ThreadLocal原理

Thread有一个成员

ThreadLocal.ThreadLocalMap threadLocals = null;

在创建ThreadLocal时会创建这个对象

    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

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

精髓尽在ThreadLocalMap中

现在相当于Thread 一对一ThreadLocalMap

而ThreadLocalMap 以ThreadLocal（的HashCode）为Key，也就是说 ThreadLocalMap 一对多 ThreadLocal，也可以有结论

Thread 一对多ThreadLocal

//ThreadLocalMap 方法       
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;
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal<?> k = e.get();

                if (k == key) {
                    e.value = value;
                    return;
                }

                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }

            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

所以一个线程可以有多个ThreadLocal对象，同时每个ThreadLocal都能取到自己对应的值