👀 Views: 32 💬 Answers: 1 📅 Created: 2025-05-31

java concurrency concurrenthashmap Java

I'm having a hard time understanding I've been banging my head against this for hours... I'm experiencing unexpected behavior when using the `computeIfAbsent` method of `ConcurrentHashMap` in Java 11. My goal is to populate a map with a default value if the key is absent. However, it seems that the lambda I've provided is being executed multiple times, leading to performance issues and unexpected results. Here’s a simplified version of my code: ```java import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.TimeUnit; public class ConcurrentMapExample { private static final ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>(); public static void main(String[] args) throws InterruptedException { Runnable task = () -> { for (int i = 0; i < 10; i++) { String key = "key" + i; String value = map.computeIfAbsent(key, k -> { System.out.println(Thread.currentThread().getName() + " is computing value for " + k); try { TimeUnit.SECONDS.sleep(1); // Simulating expensive computation } catch (InterruptedException e) { Thread.currentThread().interrupt(); } return "value" + i; }); System.out.println(Thread.currentThread().getName() + " found value: " + value); } }; Thread thread1 = new Thread(task); Thread thread2 = new Thread(task); thread1.start(); thread2.start(); thread1.join(); thread2.join(); } } ``` When I run this code, I see that the `computeIfAbsent` lambda is triggered multiple times for the same key when both threads try to access it simultaneously. This results in multiple prints like: ``` Thread-0 is computing value for key0 Thread-1 is computing value for key0 ``` I expected that `computeIfAbsent` would ensure that the lambda is executed only once per key, but it seems that both threads are entering the lambda simultaneously. I even tried using `synchronize` around the task, but that led to performance degradation. Am I missing something in my understanding of how `ConcurrentHashMap` operates, or is there a better way to handle this situation? Any insights or best practices would be greatly appreciated! Am I missing something obvious? I'm working on a service that needs to handle this. I'm using Java 3.11 in this project. Hoping someone can shed some light on this.