Breaking change in 1.5.0

[drewfeelsblue]

Hi,

doesn't 1.5.0 introduce breaking change since from implementation perspective changing method access from protected to public is not compatible (one who implemented it as protected can't do this anymore)?

see https://github.com/prometheus/client_java/pull/1800/changes#diff-e2e698a852fe81e25539a14fb61eb5d7f987750b704bcbc7df7f474e8821e1baL30

[zeitlinger]

Thanks for reporting this.

You are correct that this is technically a source-incompatible change — if a user had a subclass of MetricWithFixedMetadata (or any concrete metric type) that overrides getMetadata() with protected visibility, recompiling against 1.5.0 would fail with "attempting to assign weaker access privileges; was public".

However, it's worth distinguishing this from a binary-incompatible change:

• Binary compatibility (existing compiled JARs): Preserved. Code compiled against 1.4.x will continue to work at runtime with 1.5.0.
• Source compatibility (recompiling against 1.5.0): Potentially broken if someone overrides getMetadata() as protected.

The practical impact is likely low because:

1. MetricWithFixedMetadata is an internal base class — users rarely subclass it directly.
2. getMetadata() was protected and undocumented — unlikely to be overridden by users.
3. The method now implements the new Collector.getMetadata() default method from the interface, which must be public.

That said, we should have caught this, and we should add API compatibility checking to CI to prevent this class of issue in the future (see linked issue below).

Do you have a concrete case where this broke your code? If so, that would help us assess priority for a fix.

[zeitlinger]

Tracking API compatibility linting in #1901.

[drewfeelsblue]

thank you for detailed reply

yes, we had unfortunate but completely possible from logical pov setup with 1.4.3 core and 1.5.0 model which led to IllegalAccessError at runtime

[error] java.lang.IllegalAccessError: 'io.prometheus.metrics.model.snapshots.MetricMetadata io.prometheus.metrics.core.metrics.Gauge.getMetadata()'
[error]         at io.prometheus.metrics.model.registry.PrometheusRegistry.register(PrometheusRegistry.java:203)
[error]         at io.prometheus.metrics.core.metrics.Metric$Builder.register(Metric.java:58)
[error]         at io.prometheus.metrics.core.metrics.MetricWithFixedMetadata$Builder.register(MetricWithFixedMetadata.java:51)
...

[drewfeelsblue]

so seems we observed binary incompatibility not source