线程崩溃

我有一个简单的项目，旨在更好地理解Java中的锁定和线程。本质上，我有一个缓存对象，该对象启动清理线程以删除超过给定期限的项目。测试类“ Tester”运行两个附加线程，一个用于添加要缓存的项目，另一个用于打印出缓存的内容。由于某些原因，当清理线程修改了嵌入在Cache中的HashMap时，它将停止任何进一步的迭代。我尝试同步访问器/更改器方法以及在Cache中围绕LOCK对象进行同步。任何想法或帮助都是muy beueno。;）

public class Cache 
{
    private HashMap<Object, ObjectWrapper> cachedObjects = new HashMap<>();
    private static Cache cache = null;
    private static int TIME_TO_KEEP = 60000; // 60 seconds
    private final static Object LOCK = new Object();

    public static Cache getInstance()
    {
        if (cache == null)
        {
            cache = new Cache();
        }
        return cache;
    }

    private Cache()
    {
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(2);

        Runnable task = () -> {
            synchronized(LOCK)
            {
                System.out.println("Cleanup");
                Set<Object> keys = cachedObjects.keySet();
                final long now = System.currentTimeMillis();
                keys.forEach(k -> {
                    try
                    {

                        {
                            ObjectWrapper ow = cachedObjects.get(k);
                            if(ow.getExpireTime() < now)
                            {
                                int size = cachedObjects.size();
                                cachedObjects.remove(k, ow);
                                System.out.println("DEL:" + k + ", from " + size + " to " + cachedObjects.size());
                            }
                        }
                    }
                    catch (Throwable t)
                    {
                        t.printStackTrace();
                    }
                });
            }
        };

        executor.scheduleWithFixedDelay(task, 5, 15, TimeUnit.SECONDS);
    }

    public void addObject(Object key, Object obj)
    {
        synchronized(LOCK)
        {
            ObjectWrapper ow = new ObjectWrapper(obj, System.currentTimeMillis() + TIME_TO_KEEP);
            cachedObjects.put(key, ow);
        }
    }

    public ObjectWrapper getObj(Object key)
    {
        synchronized(LOCK)
        {
            return cachedObjects.get(key);
        }
    }

    public Collection<ObjectWrapper> getValues()
    {
        synchronized(LOCK)
        {
            return cachedObjects.values();
        }
    }

    public Set<Object> getKeys()
    {
        synchronized(LOCK)
        {
            return cachedObjects.keySet();
        }
    }

    public static void main(String[] args)
    {
        Cache cache = Cache.getInstance();
    }
}

import java.util.*;
import java.util.concurrent.*;

public class Tester 
{
    private static Integer id = 0;
    private static Cache cache = Cache.getInstance();

    public static void main(String[] args)
    {
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(2);

        Runnable adder = () -> {
            System.out.println("Adding id:" + ++id);
            Object o = new Object();
            cache.addObject(id, o);
        };

        executor.scheduleWithFixedDelay(adder, 5, 10, TimeUnit.SECONDS);


        Runnable tester = () -> {
            long currTime = System.currentTimeMillis();
            System.out.println("Test: ");
            Set<Object> keys = cache.getKeys();
            keys.forEach(k -> {
                ObjectWrapper o = cache.getObj(k);
                System.out.println(k + ">" + currTime + "::" + o.getExpireTime());
            });
        };

        executor.scheduleWithFixedDelay(tester, 20, 10, TimeUnit.SECONDS);
    }
}

public class ObjectWrapper
{
    private Object obj;
    private long expireTime;

    public ObjectWrapper(Object obj, long expireTime)
    {
        this.obj = obj;
        this.expireTime = expireTime;
    }

    public Object getObj()
    {
        return obj;
    }

    public void setObj(Object obj)
    {
        this.obj = obj;
    }

    public long getExpireTime()
    {
        return expireTime;
    }

    public void setExpireTime(long expireTime)
    {
        this.expireTime = expireTime;
    }
}