ThreadLocal原理

从ThreadLocal的get和set方法入手

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

class Thread implements Runnable {
    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

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

可以看到在set方法中首先会使用getMap方法得到一个ThreadLocalMap，如果map为空就进行初始化调用createMap方法，不为空就网map里面添加数据。所以ThreadLocal里面是靠ThreadLocalMap储存数据的。
get方法也是用ThreadLocalMap里面的方法。