val threadLocal= ThreadLocal<Any>()
 threadLocal.set("hhahah")
 threadLocal.get()

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

1.可见是由map中取值，如果getMap(t)存在则直接set，否者则创建

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

2.getmap() 是从当前线程中取出自己的 ThreadLocalMap

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

 ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }

3.可知ThreadLocalMap里面是一个散列表，下标是ThreadLocal， 值是set里面的value

4.至此set跟踪完毕，由此可知， ThreadLocal的set实际上是，
初始化了当前线程的threadLocals，并且把值通过散列表存入自己内部

5.每一个ThreadLocal  都是线程里面散列表的下标