Ensuring task execution order in threadpool

Question

I have been reading about the thread-pool pattern and I can\'t seem to find the usual solution for the following problem.

I sometimes want tasks to be executed serial

Answer 1

Thread Pool with ordered and unordered execute methods:

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class OrderedExecutor {
    private ExecutorService multiThreadExecutor;
    // for single Thread Executor
    private ThreadLocal threadLocal = new ThreadLocal<>();

    public OrderedExecutor(int nThreads) {
        this.multiThreadExecutor = Executors.newFixedThreadPool(nThreads);
    }

    public void executeUnordered(Runnable task) {
        multiThreadExecutor.submit(task);
    }

    public void executeOrdered(Runnable task) {
        multiThreadExecutor.submit(() -> {
            ExecutorService singleThreadExecutor = threadLocal.get();
            if (singleThreadExecutor == null) {
                singleThreadExecutor = Executors.newSingleThreadExecutor();
                threadLocal.set(singleThreadExecutor);
            }
            singleThreadExecutor.submit(task);
        });
    }

    public void clearThreadLocal() {
        threadLocal.remove();
    }

}

After filling all queues the threadLocal should be cleared. The only drawback is that singleThreadExecutor will be created each time the method

executeOrdered(Runnable task)

invoked in separate thread