Java中ThreadLocal共享变量的使用

public class ThreadLocalTest {
    private static ThreadLocal<String> variable = new ThreadLocal<>(); // (1) 
 
    public static void main(String[] args) throws InterruptedException {
        variable.set(Thread.currentThread().getName());  // (2)
        // 创建线程一
        var thread1 = new Thread(() -> {
            System.err.println("Thread Name before set: " + Thread.currentThread().getName() + " " + variable.get());  // (3)
            variable.set(Thread.currentThread().getName()); // (4)
            System.err.println("Thread Name after set: " + Thread.currentThread().getName() + " " + variable.get()); // (5)
        });


        var thread2 = new Thread(() -> {
            System.err.println("Thread Name before set: " + Thread.currentThread().getName() + " " + variable.get()); // (6)
            variable.set(Thread.currentThread().getName()); // (7)
            System.err.println("Thread Name after set: " + Thread.currentThread().getName() + " " + variable.get()); // (8)
        });
        thread1.start();  // (9)
        thread2.start(); // (10)
        Thread.sleep(2000); // (11)
        System.err.println("main thread: " + variable.get()); // (12)
    }
}

variable.set(Thread.currentThread().getName()); // 设置ThreadLocal变量
variable.get();    // 读取ThreadLocal变量

public T get() {
  return get(Thread.currentThread()); // (1)
}

private T get(Thread t) {
  ThreadLocalMap map = getMap(t);  // (2)
  if (map != null) {
    ThreadLocalMap.Entry e = map.getEntry(this); // (3)
    if (e != null) {
      @SuppressWarnings("unchecked")
      T result = (T) e.value;
      return result;
    }
  }
  return setInitialValue(t);   // (4)
}

ThreadLocalMap getMap(Thread t) {
  return t.threadLocals;    // (5)
}

private T setInitialValue(Thread t) {
  T value = initialValue();
  ThreadLocalMap map = getMap(t);
  if (map != null) {
    map.set(this, value);
  } else {
    createMap(t, value);
  }
  if (this instanceof TerminatingThreadLocal<?> ttl) {
    TerminatingThreadLocal.register(ttl);
  }
  if (TRACE_VTHREAD_LOCALS) {
    dumpStackIfVirtualThread();
  }
  return value;
}

void createMap(Thread t, T firstValue) {
  t.threadLocals = new ThreadLocalMap(this, firstValue); // (1)
}

public void set(T value) {
  set(Thread.currentThread(), value);   // (1)
  if (TRACE_VTHREAD_LOCALS) {
    dumpStackIfVirtualThread();
  }
}

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