Volatile to be used with Concurrent Collection?

Question

I am developing a metrics store (Map) , which basically collects metrics about some operations such as

• mix
• max
• counter
• timeElapsed[] etc

Here Key is the name of the method and value are metrics about it.

Spring can help me create singleton object of MetricStore, i am using ConcurrentHashMap to avoid race condition when multiple REST request comes in parallel.

My query 1- Do i need to make MetricStore variable store volatile? to improve the visibility among multiple requests. 2- I am using Map as the base class and ConcurrentHashMap as Implemetnation, does it affect as Map is not ThreadSafe. -

@Component
class MetricStore{
    public Map<String, Metric> store = new ConcurrentHashMap<>();
    //OR  public volatile Map<String, Metric> store = new ConcurrentHashMap<>();
}

@RestController
class MetricController{
    @Autowired
    private MetricStore metricStore;

    @PostMapping(name="put")
    public void putData(String key, Metric metricData) {
        if(metricStore.store.containsKey(key)) {
            // udpate data
        }
        else {
            metricStore.store.put(key, metricData);
        }
    }

    @PostMapping(name="remove")
    public void removeData(String key) {
        if(metricStore.store.containsKey(key)) {
            metricStore.store.remove(key);
        }
    }

}

Answer 1

Do i need to make MetricStore variable store volatile?

No, because you are not changing the value of store (i.e. store can be marked as final and the code should still compile).

I am using Map as the base class and ConcurrentHashMap as Implemetnation, does it affect as Map is not ThreadSafe

Because you're using a ConcurrentHashMap as the implementation of Map, it is thread-safe. If you want the declared type to be more specific, Map can be changed to ConcurrentMap.

---

The bigger issue here is that you're using containsKey before calling put and remove when you should be using compute and computeIfPresent, which are atomic operations:

@PostMapping(name="put")
public void putData(String key, Metric metricData) {
    metricStore.store.compute(key, (k, v) -> {
        if (v == null) {
            return metricData;
        }

        // update data
    });
}

@PostMapping(name="remove")
public void removeData(String key) {
    metricStore.store.computeIfPresent(key, (k, v) -> null);
}