Java 多线程同步问题求助

想用 wait/notify 和 map 来实现一个线程写入，另一个线程随后读出的同步处理。

但是遇到了 happens-before 问题。

一个可以复现的代码如下：

import java.time.LocalDateTime;
import java.util.Map;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class MainConcurrentHashmapTest {

	private static ExecutorService executorService = Executors.newFixedThreadPool(2);

	public static void main(String[] args) {

		for (int i = 0; i != 100000; ++i) {
			test();
		}

		executorService.shutdown();
	}

	public static void test() {

		final Map<String, Object> map = new ConcurrentHashMap<String, Object>();

		final Object mutex = new Object();
		final String key = UUID.randomUUID().toString();

		Runnable consumer = new Runnable() {

			@Override
			public void run() {
				synchronized (mutex) {
					try {
						mutex.wait(10000L);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					Object value = map.remove(key);
					if (value == null) {
						System.out.println("got null value for key = " + key);
					}
				}
			}
		};

		Runnable producer = new Runnable() {

			@Override
			public void run() {
				map.put(key, LocalDateTime.now().toString());
				synchronized (mutex) {
					mutex.notify();
				}
			}
		};

		executorService.submit(consumer);
		executorService.submit(producer);
	}

}

运行的时候会发现，有时候得不到最新的 value 值，如下：

got null value for key = f972a078-c314-4cd1-a5e4-d3ce11195a7e
got null value for key = 6ef9476c-8ded-43ec-900e-5f859eeaec1e
got null value for key = 8ecff482-eb9a-4e63-8eee-c950a485fbb0

为什么呢？有什么改进建议么？