Object o = new Object();

public class SoftReference<T> extends Reference<T> {...} 

public class WeakReference<T> extends Reference<T> {...} 

public class WeakHashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>

/**
  * The default initial capacity -- MUST be a power of two.
  * 默認(rèn)容量，必須是2次冪
  */
private static final int DEFAULT_INITIAL_CAPACITY = 16;
/**
  * The maximum capacity, used if a higher value is implicitly specified by either of the
  * constructors with arguments. MUST be a power of two <= 1<<30.
  * 最大容量，必須為2次冪且 <= 1<<30
  */
private static final int MAXIMUM_CAPACITY = 1 << 30;
/**
  * The load factor used when none specified in constructor.
  * 負(fù)載因子
  */
private static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
  * The table, resized as necessary. Length MUST Always be a power of two.
  * 容量必須為2次冪的數(shù)組
  */
Entry<K,V>[] table;
/**
  * The number of key-value mappings contained in this weak hash map.
  * 擁有鍵值對(duì)的數(shù)量
  */
private int size;
/**
  * The next size value at which to resize (capacity * load factor).
  * 閾值 -- 擴(kuò)容判斷依據(jù)
  */
private int threshold;
/**
  * The load factor for the hash table.
  */
private final float loadFactor;
/**
  * Reference queue for cleared WeakEntries
  * 引用隊(duì)列，用于存儲(chǔ)已被GC清除的WeakEntries
  */
private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

// 默認(rèn)構(gòu)造函數(shù)
WeakHashMap()
// 指定"容量大小"的構(gòu)造函數(shù)
WeakHashMap(int capacity)
// 指定"容量大小"和"負(fù)載因子"的構(gòu)造函數(shù)
WeakHashMap(int capacity, float loadFactor)
// 包含"子Map"的構(gòu)造函數(shù)
WeakHashMap(Map<? extends K, ? extends V> map)

/**
  * The entries in this hash table extend WeakReference, using its main ref field as the key.
  * 該Enty繼承WeakReference，從而具備弱引用的特性
  */
private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
    V value;
    int hash;
    Entry<K,V> next;//鏈表
    /**
      * Creates new entry.
      */
    Entry(Object key, V value,
            ReferenceQueue<Object> queue,
            int hash, Entry<K,V> next) {
        super(key, queue);
        this.value = value;
        this.hash  = hash;
        this.next  = next;
    }
    ....
}  

/**
  * Expunges stale entries from the table. -- 刪除過時(shí)的entry
  * 該方法是實(shí)現(xiàn)弱鍵回收的最關(guān)鍵方法，也是區(qū)分HashMap的根本方法
  * 核心：移除table和queue的并集元素（queue中存儲(chǔ)是已被GC的key，注意是key?。。?
  * 效果：key在GC的時(shí)候被清除，value在key清除后訪問WeakHashMap被清除
  */
private void expungeStaleEntries() {
    //從隊(duì)列中出隊(duì)遍歷
    //poll 移除并返問隊(duì)列頭部的元素；如果隊(duì)列為空，則返回null
    for (Object x; (x = queue.poll()) != null; ) {
        synchronized (queue) {
            //值得一提的是WeakHashMap是非線程安全的，這里卻同步了一下
            //大神原本的用意是保證在多線程時(shí)能不破壞數(shù)據(jù)結(jié)構(gòu)，但JavaDoc已經(jīng)強(qiáng)調(diào)這類非安全，如下文
            //http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6425537
            @SuppressWarnings("unchecked")
                Entry<K,V> e = (Entry<K,V>) x;
            //找到該隊(duì)列中的元素在數(shù)組中的位置，并移除該元素（table和queue都需要移除）    
            int i = indexFor(e.hash, table.length);
            Entry<K,V> prev = table[i];
            Entry<K,V> p = prev;
            while (p != null) {
                Entry<K,V> next = p.next;
                if (p == e) {
                    if (prev == e)
                        table[i] = next;
                    else
                        prev.next = next;
                    // Must not null out e.next;
                    // stale entries may be in use by a HashIterator
                    e.value = null; // Help GC 移除value
                    size--;
                    break;
                }
                prev = p;
                p = next;
            }
        }
    }
}