java - 如何让ScheduleTask在某个时间执行一次任务？

Question

最近在使用ScheduleTask，有这样的问题。我希望本地图大小超过 MAX_SIZE 字段时，我的 ScheduleTask 每 5 秒从 map 中删除元素。我正在尝试这样做:

public class RemoverThread extends TimerTask {

    private AbstractCustomCache customCache;
    private static final int MAX_SIZE = 2;

    public RemoverThread(AbstractCustomCache customCache) {
        this.customCache = customCache;
    }

    @Override
    public void run() {
        if (customCache.getCacheEntry().size() > MAX_SIZE) {
            List<String> gg = new ArrayList<>(customCache.getCacheEntry().keySet());
            for (String s : gg) {
                System.out.println("Element was remove: " + customCache.getCacheEntry().remove(s));
                System.out.println("Time: " + new SimpleDateFormat("yyyy/MM/dd HH:mm:ss").format(Calendar.getInstance().getTime()));
                if (customCache.getCacheEntry().size() <= MAX_SIZE) {
                    break;
                }
            }
        }
    }
}

我的主要:

public class Main {
    public static void main(String[] args) {
        AbstractCustomCacheStatistics gh = new MapCacheStatistics();
        AbstractCustomCache gg = new MapCache(gh);


        for (int i = 0; i < 5; i++){
            gg.put("ab" + i);
        }
        RemoverThread removerThread = new RemoverThread(gg);
        Executors.newScheduledThreadPool(2).scheduleAtFixedRate(removerThread, 5, 5, TimeUnit.SECONDS);

        for (int i = 0; i < 3; i++){
            gg.put("abc" + i);
        }
    }
}

抽象自定义缓存:

public abstract class AbstractCustomCache implements CustomCache{

    private Map<String, CacheEntry> cacheEntry = new LinkedHashMap<>();

    public Map<String, CacheEntry> getCacheEntry() {
        return Collections.synchronizedMap(cacheEntry);
    }
}

我的输出中有这样的内容:

Element was remove: CacheEntry{field='ab0'}
Time: 2019/01/31 11:39:11
Element was remove: CacheEntry{field='ab1'}
Time: 2019/01/31 11:39:11
Element was remove: CacheEntry{field='ab2'}
Time: 2019/01/31 11:39:11
Element was remove: CacheEntry{field='ab3'}
Time: 2019/01/31 11:39:11
Element was remove: CacheEntry{field='ab4'}
Time: 2019/01/31 11:39:11
Element was remove: CacheEntry{field='abc0'}
Time: 2019/01/31 11:39:11

我做错了什么？可以怎样改进呢？我想从 map 上删除每 5 秒发生一次的事件。例如，我添加了ab0、ab1、ab2、ab3、ab4。该流应删除 ab0、ab1、ab2。由于 map 中的元素大于 MAX_SIZE。然后我添加abc0、abc1、abc2。删除第一个元素 5 秒后，ab3, ab4, abc0 应被删除。但是，正如您所看到的，所有项目都会同时删除。

Answer 1

What am I doing wrong?

1. 使用Executors.newScheduledThreadPool(2).scheduleAtFixedRate(removerThread, 5, 5, TimeUnit.SECONDS);，第一次执行时间将为 delayed到5秒时，此时缓存包含:ab0 ab1 ab2 ab3 ab4 abc0 abc1 ab2，前5个将被删除。

2. 您需要一个线程安全版本 AbstractCustomCache customCache，因为它是由多个线程修改的。