啥也不说，直接上代码，欢迎指正。

package com.example.demo.cache;

/**
 * @className: JdkCache
 * @description: TODO 类描述
 * @author: Yang.H
 * @date: 2021/8/2014:26
 **/

import lombok.extern.slf4j.Slf4j;

import java.util.Date;
import java.util.Map;
import java.util.concurrent.*;

/**
 * 设计一个JDK缓存
 */
@Slf4j
public class JdkCache {
    private static final long NO_EXPIRE_TIME = -1;
    // Key,Value 容器
    private static final Map<String, ValueHolder> LOCAL_CACHE = new ConcurrentHashMap<>();

    private static class ValueHolder {
        // 缓存值
        private String value;
        // 修改时间
        private Date modifyTime;
        // 过期时间
        private long expireTime;
    }

    private static final ScheduledExecutorService exeService = new ScheduledThreadPoolExecutor(1, (r) -> {
        Thread thread = new Thread(r);
        thread.setDaemon(true);
        thread.setName("JDK失效缓存清空守护线程");
        return thread;
    });

    static {
        // 类初始化时，创建一个线程，用于不断的去判断key是否过期，然后删除。
        exeService.scheduleAtFixedRate(() -> {
            for (Map.Entry<String, ValueHolder> entry : LOCAL_CACHE.entrySet()) {
                if (isExpire(entry.getValue())) {
                    String value = removeKey(entry.getKey());
                    log.info("缓存已失效。键:{}, 缓存值:{}", entry.getKey(), value);
                } else {
                    log.info("缓存未失效。键:{}", entry.getKey());
                }
            }
        }, 0, 500, TimeUnit.MILLISECONDS);
    }


    private static boolean isExpire(ValueHolder holder) {
        if (holder.expireTime <= 0) { //不失效
            return false;
        }
        return new Date().getTime() > holder.modifyTime.getTime() + holder.expireTime;
    }

    /**
     * 删除过期的key，并返回值。返回null表示不符合要求
     *
     * @param key
     * @return
     */
    private static String removeExpireKey(String key) {
        ValueHolder holder = LOCAL_CACHE.get(key);
        if (holder != null && isExpire(holder)) {
            log.info("缓存已失效。键:{}", key);
            return (holder = LOCAL_CACHE.remove(key)) != null ? holder.value : null;
        }
        return null;
    }

    /**
     * 删除key
     *
     * @param key
     * @return
     */
    public static String removeKey(String key) {
        ValueHolder holder;
        return (holder = LOCAL_CACHE.remove(key)) != null ? holder.value : null;
    }


    /**
     * 存储，KV，但是过期时间不刷新
     * @param key
     * @param value
     * @param millSecond
     * @return
     */
    public static String put(String key, String value, long millSecond) {
        removeExpireKey(key);
        ValueHolder valueHolder = LOCAL_CACHE.get(key);
        if (valueHolder != null){
            valueHolder.value = value;
        }else {
            valueHolder = new ValueHolder();
            valueHolder.value = value;
            valueHolder.expireTime = millSecond;
            valueHolder.modifyTime = new Date();
        }
        return (valueHolder = LOCAL_CACHE.put(key, valueHolder)) == null ? null : valueHolder.value;
    }

    /**
     * 存储Key，value，并指定过期时间. 刷新过期时间
     *
     * @param key
     * @param value
     * @param millSecond
     * @return
     */
    public static String putAndUpdateExpireTime(String key, String value, long millSecond){
        removeExpireKey(key);
        ValueHolder valueHolder = new ValueHolder();
        valueHolder.value = value;
        valueHolder.modifyTime = new Date();
        valueHolder.expireTime = millSecond;
        ValueHolder oldHolder = LOCAL_CACHE.put(key, valueHolder);
        return oldHolder != null ? oldHolder.value : null;
    }

    /**
     * 添加不过期的key
     *
     * @param key
     * @param value
     * @return
     */
    public static String put(String key, String value) {
        return putAndUpdateExpireTime(key, value, NO_EXPIRE_TIME);
    }

    /**
     * 删除key
     *
     * @param key
     * @return
     */
    public static String remove(String key) {
        ValueHolder valueHolder;
        return (valueHolder = LOCAL_CACHE.remove(key)) == null ? null : valueHolder.value;
    }

    /**
     * 获取key
     *
     * @param key
     * @return
     */
    public static String get(String key) {
        removeExpireKey(key);
        ValueHolder valueHolder;
        return (valueHolder = LOCAL_CACHE.get(key)) == null ? null : valueHolder.value;
    }

    /**
     * 获取，如果不存在，则返回默认值
     *
     * @param key
     * @param defaultValue
     * @return
     */
    public static String getOrDefault(String key, String defaultValue) {
        removeExpireKey(key);
        ValueHolder valueHolder;
        return (valueHolder = LOCAL_CACHE.get(key)) == null ? defaultValue : valueHolder.value;
    }

}