NullPointerException in Java API

In the Java API, the following code works:

private final Gauge fillLevelGauge = Gauge.build()
        .name("fill_level")
        .help("Fill level")
        .register();

    // ...

    fillLevelGauge.set(1d);

… but the following code crashes with a NullPointerException:

private final Gauge fillLevelGauge = Gauge.build()
        .name("fill_level")
        .labelNames("mylabel")
        .help("Fill level")
        .register();

    // ...

    fillLevelGauge.set(1d);

I found the following issue which states “for a labelled metric you must specify the labels”. However I cannot believe that you actually intend for the program to crash when a programmer makes this mistake? And not even with any explanation of the reason for the crash?

At the very least I believe this should be an IllegalArgumentException with a message attached, e.g. “for a labelled metric you must specify the labels”.

NB PLEASE don’t just close this with your common response to ask questions in the user mailing list. This is not a usage question, I am reporting an actual bug in the software.

[njbartlett]

My fix will be roughly as follows:

1. Make SimpleCollector#noLabelsChild private.
2. Write a protected method on SimpleCollector called getNoLabelsChild, in which the null check and throw new IllegalArgumentException occurs.
3. Make SimpleCollector subclasses access noLabelsChild via that method.

The getter method will likely be rapidly inlined by Hotspot so I do not believe any noticeable performance degradation would occur.

[ievgen-kapinos]

For those who tired to investigate problem root. This feature allows to add few series in one gauge.

val collectorRegistry = CollectorRegistry()
val syncGauge = Gauge.build()
      .subsystem("app")
      .name("synchronizer")
      .labelNames("state")
      .help("Consumer/exporter synchronizer state")
      .register(collectorRegistry)

val gaugeConsumedIndex = syncGauge.labels("consumed_index")
val gaugeExportedIndex = syncGauge.labels("exported_index")
val gaugePendingExportedIndex = syncGauge.labels("pending_exported_index")

you will get

# HELP app_synchronizer Consumer/exporter synchronizer state
# TYPE app_synchronizer gauge
app_synchronizer{state="exported_index",} 0.0
app_synchronizer{state="pending_exported_index",} 0.0
app_synchronizer{state="consumed_index",} 0.0