基础数据机构之WeakHashMap源码分析

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;
}