一个ThreadLocal 在不同ClassLoader下失效的问题

ThreadId.java

import java.util.concurrent.atomic.AtomicInteger;

public class ThreadId {
	// Atomic integer containing the next thread ID to be assigned
	private static final AtomicInteger nextId = new AtomicInteger(0);

	// Thread local variable containing each thread's ID
	private static final ThreadLocal<Integer> threadId = new ThreadLocal<Integer>() {
		@Override
		protected Integer initialValue() {
			return nextId.getAndIncrement();
		}
	};

	// Returns the current thread's unique ID, assigning it if necessary
	public static int get() {
		return threadId.get();
	}
}

ThreadLocalTest.java

import java.lang.reflect.Method;
import java.net.URLClassLoader;

public class ThreadLocalTest {
	static private URLClassLoader userClassLoader;
	static {
		URLClassLoader systemClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
		userClassLoader = new URLClassLoader(systemClassLoader.getURLs(), systemClassLoader.getParent());
	}

	public static void main(String[] args) throws Exception {
		System.err.println(ThreadId.get());

		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				System.err.println(ThreadId.get());
			}
		});

		t1.start();
		t1.join();

		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				try {
					Class<?> clazz = loadClassFromUserClassLoader(ThreadId.class.getName());
					Method getMethod = clazz.getMethod("get");
					System.err.println(getMethod.invoke(null));
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});

		t2.start();

	}

	public static Class<?> loadClassFromUserClassLoader(String name) throws ClassNotFoundException {
		return userClassLoader.loadClass(name);
	}

}