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();

        }

    }

}