谁问你ThreadLocal，就把这篇文章丢给他

 static class ThreadLocalMap {
        //.....
    }

//每个线程都维护着一个 存储局部变量的Map
    ThreadLocal.ThreadLocalMap threadLocals = null;

 static class ThreadLocalMap {
         /**
         * The initial capacity -- MUST be a power of two.
         */
        private static final int INITIAL_CAPACITY = 16;

        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        private Entry[] table;

        /**
         * The number of entries in the table.
         */
        private int size = 0;

        /**
         * The next size value at which to resize.
         */
        private int threshold; // Default to 0
        //........
    }

//当前的ThreadLocal 对象的hash值
private final int threadLocalHashCode = nextHashCode();

//静态变量，用于计算下一个hash值
private static AtomicInteger nextHashCode = new AtomicInteger();

//hash增量值，参与下一个hash值的计算
private static final int HASH_INCREMENT = 0x61c88647;

/**
     * Returns the next hash code.
     */
    private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }

 public void set(T value) {
        Thread t = Thread.currentThread();
        //获取当前线程的ThreadLocalMap 对象
        ThreadLocalMap map = getMap(t);
        if (map != null)//判断Map是否创建
            map.set(this, value);//this 指代当前 threadLocl对象
        else
            createMap(t, value);//为当前线程创建ThreadLocalMap对象
    }

  ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
  }

  void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

    public T get() {
        Thread t = Thread.currentThread();
        //获取当前线程的ThreadLocalMap对象
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();//初始的value值为null
    }

//继承了 WeakReference
static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                //创建了一个ThreadLocal对象的弱引用
                super(k);
                value = v;
            }
        }

 private void set(ThreadLocal<?> key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            //通过key值（ThreadLocal对象）的散列值threadLocalHashCode计算出 Entry的索引位置
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i]; e != null;e = tab[i = nextIndex(i, len)]) {
                //获取元素的key值
                ThreadLocal<?> k = e.get();

                if (k == key) {//hash命中，直接设置value值
                    e.value = value;
                    return;
                }

                if (k == null) {//没有命中，但找到了过时的Entry对象，即key值为null
                    //替换掉此过时的Entry
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }

            //如果即没有命中，而且表中也没有发现过时的Entry对象，则在当前空的位置创建并插入一个新的Entry来吃存储
            tab[i] = new Entry(key, value);
            //表的大小增加
            int sz = ++size;
            //threshold = len * 2 / 3;判断是否需要重hash
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();//重hash
        }

private void replaceStaleEntry(ThreadLocal<?> key, Object value,
                                       int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            Entry e;

            // Back up to check for prior stale entry in current run.
            // We clean out whole runs at a time to avoid continual
            // incremental rehashing due to garbage collector freeing
            // up refs in bunches (i.e., whenever the collector runs).
            //slotToExpunge记录过时Entry的索引值
            int slotToExpunge = staleSlot;
            //以当前的过时Entry的索引staleSlot为起点，往后遍历，寻找过时的Entry
            for (int i = prevIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = prevIndex(i, len))
                if (e.get() == null)//判断是否是过时的Entry
                    slotToExpunge = i;

            // Find either the key or trailing null slot of run, whichever
            // occurs first。以staleSlot为起点，继续往后遍历
            for (int i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal<?> k = e.get();

                // If we find key, then we need to swap it
                // with the stale entry to maintain hash table order.
                // The newly stale slot, or any other stale slot
                // encountered above it, can then be sent to expungeStaleEntry
                // to remove or rehash all of the other entries in run.
                if (k == key) {//发现hash命中
                    e.value = value;//直接修改value值
                    //命中的Entry与过时的Entry交换位置
                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;

                    // 判断前面的往后遍历循环是否发现新的过时的Entry对象，即slotToExpunge记录了新的索引值
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);//清除这个新发现的过时Entry
                    return;
                }

                // If we didn't find stale entry on backward scan, the
                // first stale entry seen while scanning for key is the
                // first still present in the run.
                //发现了过时的Entry对象，如果前一个往后遍历的循环没有发现过时的Entry对象，才记录当前的索引值。
                //优先释放靠前的过时Entry对象
                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }

            // If key not found, put new entry in stale slot
           //hash依旧没有命中，那么就将当前的过时Entry给替换成新的Entry对象
            tab[staleSlot].value = null;
            tab[staleSlot] = new Entry(key, value);

            // If there are any other stale entries in run, expunge them
            if (slotToExpunge != staleSlot)//判断是否发现新的过时Entry对象
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }