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一句话，ThreadLocal并不是把线程作为key，值作为value的类似一种HashMap的东西。而是每个Thread里面都有一个ThreadLocalMap的集合，ThreadLocal只是操作每个线程的ThreadLocalMap而已。

转：http://www.iteye.com/topic/1007515

1.ThreadLocal. 
真正关键的类是它的内部类ThreadLocalMap,ThreadLocal 基本上相当于一个代理，或者算是Facade模式的应用，还没想清楚这种设计的妙处。(经过分析，这种安排与弱引用的特性有关) 

2.同时，Thread类中包含一个ThreadLocalMap类型的成员变量。 

3.ThreadLocalMap的实现原理跟HashMap差不多，内部有一个Entry数组，一个Entry通常至少包括key,value, 查找时通过一定的运算规则运算Key的HASH值，来得到Entry在数组中的位置，进而得到相应的value。但是比较特殊的是， 
<1>这个Entry继承了WeakReference， 

static class Entry extends WeakReference
   
     {            /** The value associated with this ThreadLocal. */            Object value;            Entry(ThreadLocal k, Object v) {                super(k);                value = v;            }        }
   

public T get() {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null) {            ThreadLocalMap.Entry e = map.getEntry(this);            if (e != null)                return (T)e.value;        }        return setInitialValue();    }

public void set(T value) {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);    }

/**         * Expunge a stale entry by rehashing any possibly colliding entries         * lying between staleSlot and the next null slot.  This also expunges         * any other stale entries encountered before the trailing null.  See         * Knuth, Section 6.4         *         * @param staleSlot index of slot known to have null key         * @return the index of the next null slot after staleSlot         * (all between staleSlot and this slot will have been checked         * for expunging).         */        private int expungeStaleEntry(int staleSlot) {            Entry[] tab = table;            int len = tab.length;            // expunge entry at staleSlot            tab[staleSlot].value = null;               tab[staleSlot] = null;            size--;            // Rehash until we encounter null            Entry e;            int i;            for (i = nextIndex(staleSlot, len);		 (e = tab[i]) != null;                 i = nextIndex(i, len)) {                  ThreadLocal k = e.get();                  if (k == null) {                    e.value = null;                    tab[i] = null;                    size--;                } else {                    int h = k.threadLocalHashCode & (len - 1);                    if (h != i) {                        tab[i] = 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;        }

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