Java 21 - Issues with Thread Safety in Custom Caching Implementation Using ConcurrentHashMap
I'm collaborating on a project where I've been struggling with this for a few days now and could really use some help. I'm experiencing unexpected behavior in my custom caching implementation, where I store objects of varying types using a `ConcurrentHashMap`. Despite using `ConcurrentHashMap`, I'm facing issues with stale data being returned intermittently. For instance, when I update an entry in the cache, the previous thread seems to read the old value before the update is visible. Here’s the relevant code snippet: ```java import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.TimeUnit; public class MyCache { private final ConcurrentHashMap<String, Object> cache = new ConcurrentHashMap<>(); public void put(String key, Object value) { cache.put(key, value); } public Object get(String key) { return cache.get(key); } } ``` In my application, I’m running multiple threads that simultaneously update and read from the cache. Here’s an example of how I’m using the cache: ```java public class CacheTest { public static void main(String[] args) throws InterruptedException { MyCache myCache = new MyCache(); Runnable updater = () -> { for (int i = 0; i < 10; i++) { myCache.put("key", "value" + i); try { Thread.sleep(50); } catch (InterruptedException e) { e.printStackTrace(); } } }; Runnable reader = () -> { for (int i = 0; i < 10; i++) { String value = (String) myCache.get("key"); System.out.println("Read value: " + value); try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } } }; Thread t1 = new Thread(updater); Thread t2 = new Thread(reader); t1.start(); t2.start(); t1.join(); t2.join(); } } ``` When I run the above code, I sometimes see the output showing stale values, like `value0` even after multiple updates. I’m aware that `ConcurrentHashMap` provides thread safety for individual operations, but it doesn’t prevent issues related to compound actions. I suspect that the reader thread might be executing before the updater commits the new values. How can I ensure that reads always get the most up-to-date values? Is there a better pattern or approach I should consider for ensuring thread safety in this context? I’ve also looked into using `AtomicReference` for individual entries, but I’m worried about the added complexity and some performance overhead. Any advice would be greatly appreciated! My development environment is macOS. Am I missing something obvious? What's the best practice here?