‏ScheduledExecutorService للمهام المتكررة

لماذا نستخدم ScheduledExecutorService؟

import java.util.concurrent.*;

ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(2);

// One-shot delay
scheduler.schedule(
    () -> System.out.println("Runs once after 3 seconds"),
    3, TimeUnit.SECONDS
);

// Don't forget to shut down eventually:
// scheduler.shutdown();

schedule: تأخير لمرة واحدة

ScheduledExecutorService s = Executors.newScheduledThreadPool(1);

// Runnable:
ScheduledFuture<?> sf1 = s.schedule(
    () -> System.out.println("Runnable fired!"), 2, TimeUnit.SECONDS);

// Callable:
ScheduledFuture<Integer> sf2 = s.schedule(
    () -> { System.out.println("Callable fired!"); return 42; },
    2, TimeUnit.SECONDS);

System.out.println(sf2.get()); // 42 (after 2 seconds)

scheduleAtFixedRate

ScheduledExecutorService s = Executors.newScheduledThreadPool(1);
AtomicInteger count = new AtomicInteger();

ScheduledFuture<?> sf = s.scheduleAtFixedRate(
    () -> System.out.println("Tick " + count.incrementAndGet()),
    0,   // initial delay
    1,   // period
    TimeUnit.SECONDS
);

Thread.sleep(5000);
sf.cancel(false); // stop after 5 ticks
s.shutdown();

scheduleWithFixedDelay

s.scheduleWithFixedDelay(
    () -> {
        System.out.println("Polling... " + Instant.now());
        // variable-length polling task
    },
    0,  // no initial delay
    5,  // 5-second gap AFTER each completion
    TimeUnit.SECONDS
);
// Next execution = task end + 5 seconds (not task start)

المعدل الثابت مقابل التأخير الثابت

ScheduledFuture: الإلغاء

ScheduledFuture<?> sf = scheduler.scheduleAtFixedRate(
    () -> System.out.println("Heartbeat"), 0, 1, TimeUnit.SECONDS);

// Cancel after 5 seconds
scheduler.schedule(() -> sf.cancel(false), 5, TimeUnit.SECONDS);

معالجة الاستثناءات في المهام المجدولة

scheduler.scheduleAtFixedRate(() -> {
    try {
        // risky work
        if (Math.random() < 0.3) throw new RuntimeException("transient");
        System.out.println("Success");
    } catch (Exception e) {
        System.err.println("Task error: " + e.getMessage());
        // task continues to run on next interval
    }
}, 0, 2, TimeUnit.SECONDS);

تطبيق عملي: تحديث ذاكرة التخزين المؤقت

class PriceCache {
    private volatile Map<String, Double> prices = new HashMap<>();
    
    PriceCache(ScheduledExecutorService scheduler) {
        scheduler.scheduleAtFixedRate(this::refresh, 0, 30, TimeUnit.SECONDS);
    }
    
    private void refresh() {
        System.out.println("Refreshing prices...");
        // prices = fetchFromDatabase();
    }
    
    Double getPrice(String symbol) { return prices.get(symbol); }
}

تطبيق عملي: إعادة المحاولة مع التراجع التدريجي

void retryWithBackoff(Runnable task, int maxRetries, ScheduledExecutorService s) {
    AtomicInteger attempts = new AtomicInteger(0);
    long[] delay = {1}; // seconds
    
    Runnable wrapper = new Runnable() {
        public void run() {
            try {
                task.run();
            } catch (Exception e) {
                int n = attempts.incrementAndGet();
                if (n < maxRetries) {
                    delay[0] *= 2;
                    s.schedule(this, delay[0], TimeUnit.SECONDS);
                } else System.err.println("Max retries reached");
            }
        }
    };
    s.execute(wrapper);
}

المراقبة: getDelay وgetQueue

ScheduledFuture<?> sf = scheduler.schedule(() -> {}, 10, TimeUnit.SECONDS);
System.out.println("Delay remaining: " + sf.getDelay(TimeUnit.SECONDS) + "s");

// ThreadPoolExecutor provides queue inspection:
if (scheduler instanceof ScheduledThreadPoolExecutor ste) {
    System.out.println("Queue size: " + ste.getQueue().size());
}

إيقاف التشغيل وتصريف المهام

ScheduledThreadPoolExecutor ste = (ScheduledThreadPoolExecutor)
    Executors.newScheduledThreadPool(2);

// Cancel scheduled tasks on shutdown (default: false = they run)
ste.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
ste.setContinueExistingPeriodicTasksAfterShutdownPolicy(false);

ste.shutdown();

اختبار سريع

مراجعة: ScheduledExecutorService