ThreadLocal的场景是用来隔离各个线程的局部变量，各个线程之间的数值互不干扰。
先查看个例子：

    final ThreadLocal<Boolean> threadLocal = new ThreadLocal<>();
        threadLocal.set(true);
        Log.e(TAG, Thread.currentThread()+"="+threadLocal.get());
        new Thread(new Runnable() {
            @Override
            public void run() {
                Log.e(TAG, Thread.currentThread()+"="+threadLocal.get());
            }
        }).start();

执行的结果如下:

Thread[main,5,main]=true
Thread[Thread-5,5,main]=null

从结果看 主线程和子线程获取的threadLocal值不同，所以应用于线程数据隔离

源码查看Set(true)方法

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

//获取线程对应的ThreadLocalMap
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

//ThreadLocal变量跟对象实例firstValue由线程的threadLocals来维护
   void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

  private void set(ThreadLocal<?> key, Object value) {

            // We don't use a fast path cas with get() beause 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;  //table是ThreadMap内的Entry数组
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);  //获取hash值，用于数组中的小标

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
  //如果数组中有对应的值则进入
                ThreadLocal<?> k = e.get();
        //key值相等则覆盖值
                if (k == key) {
                    e.value = value;
                    return;
                }
 //此时说明此处 Entry 的 k 中的对象实例已经被回收了，需要替换掉这个位置的 key 和 value
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
//创建 Entry 对象
            tab[i] = new Entry(key, value);
            int sz = ++size;
//超过负载因子，则扩充
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

源码查看get（）方法

    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();
    }

get()方法就是根据所在线程获取ThreadLocalMap，根据TheadLocal获取对应的ThreadLocalMap.Entry，然后获取对应的value值，否则返回setInitialValue()=null