源码未看完，先占坑

每个Thread维护一个ThreadLocalMap

获取ThreadLocalMap

/**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */    
ThreadLocalMap getMap(Thread t) {
            return t.threadLocals;
        }

获取ThreadLocal实例

/**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);//先获取当前线程维护的ThreadLocalMap
        if (map != null) {
            //this:外部的ThreadLocal(如ThreadLocal<Session> stl;stl.get())那么该this就是该ThreadLocal对象stl;
            //Entry<key:this对象指向的ThreadLocal对象,value:我们所设置的对象，这里是Session>
            ThreadLocalMap.Entry e = map.getEntry(this);//得到该线程下map中的ThreadLocal对象stl
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

设置ThreadLocal实例

/**
     * Sets the current thread's copy of this thread-local variable
     * to the specified value.  Most subclasses will have no need to
     * override this method, relying solely on the {@link #initialValue}
     * method to set the values of thread-locals.
     *
     * @param value the value to be stored in the current thread's copy of
     *        this thread-local.
     */
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);//不存在map,则创建该线程绑定的一个map
    }

JDK1.4后用WeakReference的好处

不知道ThreadLocal干嘛用的，参考

http://www.iteye.com/topic/103804
[荐]http://qifuguang.me/2015/09/02/[Java%E5%B9%B6%E5%8F%91%E5%8C%85%E5%AD%A6%E4%B9%A0%E4%B8%83]%E8%A7%A3%E5%AF%86ThreadLocal/