Java中的HashMap弱引用之WeakHashMap详解

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.
  * 默认容量，必须是2次幂
  */
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，必须为2次幂且 <= 1<<30
  */
private static final int MAXIMUM_CAPACITY = 1 << 30;
/**
  * The load factor used when none specified in constructor.
  * 负载因子
  */
private static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
  * The table, resized as necessary. Length MUST Always be a power of two.
  * 容量必须为2次幂的数组
  */
Entry<K,V>[] table;
/**
  * The number of key-value mappings contained in this weak hash map.
  * 拥有键值对的数量
  */
private int size;
/**
  * The next size value at which to resize (capacity * load factor).
  * 阈值 -- 扩容判断依据
  */
private int threshold;
/**
  * The load factor for the hash table.
  */
private final float loadFactor;
/**
  * Reference queue for cleared WeakEntries
  * 引用队列，用于存储已被GC清除的WeakEntries
  */
private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

// 默认构造函数
WeakHashMap()
// 指定"容量大小"的构造函数
WeakHashMap(int capacity)
// 指定"容量大小"和"负载因子"的构造函数
WeakHashMap(int capacity, float loadFactor)
// 包含"子Map"的构造函数
WeakHashMap(Map<? extends K, ? extends V> map)

/**
  * The entries in this hash table extend WeakReference, using its main ref field as the key.
  * 该Enty继承WeakReference，从而具备弱引用的特性
  */
private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
    V value;
    int hash;
    Entry<K,V> next;//链表
    /**
      * 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. -- 删除过时的entry
  * 该方法是实现弱键回收的最关键方法，也是区分HashMap的根本方法
  * 核心：移除table和queue的并集元素（queue中存储是已被GC的key，注意是key！！）
  * 效果：key在GC的时候被清除，value在key清除后访问WeakHashMap被清除
  */
private void expungeStaleEntries() {
    //从队列中出队遍历
    //poll 移除并返问队列头部的元素；如果队列为空，则返回null
    for (Object x; (x = queue.poll()) != null; ) {
        synchronized (queue) {
            //值得一提的是WeakHashMap是非线程安全的，这里却同步了一下
            //大神原本的用意是保证在多线程时能不破坏数据结构，但JavaDoc已经强调这类非安全，如下文
            //http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6425537
            @SuppressWarnings("unchecked")
                Entry<K,V> e = (Entry<K,V>) x;
            //找到该队列中的元素在数组中的位置，并移除该元素（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;
            }
        }
    }
}