Example how to synchronize task by an indentifier

LockingTest

package locking;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.atomic.AtomicInteger;

public class LockingTest
{
	public static void main(String[] args)
	{
		new LockingTest().start();
	}
	
	volatile long startTs;
	
	public void start()
	{
		List<Thread> threads=new ArrayList<>();
		for (int i=0;i<100;i++)
		{
			final int workId=i % 2;
			threads.add(new Thread(()->doWork(workId)));
		}
		startTs=System.currentTimeMillis();
		threads.stream().forEach((t)->t.start());
	}
	

	protected final Map<Integer,AtomicInteger> LOCK_MAP=new HashMap<>();
	
	public void doWork(Integer workId)
	{
		String id="#"+workId+" with thread "+Thread.currentThread().getName();
		
		AtomicInteger lock;
		synchronized(LOCK_MAP) // globaler Lock
		{
			lock=LOCK_MAP.get(workId);
			if (lock==null)
			{
				System.err.println("New lock for "+id+" ts: "+(System.currentTimeMillis()-startTs));
				lock=new AtomicInteger(1);
				LOCK_MAP.put(workId,lock);
			}
			else
			{
				lock.incrementAndGet();
			}
		}

		System.err.println("About to start work "+id+" ts: "+(System.currentTimeMillis()-startTs));
		System.err.println("Workers for this id: "+lock.get());
		
		synchronized(lock)
		{
			try
			{
				System.err.println("Start work "+id+" ts: "+(System.currentTimeMillis()-startTs));
				doActualWork();
				System.err.println("Done work "+id+" ts: "+(System.currentTimeMillis()-startTs));
			}
			finally
			{
				synchronized(LOCK_MAP) // globaler Lock
				{
					if (lock.decrementAndGet()==0)
					{
						System.err.println("Remove lock for "+id+" ts: "+(System.currentTimeMillis()-startTs));
						LOCK_MAP.remove(workId);
					}
				}
			}
		}
	}
	
	protected void doActualWork()
	{
		try
		{
			Thread.sleep(100);
		}
		catch (Exception ex)
		{
			// ignored
		}
	}

}