Java 8'de engelleme arka plan yükleyicisi nasıl oluşturulur?

Soru

Java 8'de uygun bir arka plan yükleyiciyi nasıl oluşturursunuz? Koşullar:

• veriler arka planda yüklenmelidir
• Yüklemeden sonra veriler gösterilmelidir
• veriler yüklenirken başka hiçbir istek kabul edilmemelidir
• veriler yüklenirken istekler varsa, belirli bir zaman aşımından sonra başka bir yükleme planlanmalıdır (örn. 5 saniye)

Amaç, yeniden yükleme isteklerini kabul ettirmektir, ancak veritabanı isteklerle dolmaz.

MCVE

İşte bir MCVE. Thread.sleep'ı 2 saniye boyunca çağırarak yüklemeyi simüle eden bir arka plan görevinden oluşur. Görev, doğal olarak kaçınılması gereken arka plan yükleme görevlerinin çakışmasına yol açan her saniye zamanlanır.

public class LoadInBackgroundExample {

  /**
   * A simple background task which should perform the data loading operation. In this minimal example it simply invokes Thread.sleep
   */
  public static class BackgroundTask implements Runnable {

    private int id;

    public BackgroundTask(int id) {
      this.id = id;
    }

    /**
     * Sleep for a given amount of time to simulate loading.
     */
    @Override
    public void run() {

      try {

        System.out.println("Start #" + id + ": " + Thread.currentThread());

        long sleepTime = 2000; 
        Thread.sleep( sleepTime);

      } catch (InterruptedException e) {
        e.printStackTrace();
      } finally {
        System.out.println("Finish #" + id + ": " + Thread.currentThread());
      }

    }
  }

  /**
   * CompletableFuture which simulates loading and showing data.
   * @param taskId Identifier of the current task
   */
  public static void loadInBackground( int taskId) {

    // create the loading task
    BackgroundTask backgroundTask = new BackgroundTask( taskId);

    // "load" the data asynchronously
    CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(new Supplier<String>() {

      @Override
      public String get() {

        CompletableFuture<Void> future = CompletableFuture.runAsync(backgroundTask);

        try {

          future.get();

        } catch (InterruptedException | ExecutionException e) {
          e.printStackTrace();
        }

        return "task " + backgroundTask.id;
      }
    });

    // display the data after they are loaded
    CompletableFuture<Void> future = completableFuture.thenAccept(x -> {

      System.out.println( "Background task finished:" + x);

    });

  }


  public static void main(String[] args) {

    // runnable which invokes the background loader every second
    Runnable trigger = new Runnable() {

      int taskId = 0;

      public void run() { 

        loadInBackground( taskId++);

      }
    };

    // create scheduler
    ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
    ScheduledFuture<?> beeperHandle = scheduler.scheduleAtFixedRate(trigger, 0, 1, TimeUnit.SECONDS);

    // cancel the scheudler and the application after 10 seconds
    scheduler.schedule(() -> beeperHandle.cancel(true), 10, TimeUnit.SECONDS);

    try {
      beeperHandle.get();
    } catch (Throwable th) {
    }

    System.out.println( "Cancelled");
    System.exit(0);
  }

}

Çıktı şudur:

Start #0: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Start #1: Thread[ForkJoinPool.commonPool-worker-4,5,main]
Start #2: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Finish #0: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Background task finished:task 0
Finish #1: Thread[ForkJoinPool.commonPool-worker-4,5,main]
Background task finished:task 1
Start #3: Thread[ForkJoinPool.commonPool-worker-4,5,main]
Finish #2: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Background task finished:task 2
Start #4: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Start #5: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Finish #3: Thread[ForkJoinPool.commonPool-worker-4,5,main]
Background task finished:task 3
Start #6: Thread[ForkJoinPool.commonPool-worker-4,5,main]
Finish #4: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Background task finished:task 4
Finish #5: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Background task finished:task 5
Start #7: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Finish #6: Thread[ForkJoinPool.commonPool-worker-4,5,main]
Start #8: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Background task finished:task 6
Start #9: Thread[ForkJoinPool.commonPool-worker-4,5,main]
Finish #7: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Background task finished:task 7
Start #10: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Finish #8: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Background task finished:task 8
Cancelled

Amaç örneğin # 1 ve # 2 atlamaktır çünkü # 0 hala çalışıyor.

Sorun

Engelleme mekanizmasını nerede doğru şekilde ayarladınız? Senkronizasyon kullanılmalı mı? Veya bazıları AtomicBoolean? Ve eğer öyleyse, get()yöntemin içinde mi yoksa başka bir yerde mi olmalı ?

Görevi yürütmek için zaten bir iş parçacığınız var. Zorunlu değildir ve görevi başka bir ForkJoinPoolzaman uyumsuz yürütücüde çalıştırmak için karmaşık bir şey yapmak ( kullandığınızda CompletableFuture)

Basitleştir:

public static void loadInBackground(int taskId) {
    // create the loading task
    BackgroundTask backgroundTask = new BackgroundTask(taskId);
    // No need to run in async, as it already in executor
    backgroundTask.run();
}

ScheduledExecutorService, scheduleAtFixedRate ile çağırdığınızda aynı anda yalnızca bir görevin çalıştırılmasını sağlar

Önce verilen ilk gecikmeden sonra ve sonra verilen süre ile etkinleştirilen periyodik bir eylem oluşturur ve yürütür; yani yürütme, initialDelay sonra initialDelay + periyodu, sonra initialDelay + 2 * periyodu vb. sonra başlayacaktır. Görevin herhangi bir yürütülmesi bir istisna ile karşılaşırsa, sonraki yürütmeler bastırılır. Aksi takdirde, görev yalnızca yürütmenin iptali veya feshi ile sona erer. Bu görevin yürütülmesi süresinden daha uzun sürerse, sonraki yürütmeler geç başlayabilir, ancak eşzamanlı olarak yürütülmez .

Aşağıdakileri gereksinimler olarak alarak:

• veriler arka planda yüklenmelidir
• Yüklemeden sonra veriler gösterilmelidir
• veriler yüklenirken başka hiçbir istek kabul edilmemelidir
• veriler yüklenirken istekler varsa, belirli bir zaman aşımından sonra başka bir yükleme planlanmalıdır (örn. 5 saniye)

Çözelti inşa dayanabilir ca Executors.newSingleThreadExecutor(), CompletableFutureve LinkedBlockingQueue:

public class SingleThreadedLoader {

  private static class BackgroundTask extends CompletableFuture<String> {

    private final String query;

    private BackgroundTask(String query) {
      this.query = query;
    }

    public String getQuery() {
      return query;
    }
  }

  private final BlockingQueue<BackgroundTask> tasks = new LinkedBlockingQueue<>();
  // while data are loaded no further requests should be accepted
  private final Executor executor = Executors.newSingleThreadExecutor();

  private final int delaySeconds;

  private AtomicReference<Instant> lastExecution = new AtomicReference<>(Instant.EPOCH);

  public SingleThreadedLoader(int delaySeconds) {
    this.delaySeconds = delaySeconds;
    setupLoading();
  }

  public BackgroundTask loadInBackground(String query) {
    log("Enqueued query " + query);
    BackgroundTask task = new BackgroundTask(query);
    tasks.add(task);
    return task;
  }

  private void setupLoading() {
    // data should be loaded in background
    executor.execute(() -> {
      while (true) {
        try {
          // if there were requests while the data were loaded
          // another loading should be scheduled after a certain timeout (e. g. 5 seconds)
          Instant prev = lastExecution.get();
          long delay = Duration.between(prev, Instant.now()).toSeconds();
          if (delay < delaySeconds) {
            log("Waiting for 5 seconds before next data loading");
            TimeUnit.SECONDS.sleep(delaySeconds - delay);
          }
          BackgroundTask task = tasks.take();
          try {
            String query = task.getQuery();
            String data = loadData(query);
            task.complete(data);
          } catch (Exception e) {
            task.completeExceptionally(e);
          }
          lastExecution.set(Instant.now());
        } catch (InterruptedException e) {
          log(e.getMessage());
          return;
        }
      }
    });
  }

  private String loadData(String query) {
    try {
      log("Loading data for " + query);
      TimeUnit.SECONDS.sleep(2);
      log("Loaded data for " + query);
      return "Result " + query;
    } catch (InterruptedException e) {
      throw new RuntimeException(e);
    }
  }

  private static void log(String str) {
    String time = LocalTime.now().truncatedTo(ChronoUnit.SECONDS).format(DateTimeFormatter.ISO_TIME);
    String thread = Thread.currentThread().getName();
    System.out.println(time + ' ' + thread + ": " + str);
  }

  public static void main(String[] args) throws Exception {
    SingleThreadedLoader loader = new SingleThreadedLoader(5);
    // after the loading the data should be displayed
    loader.loadInBackground("1").thenAccept(SingleThreadedLoader::log);
    loader.loadInBackground("2").thenAccept(SingleThreadedLoader::log);
    loader.loadInBackground("3").thenAccept(SingleThreadedLoader::log);

    log("Do another work in the main thread");

    TimeUnit.SECONDS.sleep(30);
  }
}

Yürütmeden sonra stdout aşağıdaki çıktıya sahip olacaktır:

10:29:26 main: Enqueued query 1
10:29:26 pool-1-thread-1: Loading data for 1
10:29:26 main: Enqueued query 2
10:29:26 main: Enqueued query 3
10:29:26 main: Do another work in the main thread
10:29:28 pool-1-thread-1: Loaded data for 1
10:29:28 pool-1-thread-1: Result 1
10:29:28 pool-1-thread-1: Waiting for 5 seconds before next data loading
10:29:33 pool-1-thread-1: Loading data for 2
10:29:36 pool-1-thread-1: Loaded data for 2
10:29:36 pool-1-thread-1: Result 2
10:29:36 pool-1-thread-1: Waiting for 5 seconds before next data loading
10:29:41 pool-1-thread-1: Loading data for 3
10:29:43 pool-1-thread-1: Loaded data for 3
10:29:43 pool-1-thread-1: Result 3

Basit lock () ve unlock () yöntemleri orijinal kodunuza bu küçük değişiklikle çıktı çalıştırmak için bir sayaç olarak hareket edecek bir AtomicInteger ekledim çıktı:

Start #0: Thread[ForkJoinPool.commonPool-worker-2,5,main]
background task cancelled  1
background task cancelled  2
Finish #0: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Background task finished:task 0
Start #3: Thread[ForkJoinPool.commonPool-worker-2,5,main]
background task cancelled  4
Finish #3: Thread[ForkJoinPool.commonPool-worker-2,5,main]
background task cancelled  5
Background task finished:task 3
Start #6: Thread[ForkJoinPool.commonPool-worker-3,5,main]
background task cancelled  7
Finish #6: Thread[ForkJoinPool.commonPool-worker-3,5,main]
background task cancelled  8
Background task finished:task 6
Start #9: Thread[ForkJoinPool.commonPool-worker-2,5,main]
background task cancelled  10
Cancelled

İşte benim görev için benim çözüm:

public class LoadInBackgroundExample {
    //Added new exception
    public static class AlreadyIsRunningException extends RuntimeException {
        long taskId;

        public AlreadyIsRunningException(String message, long taskId) {
            super(message);
            this.taskId = taskId;
        }

        public long getTaskId() {
            return taskId;
        }

        public void setTaskId(long taskId) {
            this.taskId = taskId;
        }
    }


    /**
     * A simple background task which should perform the data loading operation. In this minimal example it simply invokes Thread.sleep
     */
    public static class BackgroundTask implements Runnable {


        //this atomicInteger acts as a global lock counter for BackgroundTask objects
        private static AtomicInteger counter = new AtomicInteger(0);


        private int id;

        public BackgroundTask(int id) {
            this.id = id;
        }

        private void unlock() {
            counter.decrementAndGet();
        }

        private void lock() {
            //we need to check this way to avoid some unlucky timing between threads
            int lockValue = counter.incrementAndGet();
            //if we got counter different than 1 that means that some other task is already running (it has already acquired the lock)
            if (lockValue != 1) {
                //rollback our check
                counter.decrementAndGet();
                //throw an exception
                throw new AlreadyIsRunningException("Some other task already is running", id);
            }
        }

        /**
         * Sleep for a given amount of time to simulate loading.
         */
        @Override
        public void run() {
            //Check if we can acquire lock

            lock();
            //we have a lock to
            try {
                System.out.println("Start #" + id + ": " + Thread.currentThread());
                long sleepTime = 2000;
                Thread.sleep(sleepTime);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                System.out.println("Finish #" + id + ": " + Thread.currentThread());
                unlock();
            }
        }
    }

    /**
     * CompletableFuture which simulates loading and showing data.
     *
     * @param taskId Identifier of the current task
     */


    public static void loadInBackground(int taskId) {
        // create the loading task
        BackgroundTask backgroundTask = new BackgroundTask(taskId);
        // "load" the data asynchronously
        CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(new Supplier<String>() {
            @Override
            public String get() {
                CompletableFuture<Void> future = CompletableFuture.runAsync(backgroundTask);

                try {
                    future.get();
                } catch (ExecutionException e) {
                    if (e.getCause() instanceof AlreadyIsRunningException) {
                        System.out.println("background task cancelled  " + ((AlreadyIsRunningException) e.getCause()).getTaskId());
                        throw (AlreadyIsRunningException) e.getCause();
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                return "task " + backgroundTask.id;
            }
        });
        // display the data after they are loaded
        CompletableFuture<Void> future = completableFuture.thenAccept(x -> {
            System.out.println("Background task finished:" + x);
        });
    }

    ArrayList<BackgroundTask> backgroundTasks = new ArrayList<>();


    public static void main(String[] args) {
        // runnable which invokes the background loader every second
        Runnable trigger = new Runnable() {
            int taskId = 0;

            public void run() {
                loadInBackground(taskId++);
            }
        };

        // create scheduler
        ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
        ScheduledFuture<?> beeperHandle = scheduler.scheduleAtFixedRate(trigger, 0, 1, TimeUnit.SECONDS);

        // cancel the scheudler and the application after 10 seconds
        scheduler.schedule(() -> beeperHandle.cancel(true), 10, TimeUnit.SECONDS);

        try {
            beeperHandle.get();
        } catch (Throwable th) {
        }

        System.out.println("Cancelled");
        System.exit(0);
    }

GÜNCELLEME

Lock () ve unlock () yöntemlerini daha basit bir forma değiştirdim:

private static  AtomicBoolean atomicBoolean = new AtomicBoolean(false);
        private void unlock() {
            atomicBoolean.set(false);
        }
        private void lock() {
            //if 'changed' is false that means some other task is already running
            boolean changed = atomicBoolean.compareAndSet(false,true);
            if (!changed) {
                throw new AlreadyIsRunningException("Some other task  is already running", id);
            }
        }

Eğer anlıyorsanız, aynı anda arka planda birkaç göreviniz var. Bu görevler aynı işi yapıyor olduğundan, bunları paralel olarak yürütmek istemezsiniz, işi bitirmek ve sonuçlarını başkalarıyla paylaşmak için bir göreve ihtiyacınız vardır. Yani CompletableFutureaynı anda 10 alırsanız, bunlardan birinin db'ye 'yeniden yükle' demenizi ve yürütme sonuçlarını diğerleriyle CompletableFuturenormal olarak sonuçla tamamlanacağı şekilde paylaşmasını istersiniz . Bunu sanırım

Amaç örneğin # 1 ve # 2 atlamaktır çünkü # 0 hala çalışıyor.

ve

Yüklemeden sonra veriler gösterilmelidir

tahminlerim doğruysa çözümümü deneyebilirsiniz.

Görevler arasında bir tür ebeveyn-çocuk ilişkim var. ana görev, işini gerçekten yapan ve çocuklarına paylaşımları paylaşan görevdir. alt görev, üst görev hala yürütülürken eklenen bir görevdir, alt görev üst görev yürütülmesini tamamlayana kadar bekler. Ebeveynler görevinin sonuçları hala 'taze' olduğundan, her bir çocuğa kopyalanır ve hepsi geleceklerini tamamlar.

public class BackgroundService {


    public static class BackgroundJob implements Callable<String> {

        private static BackgroundJob ROOT_JOB = null;

        private synchronized static void addBackgroundJob(BackgroundJob backgroundJob) {
            if (ROOT_JOB != null) {
                ROOT_JOB.addChild(backgroundJob);
            } else {
                System.out.println();
                System.out.println(Thread.currentThread().getName() + " RUNNING ROOT TASK-" + backgroundJob.jobId);
                ROOT_JOB = backgroundJob;
            }
        }

        private synchronized static void unlock() {
            ROOT_JOB = null;
        }

        private final int jobId;
        private List<BackgroundJob> children = new ArrayList<>();
        private BackgroundJob parent;
        private String providedResultFromParent = null;


        public BackgroundJob(int jobId) {
            this.jobId = jobId;
        }

        private  void addChild(BackgroundJob backgroundJob) {
            backgroundJob.parent = this;
            this.children.add(backgroundJob);
        }

        @Override
        public String call() throws Exception {
            addBackgroundJob(this);
            if (parent == null) {
                String result = logic();

                synchronized (ROOT_JOB) {
                    for (final BackgroundJob backgroundJob : children) {
                        backgroundJob.providedResultFromParent = result;
                        synchronized (backgroundJob) {
                            backgroundJob.notify();
                        }
                    }
                    unlock();
                }

                return "\t\tROOT task" + jobId + "'s " + result;
            } else {
                synchronized (this) {
                    System.out.println(Thread.currentThread().getName() + "\t\tskipping task-" + jobId + " and waiting running task-" + parent.jobId + " to finish");
                    this.wait();
                }
                return "\t\t\t\ttask-" + jobId + "'s " + providedResultFromParent;
            }
        }

        private String logic() throws InterruptedException {
            Thread.sleep(2000);
            return (int) (Math.random() * 1000) + " ";
        }
    }

    public static void main(String[] args) throws InterruptedException, ExecutionException {
        AtomicInteger atomicInteger = new AtomicInteger();
        ExecutorService pool = Executors.newCachedThreadPool();
        Supplier<String> job = () -> {
            int taskId = atomicInteger.incrementAndGet();
            BackgroundJob backgroundJob = new BackgroundJob(taskId);
            try {
                return backgroundJob.call();
            } catch (Exception e) {
                e.printStackTrace();
            }
            return "finished " + taskId;
        };

        for (int i = 100; i > 0; i--) {
            CompletableFuture.supplyAsync(job, pool).thenAccept(s -> System.out.println(Thread.currentThread().getName()+" "+ s + " result is readable"));
            Thread.sleep((long) (Math.random() * 500));
        }

        pool.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
        pool.shutdown();
    }

Ve işte çıktı:

pool-1-thread-1 RUNNING ROOT TASK-1
pool-1-thread-2     skipping task-2 and waiting running task-1 to finish
pool-1-thread-3     skipping task-3 and waiting running task-1 to finish
pool-1-thread-4     skipping task-4 and waiting running task-1 to finish
pool-1-thread-5     skipping task-5 and waiting running task-1 to finish
pool-1-thread-6     skipping task-6 and waiting running task-1 to finish
pool-1-thread-7     skipping task-7 and waiting running task-1 to finish
pool-1-thread-8     skipping task-8 and waiting running task-1 to finish
pool-1-thread-3                 task-3's 165  result is readable
pool-1-thread-8                 task-8's 165  result is readable
pool-1-thread-6                 task-6's 165  result is readable
pool-1-thread-5                 task-5's 165  result is readable
pool-1-thread-7                 task-7's 165  result is readable
pool-1-thread-2                 task-2's 165  result is readable
pool-1-thread-1         ROOT task1's 165  result is readable
pool-1-thread-4                 task-4's 165  result is readable

pool-1-thread-4 RUNNING ROOT TASK-9
pool-1-thread-1     skipping task-10 and waiting running task-9 to finish
pool-1-thread-2     skipping task-11 and waiting running task-9 to finish
pool-1-thread-7     skipping task-12 and waiting running task-9 to finish
pool-1-thread-5     skipping task-13 and waiting running task-9 to finish
pool-1-thread-8     skipping task-14 and waiting running task-9 to finish
pool-1-thread-6     skipping task-15 and waiting running task-9 to finish
pool-1-thread-3     skipping task-16 and waiting running task-9 to finish
pool-1-thread-9     skipping task-17 and waiting running task-9 to finish
pool-1-thread-10        skipping task-18 and waiting running task-9 to finish
pool-1-thread-1                 task-10's 370  result is readable
pool-1-thread-10                task-18's 370  result is readable
pool-1-thread-4         ROOT task9's 370  result is readable
pool-1-thread-9                 task-17's 370  result is readable
pool-1-thread-7                 task-12's 370  result is readable
pool-1-thread-6                 task-15's 370  result is readable
pool-1-thread-8                 task-14's 370  result is readable
pool-1-thread-2                 task-11's 370  result is readable
pool-1-thread-3                 task-16's 370  result is readable
pool-1-thread-5                 task-13's 370  result is readable

pool-1-thread-5 RUNNING ROOT TASK-19
pool-1-thread-3     skipping task-20 and waiting running task-19 to finish
pool-1-thread-2     skipping task-21 and waiting running task-19 to finish
pool-1-thread-8     skipping task-22 and waiting running task-19 to finish
pool-1-thread-6     skipping task-23 and waiting running task-19 to finish
pool-1-thread-7     skipping task-24 and waiting running task-19 to finish
pool-1-thread-9     skipping task-25 and waiting running task-19 to finish
pool-1-thread-4     skipping task-26 and waiting running task-19 to finish
pool-1-thread-10        skipping task-27 and waiting running task-19 to finish
pool-1-thread-1     skipping task-28 and waiting running task-19 to finish
pool-1-thread-5         ROOT task19's 574  result is readable
pool-1-thread-8                 task-22's 574  result is readable
pool-1-thread-4                 task-26's 574  result is readable
pool-1-thread-7                 task-24's 574  result is readable
pool-1-thread-6                 task-23's 574  result is readable
pool-1-thread-3                 task-20's 574  result is readable
pool-1-thread-9                 task-25's 574  result is readable
pool-1-thread-2                 task-21's 574  result is readable
pool-1-thread-1                 task-28's 574  result is readable
pool-1-thread-10                task-27's 574  result is readable

pool-1-thread-10 RUNNING ROOT TASK-29
pool-1-thread-1     skipping task-30 and waiting running task-29 to finish
pool-1-thread-2     skipping task-31 and waiting running task-29 to finish
pool-1-thread-9     skipping task-32 and waiting running task-29 to finish
pool-1-thread-3     skipping task-33 and waiting running task-29 to finish
pool-1-thread-6     skipping task-34 and waiting running task-29 to finish
pool-1-thread-7     skipping task-35 and waiting running task-29 to finish
pool-1-thread-4     skipping task-36 and waiting running task-29 to finish
pool-1-thread-8     skipping task-37 and waiting running task-29 to finish
pool-1-thread-5     skipping task-38 and waiting running task-29 to finish
pool-1-thread-11        skipping task-39 and waiting running task-29 to finish
pool-1-thread-1                 task-30's 230  result is readable
pool-1-thread-11                task-39's 230  result is readable
pool-1-thread-8                 task-37's 230  result is readable
pool-1-thread-5                 task-38's 230  result is readable
pool-1-thread-4                 task-36's 230  result is readable
pool-1-thread-7                 task-35's 230  result is readable

pool-1-thread-12 RUNNING ROOT TASK-40
pool-1-thread-6                 task-34's 230  result is readable
pool-1-thread-10        ROOT task29's 230  result is readable
pool-1-thread-3                 task-33's 230  result is readable
pool-1-thread-9                 task-32's 230  result is readable
pool-1-thread-2                 task-31's 230  result is readable
pool-1-thread-2     skipping task-41 and waiting running task-40 to finish
pool-1-thread-9     skipping task-42 and waiting running task-40 to finish
pool-1-thread-3     skipping task-43 and waiting running task-40 to finish
pool-1-thread-10        skipping task-44 and waiting running task-40 to finish
pool-1-thread-6     skipping task-45 and waiting running task-40 to finish
pool-1-thread-7     skipping task-46 and waiting running task-40 to finish
pool-1-thread-2                 task-41's 282  result is readable
pool-1-thread-10                task-44's 282  result is readable
pool-1-thread-6                 task-45's 282  result is readable
pool-1-thread-7                 task-46's 282  result is readable
pool-1-thread-3                 task-43's 282  result is readable
pool-1-thread-9                 task-42's 282  result is readable
pool-1-thread-12        ROOT task40's 282  result is readable

pool-1-thread-12 RUNNING ROOT TASK-47
pool-1-thread-9     skipping task-48 and waiting running task-47 to finish
pool-1-thread-3     skipping task-49 and waiting running task-47 to finish
pool-1-thread-7     skipping task-50 and waiting running task-47 to finish
pool-1-thread-6     skipping task-51 and waiting running task-47 to finish
pool-1-thread-10        skipping task-52 and waiting running task-47 to finish
pool-1-thread-2     skipping task-53 and waiting running task-47 to finish
pool-1-thread-12        ROOT task47's 871  result is readable
pool-1-thread-10                task-52's 871  result is readable
pool-1-thread-2                 task-53's 871  result is readable
pool-1-thread-3                 task-49's 871  result is readable
pool-1-thread-6                 task-51's 871  result is readable
pool-1-thread-7                 task-50's 871  result is readable
pool-1-thread-9                 task-48's 871  result is readable

pool-1-thread-9 RUNNING ROOT TASK-54
pool-1-thread-7     skipping task-55 and waiting running task-54 to finish
pool-1-thread-6     skipping task-56 and waiting running task-54 to finish
pool-1-thread-3     skipping task-57 and waiting running task-54 to finish
pool-1-thread-2     skipping task-58 and waiting running task-54 to finish
pool-1-thread-10        skipping task-59 and waiting running task-54 to finish
pool-1-thread-12        skipping task-60 and waiting running task-54 to finish
pool-1-thread-4     skipping task-61 and waiting running task-54 to finish
pool-1-thread-5     skipping task-62 and waiting running task-54 to finish
pool-1-thread-9         ROOT task54's 345  result is readable
pool-1-thread-2                 task-58's 345  result is readable
pool-1-thread-5                 task-62's 345  result is readable
pool-1-thread-7                 task-55's 345  result is readable
pool-1-thread-10                task-59's 345  result is readable
pool-1-thread-6                 task-56's 345  result is readable
pool-1-thread-3                 task-57's 345  result is readable
pool-1-thread-4                 task-61's 345  result is readable
pool-1-thread-12                task-60's 345  result is readable

pool-1-thread-12 RUNNING ROOT TASK-63
pool-1-thread-4     skipping task-64 and waiting running task-63 to finish
pool-1-thread-3     skipping task-65 and waiting running task-63 to finish
pool-1-thread-6     skipping task-66 and waiting running task-63 to finish
pool-1-thread-10        skipping task-67 and waiting running task-63 to finish
pool-1-thread-7     skipping task-68 and waiting running task-63 to finish
pool-1-thread-5     skipping task-69 and waiting running task-63 to finish
pool-1-thread-2     skipping task-70 and waiting running task-63 to finish
pool-1-thread-12        ROOT task63's 670  result is readable
pool-1-thread-2                 task-70's 670  result is readable
pool-1-thread-5                 task-69's 670  result is readable
pool-1-thread-7                 task-68's 670  result is readable
pool-1-thread-10                task-67's 670  result is readable
pool-1-thread-6                 task-66's 670  result is readable
pool-1-thread-3                 task-65's 670  result is readable
pool-1-thread-4                 task-64's 670  result is readable

pool-1-thread-4 RUNNING ROOT TASK-71
pool-1-thread-3     skipping task-72 and waiting running task-71 to finish
pool-1-thread-6     skipping task-73 and waiting running task-71 to finish
pool-1-thread-10        skipping task-74 and waiting running task-71 to finish
pool-1-thread-7     skipping task-75 and waiting running task-71 to finish
pool-1-thread-5     skipping task-76 and waiting running task-71 to finish
pool-1-thread-2     skipping task-77 and waiting running task-71 to finish
pool-1-thread-12        skipping task-78 and waiting running task-71 to finish
pool-1-thread-9     skipping task-79 and waiting running task-71 to finish
pool-1-thread-8     skipping task-80 and waiting running task-71 to finish
pool-1-thread-4         ROOT task71's 445  result is readable
pool-1-thread-6                 task-73's 445  result is readable
pool-1-thread-9                 task-79's 445  result is readable
pool-1-thread-3                 task-72's 445  result is readable
pool-1-thread-8                 task-80's 445  result is readable
pool-1-thread-12                task-78's 445  result is readable
pool-1-thread-5                 task-76's 445  result is readable
pool-1-thread-10                task-74's 445  result is readable
pool-1-thread-2                 task-77's 445  result is readable
pool-1-thread-7                 task-75's 445  result is readable

pool-1-thread-7 RUNNING ROOT TASK-81
pool-1-thread-2     skipping task-82 and waiting running task-81 to finish
pool-1-thread-10        skipping task-83 and waiting running task-81 to finish
pool-1-thread-5     skipping task-84 and waiting running task-81 to finish
pool-1-thread-12        skipping task-85 and waiting running task-81 to finish
pool-1-thread-8     skipping task-86 and waiting running task-81 to finish
pool-1-thread-3     skipping task-87 and waiting running task-81 to finish
pool-1-thread-9     skipping task-88 and waiting running task-81 to finish
pool-1-thread-6     skipping task-89 and waiting running task-81 to finish
pool-1-thread-7         ROOT task81's 141  result is readable
pool-1-thread-6                 task-89's 141  result is readable
pool-1-thread-9                 task-88's 141  result is readable
pool-1-thread-3                 task-87's 141  result is readable
pool-1-thread-10                task-83's 141  result is readable
pool-1-thread-5                 task-84's 141  result is readable
pool-1-thread-12                task-85's 141  result is readable
pool-1-thread-8                 task-86's 141  result is readable
pool-1-thread-2                 task-82's 141  result is readable

pool-1-thread-2 RUNNING ROOT TASK-90
pool-1-thread-8     skipping task-91 and waiting running task-90 to finish
pool-1-thread-12        skipping task-92 and waiting running task-90 to finish
pool-1-thread-5     skipping task-93 and waiting running task-90 to finish
pool-1-thread-10        skipping task-94 and waiting running task-90 to finish
pool-1-thread-3     skipping task-95 and waiting running task-90 to finish
pool-1-thread-9     skipping task-96 and waiting running task-90 to finish
pool-1-thread-6     skipping task-97 and waiting running task-90 to finish
pool-1-thread-7     skipping task-98 and waiting running task-90 to finish
pool-1-thread-4     skipping task-99 and waiting running task-90 to finish
pool-1-thread-11        skipping task-100 and waiting running task-90 to finish
pool-1-thread-2         ROOT task90's 321  result is readable
pool-1-thread-3                 task-95's 321  result is readable
pool-1-thread-7                 task-98's 321  result is readable
pool-1-thread-8                 task-91's 321  result is readable
pool-1-thread-11                task-100's 321  result is readable
pool-1-thread-4                 task-99's 321  result is readable
pool-1-thread-5                 task-93's 321  result is readable
pool-1-thread-9                 task-96's 321  result is readable
pool-1-thread-12                task-92's 321  result is readable
pool-1-thread-10                task-94's 321  result is readable
pool-1-thread-6                 task-97's 321  result is readable

sadece tek bir erişim iş parçacığı istiyorsanız basit bir senkronize işi yapardı ...

çıktı:

Start #2: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Finish #0: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Background task finished:task 0 finished getting data...
Start #3: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Finish #2: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Start #4: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Background task finished:task 2 finished getting data...
Finish #1: Thread[ForkJoinPool.commonPool-worker-4,5,main]
Background task finished:task 1 finished getting data...
Start #6: Thread[ForkJoinPool.commonPool-worker-3,5,main]
Finish #4: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Start #5: Thread[ForkJoinPool.commonPool-worker-6,5,main]
Background task finished:task 4 finished getting data...
Finish #3: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Start #7: Thread[ForkJoinPool.commonPool-worker-2,5,main]
Background task finished:task 3 finished getting data...
Cancelled

kod:

package queue;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.function.Supplier;

public class LoadInBackgroundExample {

    public static class SyncronizedBackend {
        public synchronized String getData() {

            long sleepTime = 2000;
            try {
                Thread.sleep(sleepTime);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            return new String("finished getting data...");
        }
    }

    /**
     * A simple background task which should perform the data loading operation. In
     * this minimal example it simply invokes Thread.sleep
     */
    public static class BackgroundTask implements Runnable {

        private int id;
        private SyncronizedBackend syncronizedBackend;
        private String result;

        public BackgroundTask(SyncronizedBackend syncronizedBackend, int id) {
            this.syncronizedBackend = syncronizedBackend;
            this.id = id;
        }

        /**
         * Sleep for a given amount of time to simulate loading.
         */
        @Override
        public void run() {

            System.out.println("Start #" + id + ": " + Thread.currentThread());

            result = this.syncronizedBackend.getData();

            System.out.println("Finish #" + id + ": " + Thread.currentThread());
        }

        public String getResult() {
            return result;
        }
    }

    /**
     * CompletableFuture which simulates loading and showing data.
     * @param syncronizedBackend 
     * 
     * @param taskId Identifier of the current task
     */
    public static void loadInBackground(SyncronizedBackend syncronizedBackend, int taskId) {

        // create the loading task
        BackgroundTask backgroundTask = new BackgroundTask(syncronizedBackend, taskId);

        // "load" the data asynchronously
        CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(new Supplier<String>() {

            @Override
            public String get() {

                CompletableFuture<Void> future = CompletableFuture.runAsync(backgroundTask);

                try {

                    future.get();

                } catch (InterruptedException | ExecutionException e) {
                    e.printStackTrace();
                }

                return "task " + backgroundTask.id + " " + backgroundTask.getResult();
            }
        });

        // display the data after they are loaded
        CompletableFuture<Void> future = completableFuture.thenAccept(x -> {

            System.out.println("Background task finished:" + x);

        });

    }

    public static void main(String[] args) {

        SyncronizedBackend syncronizedBackend = new SyncronizedBackend();

        // runnable which invokes the background loader every second
        Runnable trigger = new Runnable() {

            int taskId = 0;

            public void run() {

                loadInBackground(syncronizedBackend, taskId++);

            }
        };

        // create scheduler
        ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
        ScheduledFuture<?> beeperHandle = scheduler.scheduleAtFixedRate(trigger, 0, 1, TimeUnit.SECONDS);

        // cancel the scheudler and the application after 10 seconds
        scheduler.schedule(() -> beeperHandle.cancel(true), 10, TimeUnit.SECONDS);

        try {
            beeperHandle.get();
        } catch (Throwable th) {
        }

        System.out.println("Cancelled");
        System.exit(0);
    }

}

Ben bir Konu çift anahtar kullanarak bir çözüm denedim, bkz sınıf BackgroundTaskDualSwitch, kullanarak kullanarak taklit CompletableFuture. Fikir, şu anda çalışan görev bitene kadar ikinci bir görevin beklemesine izin vermektir, bkz. Değişiklik BackgroundTask. Bu, maksimum bir görev iş parçacığının çalışmasını ve maksimum bir görev iş parçacığının beklemesini sağlar. Çalışan görev tamamlanana ve bir sonraki isteği işlemek için 'serbest' olana kadar başka istekler atlanır.

public static class BackgroundTask extends Thread {

  private int id;
  private Thread pendingTask;

  public BackgroundTask(int id) {
    this.id = id;
  }

  public BackgroundTask(int id, Thread pendingTask) {
    this(id);
    this.pendingTask = pendingTask;
  }

  /**
   * Sleep for a given amount of time to simulate loading.
   */
  @Override
  public void run() {

    try {

      if (pendingTask != null && pendingTask.isAlive()) {
        pendingTask.join();
      }

      System.out.println("Start #" + id + ": " + Thread.currentThread());
      ...
    }
  }

public static class BackgroundTaskDualSwitch {

  private static BackgroundTask task1;
  private static BackgroundTask task2;

  public static synchronized boolean runTask(int taskId) {
    if (! isBusy(task1)) {
      if (isBusy(task2)) {
        task1 = new BackgroundTask(taskId, task2);
      } else {
        task1 = new BackgroundTask(taskId);
      }
      runAsync(task1);
      return true;
    } else if (! isBusy(task2)) {
      if (isBusy(task1)) {
        task2 = new BackgroundTask(taskId, task1);
      } else {
        task2 = new BackgroundTask(taskId);
      }
      runAsync(task2);
      return true;
    } else {
      return false; // SKIPPED
    }
  }

  private static void runAsync(BackgroundTask task) {
    CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(new Supplier<String>() {

      @Override
      public String get() {

        try {
          task.start();
          task.join();
        }
        catch (InterruptedException e) {
          e.printStackTrace();
        }
        return "task " + task.id;
      }
    });

    // display the data after they are loaded
    CompletableFuture<Void> future = completableFuture.thenAccept(x -> {

      System.out.println( "Background task finished:" + x); 

    });
  }

  private static boolean isBusy(BackgroundTask task) {
    return task != null && task.isAlive();
  }
}

/**
 * Simulates loading and showing data.
 * @param taskId Identifier of the current task
 */
public static void loadInBackground(int taskId) {

  // create the loading task
  if (! BackgroundTaskDualSwitch.runTask(taskId)) {
    System.out.println( "Background task ignored:task " + taskId); // SKIPPED
  }
}
...

Çıktı:

Start #0: Thread[Thread-0,5,main]
Background task ignored:task 2
Finish #0: Thread[Thread-0,5,main]
Start #1: Thread[Thread-1,5,main]
Background task finished:task 0
Background task ignored:task 4
Finish #1: Thread[Thread-1,5,main]
Start #3: Thread[Thread-2,5,main]
Background task finished:task 1
Background task ignored:task 6
Finish #3: Thread[Thread-2,5,main]
Start #5: Thread[Thread-3,5,main]
Background task finished:task 3
Background task ignored:task 8
Finish #5: Thread[Thread-3,5,main]
Start #7: Thread[Thread-4,5,main]
Background task finished:task 5
Background task ignored:task 10
Finish #7: Thread[Thread-4,5,main]
Start #9: Thread[Thread-5,5,main]
Background task finished:task 7
Cancelled

Pahalı işi yapmaya başlayan ilk iş parçacığı sonucu geri arama ile bildirir. Yürütmeye çalışan diğer iş parçacıkları ExpensiveWork.notificables içinde kaydedilecektir, bu yüzden pahalı iş bittikten sonra işi yapan iş parçacığı onları bilgilendirecektir.

Bu arada dişler sonucu her 5 saniyede bir kontrol eder.

public class ExpensiveWorkTest {

    private final static int THREADS = 20;
    private final static long THREAD_TIMEOUT = 5000L;

    @Test
    public void example() throws InterruptedException {
        ExpensiveWork<String> expensiveWork = new ExpensiveWorkImpl();
        ExecutorService service = Executors.newFixedThreadPool(THREADS);
        for(int i=0; i<THREADS;i++) {
            service.execute(() ->{
                Notificable<String> notificable = new NotificableImpl();
                expensiveWork.execute(notificable);
                while(notificable.getExpensiveResult() == null) {
                    try {
                        Thread.sleep(THREAD_TIMEOUT);
                    } catch (InterruptedException e) {}
                }
                System.out.println(Thread.currentThread().getName()+" has the message: "+notificable.getExpensiveResult());
            });
        }
        service.awaitTermination(60, TimeUnit.SECONDS);
    }

    public static abstract class ExpensiveWork<T> {

        private final AtomicBoolean runnning = new AtomicBoolean(false);
        private List<Notificable<T>> notificables = Collections.synchronizedList(new ArrayList<>());

        public void execute(Notificable<T> notificable) {
            String id = Thread.currentThread().getName();
            System.out.println("Loading data for "+id);
            notificables.add(notificable);
            if(!runnning.getAndSet(true)) {
                System.out.println("Running the expensive work "+id);
                T expensiveResult = expensiveWork();
                notificables.stream().forEach(n -> n.callback(expensiveResult));
            } else {
                System.out.println(id+" will receive the response later");
            }
        }

        protected abstract T expensiveWork();

    }

    public static class ExpensiveWorkImpl extends ExpensiveWork<String>{

        @Override
        public String expensiveWork() {
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {}
            return "<Expensive result>";
        }

    }

    public static interface Notificable<T> {
        void callback(T expensiveResult);
        T getExpensiveResult();
    }

    public static class NotificableImpl implements Notificable<String> {

        private volatile String expensiveResult;

        @Override
        public void callback(String expensiveResult) {
            this.expensiveResult = expensiveResult;
        }

        @Override
        public String getExpensiveResult() {
            return expensiveResult;
        }

    }

}