本地缓存，用SoftReference和ConcurrentHashMap实现

1、接口

package yourpackage;

import java.util.function.Function;

/**
 * @param  键类型
 * @param  值类型
 * @author huangjinzhou
 * @date 2019/8/21 19:31
 */
public interface Cache {

    /**
     * 从缓存获取指定键的值，如果不存在，返回supplier取得的值
     *
     * @param key 键
     * @param supplier 值提供器
     * @return 值
     */
    V get(K key, Function supplier);

}

2、基于SoftReference和ConcurrentHashMap实现：

package yourpackage;

import java.lang.ref.SoftReference;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.function.Function;

/**
 * @author huangjinzhou
 * @date 2019/8/21 19:32
 */
public class InMemorySoftReferenceCache implements Cache {

    private volatile SoftReference> cacheRef = null;

    /**
     * 是否有线程在临界区，true-是，false-无
     */
    private final AtomicBoolean cacheLock = new AtomicBoolean(false);

    @Override
    public V get(K key, Function supplier) {
        SoftReference> cacheRef = this.cacheRef;

        Map cache = cacheRef == null ? null : cacheRef.get();
        V value;
        if (cache != null && ((value = cache.get(key)) != null || cache.containsKey(key))) {
            return value;
        }

        if ((cacheRef = this.cacheRef) == null || (cache = cacheRef.get()) == null) {
            boolean locked;
            do {
                // 自旋等待
                locked = !cacheLock.compareAndSet(false, true);
            } while (locked);

            try {
                if ((cacheRef = this.cacheRef) == null || (cache = cacheRef.get()) == null) {
                    cache = new ConcurrentHashMap<>(8);
                    this.cacheRef = new SoftReference<>(cache);
                }
            } finally {
                cacheLock.set(false);
            }
        }

        return cache.computeIfAbsent(key, supplier);
    }
}