ITL

        ThreadLocal<String> threadLocal=new ThreadLocal();
        InheritableThreadLocal<String> inheritableThreadLocal=new InheritableThreadLocal<>();
        threadLocal.set("father threadLocal");
        inheritableThreadLocal.set("father inheritableThreadLocal");
        Thread thread= new Thread(()->{
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            //子线程通过TL对象在自己的TLMap中没有找到相关的对象：输出null
            System.out.println("子线程获取父类threadLocal数据："+threadLocal.get());
            System.out.println("子线程获取父类inheritableThreadLocal数据："+inheritableThreadLocal.get());
        });
        inheritableThreadLocal.set("father inheritableThreadLocal is change");

        thread.start();

子线程获取父类threadLocal数据：null
子线程获取父类inheritableThreadLocal数据：father inheritableThreadLocal

    // TL的set方法，如果是子类的实现，那么获取（getMap）和初始化赋值（createMap）都是ITL对象里的方法
    // 其余操作不变（因为hash计算、查找、扩容都是TLMap里需要做的，这里子类ITL只起到一个为Thread对象里哪个TLMap属性赋值的作用）
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocal.ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

    // ITL里getMap方法的实现
    ThreadLocal.ThreadLocalMap getMap(Thread t) {
        return t.inheritableThreadLocals; //返回的其实是Thread对象的inheritableThreadLocals属性
    }

    // ITL里createMap方法的实现
    void createMap(Thread t, T firstValue) {
        // 也是给Thread的inheritableThreadLocals属性赋值
        t.inheritableThreadLocals = new ThreadLocal.ThreadLocalMap(this, firstValue);
    }

// 初始化一个Thread对象时的代码段（Thread类的init方法）
Thread parent = currentThread();
if (parent.inheritableThreadLocals != null){ //可以看到，如果父线程存在inheritableThreadLocals的时候，会赋值给子线程（当前正在被初始化的线程）
    // 利用父线程的TLMap对象，初始化一个TLMap，赋值给自己的inheritableThreadLocals（这就意味着这个TLMap里的值会一直被传递下去）
    this.inheritableThreadLocals =
            ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
}

// 看下TL里对应的方法
static ThreadLocal.ThreadLocalMap createInheritedMap(ThreadLocal.ThreadLocalMap parentMap) {
    return new ThreadLocal.ThreadLocalMap(parentMap); //这里就开始初始化TLMap对象了
}

// 根据parentMap来进行初始化子线程的TLMap对象
private ThreadLocalMap(ThreadLocal.ThreadLocalMap parentMap) {
    ThreadLocal.ThreadLocalMap.Entry[] parentTable = parentMap.table; //拿到父线程里的哈希表
    int len = parentTable.length;
    setThreshold(len); // 设置阈值（具体方法参考上一篇）
    table = new ThreadLocal.ThreadLocalMap.Entry[len];

    for (int j = 0; j < len; j++) {
        ThreadLocal.ThreadLocalMap.Entry e = parentTable[j]; //将父线程里的Entry取出
        if (e != null) {
            @SuppressWarnings("unchecked")
            ThreadLocal<Object> key = (ThreadLocal<Object>) e.get(); //获取key
            if (key != null) {
                Object value = key.childValue(e.value); //获取value
                ThreadLocal.ThreadLocalMap.Entry c = new ThreadLocal.ThreadLocalMap.Entry(key, value); //根据k-v重新生成一个Entry
                int h = key.threadLocalHashCode & (len - 1); //计算哈希值
                while (table[h] != null)
                    h = nextIndex(h, len); //线性探查解决哈希冲突问题（具体方法参考上一篇）
                table[h] = c; //找到合适的位置后进行赋值
                size++;
            }
        }
    }
}

// ITL里的childValue的实现
protected T childValue(T parentValue) {
    return parentValue; //直接将父线程里的值返回
}