为ThreadLocal定制的ThreadLocalMap

ThreadLocalMap是java.lang.ThreadLocal的静态内部类。

ThreadLocalMap表面看起来至少实现类似hashmap的功能，但是仔细分析它却有下面的属性。

首先，它只是尽量的让value保持在哈希值的位置或后面，

其次，它提供了对不同key（这里是ThreadLocal）对象的但hashCode相同的存储的支持。

最后，它对Key的引用是弱引用。这个和WeakHashMap一致。但这个和hashMap有点不一样，hashMap要使用用弱引用的话，

一般是对value进行弱引用的，而不是key.

也许有一天我们在自己的程序中需要类似ThreadLocalMap的功能。

以下是从ThreadLocalMap简化而来的代码：

其中Key对应JDK中的ThreadLocal

class Key {
    final int hashCode = nextHashCode();
    static AtomicInteger nextHashCode = 
    new AtomicInteger();
    static final int HASH_INCREMENT = 0x61c88647;
    static int nextHashCode() {
    return nextHashCode.getAndAdd(HASH_INCREMENT); 
    }
}
class ThreadLocalMap {
        static class Entry extends WeakReference<Key> {
            Object value;
            Entry(Key k, Object v) {
                super(k);
                value = v;
            }
        }
         //初始值，必须是2的n次方
        private static final int INITIAL_CAPACITY = 16;
        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        private Entry[] table;
        private int size = 0;
        /**
         * The next size value at which to resize.
         */
        private int threshold; // Default to 0
        /**
         * Set the resize threshold to maintain at worst a 2/3 load factor.
         */
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }
        private static int nextIndex(int i, int len) {
            return ((i + 1 < len) ? i + 1 : 0);
        }
        private static int prevIndex(int i, int len) {
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }
        /**
         * Construct a new map initially containing (firstKey, firstValue).
         * ThreadLocalMaps are constructed lazily, so we only create
         * one when we have at least one entry to put in it.
         */
        ThreadLocalMap(Key firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.hashCode & (INITIAL_CAPACITY - 1);
            table = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }
        private Entry getEntry(Key key) {
            int i = key.hashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }
        private Entry getEntryAfterMiss(Key key, int i, Entry e) {
            Entry[] tab = table[i];
            int len = tab.length;
            while (e != null) {
                Key k = e.get();
                if (k == key)
                    return e;
                if (k == null)
                    expungeStaleEntry(i);
                else
                    i = nextIndex(i, len);
                e = tab[i];
            }
            return null;
        }
       private void set(Key key, Object value) {
            Entry[] tab = table;
            int len = tab.length;
            int i = key.hashCode & (len-1);
            for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) {
                Key k = e.get();
                if (k == key) {
                    e.value = value;
                    return;
                }
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
            tab = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }
        private void remove(Key key) {
            Entry[] tab = table;
            int len = tab.length;
            int i = key.hashCode & (len-1);
            for (Entry e = tab;
         e != null;
         e = tab[i = nextIndex(i, len)]) {
                if (e.get() == key) {
                    e.clear();
                    expungeStaleEntry(i);
                    return;
                }
            }
        }
        private void replaceStaleEntry(Key key, Object value,
                                       int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            Entry e;
            int slotToExpunge = staleSlot;
            for (int i = prevIndex(staleSlot, len);
         (e = tab) != null;
                 i = prevIndex(i, len))
                if (e.get() == null)
                    slotToExpunge = i;
            for (int i = nextIndex(staleSlot, len);
         (e = tab) != null;
                 i = nextIndex(i, len)) {
                Key k = e.get();
                if (k == key) {
                    e.value = value;

                    tab = tab[staleSlot];
                    tab[staleSlot] = e;
                    // Start expunge at preceding stale entry if it exists
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                    return;
                }
                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }
            // If key not found, put new entry in stale slot
            tab[staleSlot].value = null;   
            tab[staleSlot] = new Entry(key, value);
            // If there are any other stale entries in run, expunge them
            if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }
        /**擦除staleSlot的Entry。
        *调整所有不在哈希映射位置的value的位置，让它尽量在哈希映射的位置
        */
        private int expungeStaleEntry(int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            tab[staleSlot].value = null;   
            tab[staleSlot] = null;
            size--;
            // Rehash until we encounter null
            Entry e;
            int i;
            for (i = nextIndex(staleSlot, len);
         (e = tab) != null;
                 i = nextIndex(i, len)) {
                Key k = e.get();
                if (k == null) {
                    e.value = null;
                    tab = null;
                    size--;
                } else {
                    int h = k.hashCode & (len - 1);
                    if (h != i) {
                        tab = null;

                        // Unlike Knuth 6.4 Algorithm R, we must scan until
                        // null because multiple entries could have been stale.
                        while (tab[h] != null)
                            h = nextIndex(h, len);
                        tab[h] = e;
                    }
                }
            }
            return i;
        }
        private boolean cleanSomeSlots(int i, int n) {
            boolean removed = false;
            Entry[] tab = table;
            int len = tab.length;
            do {
                i = nextIndex(i, len);
                Entry e = tab;
                if (e != null && e.get() == null) {
                    n = len;
                    removed = true;
                    i = expungeStaleEntry(i);
                }
            } while ( (n >>>= 1) != 0);
            return removed;
        }
        private void rehash() {
            expungeStaleEntries();
            // Use lower threshold for doubling to avoid hysteresis
            if (size >= threshold - threshold / 4)
                resize();
        }
        private void resize() {
            Entry[] oldTab = table;
            int oldLen = oldTab.length;
            int newLen = oldLen * 2;
            Entry[] newTab = new Entry[newLen];
            int count = 0;

            for (int j = 0; j < oldLen; ++j) {
                Entry e = oldTab[j];
                if (e != null) {
                    Key k = e.get();
                    if (k == null) {
                        e.value = null; // Help the GC
                    } else {
                        int h = k.hashCode & (newLen - 1);
                        while (newTab[h] != null)
                            h = nextIndex(h, newLen);
                        newTab[h] = e;
                        count++;
                    }
                }
            }
            setThreshold(newLen);
            size = count;
            table = newTab;
        }
        private void expungeStaleEntries() {
            Entry[] tab = table;
            int len = tab.length;
            for (int j = 0; j < len; j++) {
                Entry e = tab[j];
                if (e != null && e.get() == null)
                    expungeStaleEntry(j);
            }
        }
    }