java 引用之软引用与弱引用

package cache;

import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class WeakCache<K, V> {
    private final Map<K, Reference<V>> cache = new WeakHashMap<>();
    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    public void put(K k, V v) {
        lock.writeLock().lock();
        try {
            if (v != null ) {
                cache.put(k, new WeakReference(v));
            } else {
                cache.remove(k);
            }
        } finally {
            lock.writeLock().unlock();
        }
    }

    public V get(K k) {
        lock.readLock().lock();

        Reference<V> value = cache.get(k);
        if (value == null ) {
            return null ;
        }
        V v = value.get();
        if (v == null ) {
            lock.readLock().unlock();
            lock.writeLock().lock();
            try {
                cache.remove(k);
            } finally {
                lock.writeLock().unlock();
            }
        } else {
            lock.readLock().unlock();
        }
        return v;
    }

    public void clear() {
        lock.writeLock().lock();
        try {
            this .cache.clear();
        } finally {
            lock.writeLock().unlock();
        }
    }
}