告诉你为什么 ThreadLocal 可以做到线程隔离

private static final ThreadLocal<String> threadLocal = new ThreadLocal<>();
public static void main(String[] args) throws Throwable {
    Thread threadOne = new Thread(()->{
        threadLocal.set("ThreadOne：" + Thread.currentThread().getName());
        log.info("线程 One 本地变量值为：{}", threadLocal.get());
        threadLocal.remove();
        log.info("线程 One remove 后本地变量值为：{}", threadLocal.get());
    });
    Thread threadTwo = new Thread(()->{
        threadLocal.set("ThreadTwo：" + Thread.currentThread().getName());
        log.info("线程 Two 本地变量值为：{}", threadLocal.get());
    });
    threadOne.start();
    threadTwo.start();
}

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

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

public class Thread implements Runnable {
// ...
   ThreadLocal.ThreadLocalMap threadLocals = null;
// ...
}

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();
}

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;
}

// THread 类中的方法
private void exit() {
   // ...
   threadLocals = null;
   // ...
}