java - future - 线程未能停止

Question

我正在使用以下代码(大纲):

ExecutorService executor = Executors.newFixedThreadPool(20);
List<Future<statusModel>> futures = new ArrayList<Future<statusModel>>();

for (Map.Entry<String, String> url : urls.entrySet())
    {
        Future<statusModel> future = executor.submit(mycallable);
        futures.add(future);
    }
for (Map.Entry<String, String> url : urls.entrySet())
    {
        try
            {
                status = (statusModel) futures.get(i).get(50, TimeUnit.MILLISECONDS);
                // do stuff with status
            }
        catch (InterruptedException | ExecutionException | TimeoutException e) 
            {
                System.out.println("Error<checkServers>: Timeout OR "+e.getMessage());
            }   
    }

executor.shutdownNow();
System.out.println("Shutdown: "+executor.isShutdown());

我的控制台显示:关闭:true

myCallable:

public statusModel call() throws Exception 
{
    InputStream in = null;
    BufferedReader br = null;
    statusModel status = new statusModel();

    try 
    {
        URL url = new URL(urlStr);          
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();

        status.setStatusCode(conn.getResponseCode());
        status.setUrl(urlStr);

        if(status.getStatusCode()/100 == 2) // Status = OK
        { // Read JSON response  } 
            }
    catch (MalformedURLException e) 
    {
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } 
    catch (JSONException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    finally
    {
        if(in != null)
            in.close();
        if(br != null)
            br.close();
    }

    return status;
}

有时，当我一遍又一遍地运行此 block 时，我会收到此错误:

Aug 20, 2013 9:35:44 AM org.apache.catalina.core.StandardWrapper unload INFO: Waiting for 1 instance(s) to be deallocated Aug 20, 2013 9:35:45 AM org.apache.catalina.loader.WebappClassLoader clearReferencesThreads SEVERE: The web application [/Server_Status] is still processing a request that has yet to finish. This is very likely to create a memory leak. You can control the time allowed for requests to finish by using the unloadDelay attribute of the standard Context implementation. Aug 20, 2013 9:35:45 AM org.apache.catalina.loader.WebappClassLoader clearReferencesThreads SEVERE: The web application [/Server_Status] appears to have started a thread named [pool-3-thread-6] but has failed to stop it. This is very likely to create a memory leak.

我已经关闭了“执行器”，也检查了同样的情况。我仍然收到此错误。我在这里做错了什么吗？

更新:我是第一次使用 Future。如果我需要发布更多内容以更好地解释，请告诉我。

更新:我尝试打印所有 future.isDone()。由于某种原因，超出超时时间的 future 仍然返回 isDone() = false - 没有被 timout 取消:(

感谢任何帮助。提前致谢。

Answer 1

shutdownNow() 终止所有仍在运行的作业，这可能是导致错误消息的原因。您可能需要 shutdown() 它不会让任何新作业提交，但让已排队的作业继续执行。

此外，您需要通过调用 future.get() 等待 futures 完成(如果 future 尚未完成，则可能会阻塞)以获取结果。

for(Future<statusModel> future : futures){
  statusModel model = future.get();
  //do stuff with statusModel
}

编辑:既然已经发布了附加代码，我已将其附加到我的答案中:

我已经重新检查了 javadoc 中的 future.get(long timeout, TimeUnit unit)，它并没有说如果发生超时，那么 future 就会被取消。我还检查了 Brian Goetz 的《Java Concurrency in Practice》(顺便说一句，这是一本很棒的书；必读)，在第 147 页上推荐了以下内容:

try{
   future.get(timeout, unit);
}catch(TimeoutException e){
    //task will be cancelled below
}catch(ExecutionException e){
    //exception thrown in task; rethrow
    //... throw new MyWrappedException(e.getCause());
}finally{
    //Harmless if task already completed
    future.cancel(true); //interrupt if still running
}