Micrometer - Prometheus Gauge displays NaN

I had the same issue with Micrometer.io gauges when I used your method #1 meterRegistry.gauge("myGauge", new AtomicDouble()). I am using Scala by the way. I noticed that after I created about 50 gauges, the new gauges after that displayed NaN.

Instead I used:

val atomicDouble = new AtomicDouble()
Gauge
  .builder("myGauge", atomicDouble, new AtomicDoubleToDoubleFunction)
  .strongReference(true)
  .register(meterRegistry)

with

class AtomicDoubleToDoubleFunction extends ToDoubleFunction[AtomicDouble] {
  override def applyAsDouble(value: AtomicDouble): Double = value.doubleValue()
}

This fixed the NaN issue, and all of my gauges appear correctly. I found the .strongReference(true) example from https://www.codota.com/code/java/classes/io.micrometer.core.instrument.Gauge .

I wasn't able to use @panser solution 'cause I'm using gauges with labels. My solution involved the creation of com.google.common.util.concurrent.AtomicDouble cache with io.micrometer.core.instrument.Tag's key and values as map key, heres goes:

    private static final Map<String, AtomicDouble> GAUGE_CACHE = new HashMap<>();

    public void handleGauge(String name, List<Tag> tags, double value) {
        String gaugeKey = this.gaugeKey(name, tags);
        if (!GAUGE_CACHE.containsKey(gaugeKey)) {
            GAUGE_CACHE.put(gaugeKey, new AtomicDouble());
        }
        Objects.requireNonNull(this.registry.gauge(name, tags, GAUGE_CACHE.get(gaugeKey))).set(value);
    }

    private String gaugeKey(String name, List<Tag> tags) {
        return name + ":" + tags.stream().map(tag -> tag.getKey() + tag.getValue()).collect(Collectors.joining(""));
    }

That worked pretty well for my needs, hopefully help other people.

In all cases, you must hold a strong reference to the observed instance. When your createGauge() method is exited, all function stack allocated references are eligible for garbage collection.

For #1, pass your atomicInteger field like this: registry.gauge("my_ai", atomicInteger);. Then increment/decrement as you wish. Whenever micrometer needs to query it, it will as long as it finds the reference.

For #2, pass your productService field and a lambda. Basically whenever the gauge is queried, it will call that lambda with the provided object: registry.gauge("product_gauge", productService, productService -> productService.getProducts().size());

(No guarantee regarding syntax errors.)