up if the task is killed
 terminateTask();
 }
}
</code></pre>

Of course, this is slightly more complicated than the traditional usage of ASyncTask, and you give up the use of publishProgress() for actual progress reporting. But if memory is your concern, then this approach will ensure only one ASyncTask remains in the heap at runtime.

blocks|key|262477|text|我已经将旋转任务设置为静态的，这有助于我在旋转更改时将它们附加、分离和重新附加到UI线程。但是回到你的问题，我所做的是创建一个标志来查看线程是否正在运行。当你想重新启动线程时，我会检查旋转任务是否正在运行，如果是，我会给出一个警告。如果不是，我将它设为空，然后创建一个新的，它将解决您看到的错误。此外，在成功完成后，我会清空已完成的旋转感知任务，以便它可以再次执行。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|262478|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I have made my rotation tasks static, which then helped me attach, detach, and reattach them to UI threads on rotation changes. But to go back to your question, what I do is create a flag to see if the thread is running. When you want to restart the thread I check if the rotation task is running if it is I toast a warning. If it is not, I make it null and then create a new one, which will work around the error you are seeing. Furthermore, upon successful completion I null out the completed rotation aware task so that it is ready to go again.

blocks|key|3172341|text|是的，这是真的，文档上说只能执行一个异步任务。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3172342|每次你需要使用它的时候，你都必须实例化：|3172343|//+Any+time+if+you+need+to+call+her
final+FirmwareDownload+fDownload+=+new+FirmwareDownload();
fDownload.execute("your+parameter");

static+class+FirmwareDownload+extends+AsyncTask<String,+String,+String>+{
}|code-block|syntax|javascript|3172344|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Yes it is true, the doc says that only can be executed one Asyntask. 

Every time that you need to use it you have to instance:

<pre><code>// Any time if you need to call her
final FirmwareDownload fDownload = new FirmwareDownload();
fDownload.execute("your parameter");

static class FirmwareDownload extends AsyncTask&lt;String, String, String&gt; {
}
</code></pre>

blocks|key|3172363|text|我也有同样的问题。在我的例子中，我有一个任务，我想用onCreate()和onResume(来完成)。所以我把我的Asynctask设为静态的，并从中获取实例。现在我们仍然有同样的问题。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3172364|所以我在onPostExecute()中做的是：|3172365|instance+=+null;|code-block|syntax|javascript|3172366|请记住，我在静态getInstance方法中检查我的实例不为空，否则我将创建它：|3172367|if+(instance+==+null){
++++instance+=+new+Task();
}
return+instance;|3172368|postExecute中的方法将清空实例并重新创建它。当然，这可以在课外完成。|3172369|entityMap^0|Q|A|11|9|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Z|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|11|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|13|8|@]|D|@]|E|$]]|$1|S|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|T|$]]

I had the same issue. In my case i have a task i want to do in <code>onCreate()</code> and in <code>onResume(</code>). So i made my Asynctask static, and get the instance from it. Now we still have the same problem. 

So what i did in the onPostExecute() is this:

<pre><code>instance = null;
</code></pre>

Keeping in mind that i check in the static getInstance method that my instance isn't null, else i create it:

<pre><code>if (instance == null){
 instance = new Task();
}
return instance;
</code></pre>

The method in postExecute will empty the instance and recreate it. Of course this can be done outside the class.

In my Activity I use a class which extends from AsyncTask and a parameter which is an instance of that AsyncTask. When I call <code>mInstanceOfAT.execute("")</code> everything is fine.
But the app crash when I press an update button which calls again the AsyncTask(In case the network job didnt work). Cause then appears an Exception which says 

<blockquote>
 Cannot execute task: the task has
 already been executed (a task can be
 executed only once)
</blockquote>

I have tried calling cancel(true) for the instance of the Asyctask, but it doesnt work either. The only solution so far it's to create new instances of the Asyntask. Is that the correct way?

Thanks.

Execute AsyncTask several times

在我的活动中，我使用了一个从AsyncTask扩展的类和一个参数，它是AsyncTask的一个实例。当我调用mInstanceOfAT.execute("")时，一切都很好。但当我按下更新按钮，再次调用AsyncTask时，应用程序崩溃(以防网络作业不起作用)。原因然后出现了一个异常，它说无法执行任务:任务已经执行(一...

问多次执行AsyncTask
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