基础数据机构之WeakHashMap源码分析

最新推荐文章于 2024-04-19 20:17:37 发布

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本文深入解析了WeakHashMap的工作原理，包括其内部结构、如何利用弱引用实现键的自动垃圾回收，以及与HashMap的区别。通过源码分析展示了WeakHashMap中Entry的具体实现，并详细解释了插入数据的过程。

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1.WeakHashMap 在HashMap实现上，在HashMap上增加Entry的SoftReference软引用功能

源码分析

Entry实体类继承WeakReference

 /**
     * The entries in this hash table extend WeakReference, using its main ref
     * field as the key.
     */
    private static class Entry<K,V> extends WeakReference<K> implements Map.Entry<K,V> {//继承WeakReference弱引用
        private V value;
        private final int hash;
        private Entry<K,V> next;

        /**
         * Creates new entry.
         */
        Entry(K key, V value,ReferenceQueue<K> queue,int hash, Entry<K,V> next) {
            super(key, queue);
            this.value = value;
            this.hash  = hash;
            this.next  = next;
        }

	public K getKey() {
            return WeakHashMap.<K>unmaskNull(get());				//get()方法，获取该key的时候，判断是否被回收掉，如果被回收掉返回null
        }

}

插入数据方式和HashMap插入数据方法一致，只是多了对WeakReference的null判断逻辑

 /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for this key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated.
     * @param value value to be associated with the specified key.
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        K k = (K) maskNull(key);	//如果该key为null，那么直接使用NULL_KEY作为填充
        int h = HashMap.hash(k.hashCode());
        Entry[] tab = getTable();
        int i = indexFor(h, tab.length);

        for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
            if (h == e.hash && eq(k, e.get())) {
                V oldValue = e.value;
                if (value != oldValue)
                    e.value = value;
                return oldValue;
            }
        }

        modCount++;
	Entry<K,V> e = tab[i];
        tab[i] = new Entry<K,V>(k, value, queue, h, e);
        if (++size >= threshold)
            resize(tab.length * 2);
        return null;
    }