聊聊Java WeakHashMap背后的事情

前言

本月还没有写过Java相关的东西，今天终于挤出点时间来了，弄一篇基础知识吧。

WeakHashMap是平时常见的HashMap的变种，它是基于弱引用（WeakReference）的。我们已经知道，不管内存是否足够，弱引用对象都会随着GC被回收，所以WeakHashMap特别适用于内存敏感的局部缓存场景。本文简单探究一下它的部分实现细节。

属性

WeakHashMap的属性比HashMap来得简单，列举如下。

public class WeakHashMap
    extends AbstractMap
    implements Map {
    /**
     * The default initial capacity -- MUST be a power of two.
     */
    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.
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;
    /**
     * The load factor used when none specified in constructor.
     */
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;
    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    Entry[] table;
    /**
     * The number of key-value mappings contained in this weak hash map.
     */
    private int size;
    /**
     * The next size value at which to resize (capacity * load factor).
     */
    private int threshold;
    /**
     * The load factor for the hash table.
     */
    private final float loadFactor;
    /**
     * Reference queue for cleared WeakEntries
     */
    private final ReferenceQueue