Afraid of Multi threading Interview ? Task Scheduler code and some Important topics
Akanksha Singh
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LinkedInRubrikConfluentConcurrencyJavaCareerMultithreading

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Hey folks, recently I appeared for a couple of companies focusing on Mutli-threading and concurrency theory.

Here I'm listing down the main concepts which you should know completely about, apart from standard one's.

You can directly jump to the code part, if you have the famous paid subscription course.

  1. Amdahl's law
  2. Re-entrant locks and condition variables
  3. Mixed Signal
  4. Spurious wakeups
  5. Blocking Queue
  6. Defog tech
    • Java Concurrency Interview playlist
    • Java Concurrency
    • Java Executor Service (Very important, this is what is asked to be implemented in coding rounds)
Now coming to the code part:
Code for Task Scheduler:

And here is the complete code: Multi-Threaded-Task-Scheduler.
P.S. After watching the videos mentioned below, you should be able to completely understand my code, hence skipping the explanation part.

The only coding pattern I could think of using was Singleton pattern and factory pattern.

public class SchedulerService implements ISchedulerService {
	private static ISchedulerService schedulerService;
    private final PriorityQueue<ScheduledTask> taskPriorityQueue;
    private final ThreadPoolExecutor taskExecutor;
    private final Lock lock;
    private final Condition newTaskScheduled;

    private SchedulerService(int thread_size) {
        this.taskPriorityQueue = new PriorityQueue<>(Comparator.comparingLong(ScheduledTask::getScheduledTime));
        this.taskExecutor = (ThreadPoolExecutor) Executors.newCachedThreadPool();
        this.lock = new ReentrantLock();
        this.newTaskScheduled = this.lock.newCondition();
    }
	
	@Override
    public void run() {
        Long time_to_sleep = Long.valueOf(0);
        while (true) {
            this.lock.lock();
            try {
                while (this.taskPriorityQueue.isEmpty()) {this.newTaskScheduled.await();}
                while(!this.taskPriorityQueue.isEmpty()) {
                    time_to_sleep = this.taskPriorityQueue.peek().getTimeUnit().toMillis(this.taskPriorityQueue.peek().getScheduledTime()) - System.currentTimeMillis();
                    if (time_to_sleep <= 0) break;
                    this.newTaskScheduled.await(time_to_sleep, TimeUnit.MILLISECONDS);
                }

                ScheduledTask scheduledTask = this.taskPriorityQueue.poll();
                Long newScheduledTime = Long.valueOf(0);

                switch (scheduledTask.getScheduledTaskType()) {
                    case RUN_ONCE -> {
                        this.taskExecutor.submit(scheduledTask.getTask());
                        break;
                    }
                    case RECUR -> {
                        newScheduledTime = System.currentTimeMillis()+ scheduledTask.getTimeUnit().toMillis(scheduledTask.getReplayTime());
                        this.taskExecutor.submit(scheduledTask.getTask());
                        scheduledTask.setScheduledTime(newScheduledTime);
                        this.taskPriorityQueue.add(scheduledTask);
                        break;
                    }
                    case RECUR_WITH_WAIT -> {
                        Future<?> future = this.taskExecutor.submit(scheduledTask.getTask());
                        future.get();
                        newScheduledTime = System.currentTimeMillis()+ scheduledTask.getTimeUnit().toMillis(scheduledTask.getReplayTime());
                        scheduledTask.setScheduledTime(newScheduledTime);
                        this.taskPriorityQueue.add(scheduledTask);
                        break;
                    }
                }
            } catch (RejectedExecutionException r) {
                r.printStackTrace();
            } catch (InterruptedException | ExecutionException e) {
                throw new RuntimeException(e);
            } finally {
                this.lock.unlock();
            }
        }
    }
}

Videos explaining some of the standard multithreaded coding examples

  1. Udit Agrawal
    • MultiThreading/Concurrency playlist
  2. Gaurav Sen's Logger implementation
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