如何扩展FutureTask并确保释放对可调用的引用？

Question

我有一个自定义的ExecutorService，其中包含一个ScheduledExecutorService，可以用来中断提交给ExecutorSerice的任务，如果它们花费的时间太长，我已经将complet类放在文章的末尾。

这是正常的，只是有时中断本身会造成问题，所以我将一个易失性的布尔值、cancel标志添加到一个新的CanceableTask类中，并使它们成为子类，这样如果布尔值被发送为true，它们就可以检查和停止自己。注意，它们是提交给executor服务precisley的每个类中布尔值的一个实例，这样就可以在不取消其他任务的情况下取消长期运行的任务。

但是，FutureTask作为参数传递给beforeExecute(Thread，Runnable r)，这不会授予对可调用类的访问权，因此我的超时代码无法设置cancel标志。

为此，我重写了newTaskFor方法以返回一个类，该类仅提供对可调用对象的引用。

public class FutureCallable<V> extends FutureTask<V>
{
    private Callable<V> callable;
    public FutureCallable(Callable<V> callable) {
        super(callable);
        this.callable = callable;
    }
    public Callable<V> getCallable() {
        return callable;
    }
}

一切都很顺利，或者说我是这么想的。

不幸的是，随着新任务提交到ExecutorService并最终耗尽内存，我的应用程序使用了越来越多的内存，当我分析应用程序时，我发现所有FutureCallables都有一个线程堆栈本地引用，即使在任务完成之后，而且由于FutureCallable引用了正在运行的类，所以它使用了大量内存。

当我查看FutureTask的代码(FutureCallable扩展)时，有一个针对私有可调用引用的注释：

/** The underlying callable; nulled out after running */

那么，我如何改进我的FutureCallable，使其对可调用的引用无效呢？或者为什么在任务完成后对维护的FutureCallable有一个引用。

我已经确认，如果我注释掉了newTaskFor方法，就不会有过多的内存使用，但不幸的是，我不能取消这个类。

全班是：

public class TimeoutThreadPoolExecutor extends ThreadPoolExecutor {
    private final long timeout;
    private final TimeUnit timeoutUnit;

    private final static int WAIT_BEFORE_INTERRUPT = 10000;
    private final static int WAIT_BEFORE_STOP      = 10000;


    private final ScheduledExecutorService timeoutExecutor = Executors.newSingleThreadScheduledExecutor();

    //Map Task to the Future of the Timeout Task that could be used to interrupt it
    private final ConcurrentMap<Runnable, ScheduledFuture> runningTasks = new ConcurrentHashMap<Runnable, ScheduledFuture>();

    public long getTimeout()
    {
        return timeout;
    }

    public TimeUnit getTimeoutUnit()
    {
        return timeoutUnit;
    }

    public TimeoutThreadPoolExecutor(int workerSize, ThreadFactory threadFactory, long timeout, TimeUnit timeoutUnit)
    {
        super(workerSize, workerSize, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(), threadFactory);
        MainWindow.logger.severe("Init:"+workerSize+":Timeout:"+timeout+":"+timeoutUnit);
        this.timeout = timeout;
        this.timeoutUnit = timeoutUnit;
    }

    public TimeoutThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, long timeout, TimeUnit timeoutUnit) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
        this.timeout = timeout;
        this.timeoutUnit = timeoutUnit;
    }

    @Override
    public <T> FutureCallable<T> newTaskFor(Callable<T> callable) {
        return new FutureCallable<T>(callable);
    }

    @Override
    public List<Runnable> shutdownNow() {
        timeoutExecutor.shutdownNow();
        return super.shutdownNow();
    }

    @Override
    protected void beforeExecute(Thread t, Runnable r) {
        if(timeout > 0) {
            //Schedule a task to interrupt the thread that is running the task after time timeout starting from now
            final ScheduledFuture<?> scheduled = timeoutExecutor.schedule(new TimeoutTask(t, r), timeout, timeoutUnit);

            //Add Mapping
            runningTasks.put(r, scheduled);
        }
    }

    @Override
    protected void afterExecute(Runnable r, Throwable t) {

        //AfterExecute will be called after the task has completed, either of its own accord or because it
        //took too long and was interrupted by corresponding timeout task
        //Remove mapping and cancel timeout task
        ScheduledFuture timeoutTask = runningTasks.remove(r);
        if(timeoutTask != null) {
            timeoutTask.cancel(false);
        }

    }

    @Override
    protected void terminated()
    {
        //All tasks have completed either naturally or via being cancelled by timeout task so close the timeout task
        MainWindow.logger.severe("---Shutdown TimeoutExecutor");
        timeoutExecutor.shutdown();
    }

    /**
     * Interrupt or possibly stop the thread
     *
     */
    class TimeoutTask implements Runnable {
        private final       Thread thread;
        private             Callable c;

        public TimeoutTask(Thread thread, Runnable c) {
            this.thread = thread;
            if(c instanceof FutureCallable)
            {
                this.c = ((FutureCallable) c).getCallable();
            }
        }

        @Override
        public void run()
        {
            String msg = "";
            if (c != null)
            {
                if (c != null && c instanceof CancelableTask)
                {
                    MainWindow.logger.severe("+++Cancelling " + msg + " task because taking too long");
                    ((CancelableTask) c).setCancelTask(true);
                }
            }
        }
    }
}

    public abstract class CancelableTask  extends ExecutorServiceEnabledAnalyser
    {
        private volatile boolean cancelTask = false;

        public boolean isCancelTask() {
            return cancelTask;
        }

        public void setCancelTask(boolean cancelTask) {
            this.cancelTask = cancelTask;
        }

        CancelableTask(final MainWindow start, boolean isSelectedRecords, boolean isUseRowSelection)
        {
            super(start, isSelectedRecords, isUseRowSelection);
        }

        CancelableTask(final MainWindow start, List<MetadataChangedWrapper> songs)
        {
            super(start, songs );
        }

    }

Answer 1

这个ThreadLocal在哪里？我觉得很奇怪，也很难相信你在说什么，它能保持对所有任务的引用，即使是在完成之后。如果是这样的话，即使没有覆盖，它最终也会耗尽内存(任务本身使用一些内存，尽管可能比可调用的内存少，但仍然不是零)。

无论如何，您可以在您的done上重写FutureCallable方法，以便在执行后将包装的对象空出。