We recently updated from Springboot 3.4.9/Micrometer 1.12.5 (which apparently was too old for that Springboot version, but worked) to Springboot 3.5.6 and Micrometer 1.15.4 We are running a dedicated management server, have remapped actuator to "/" and mapped the Prometheus endpoint to /metrics, by setting these properties:

management.server.port=8081
management.endpoints.web.base-path=/
management.endpoint.prometheus.access=read_only
management.prometheus.metrics.export.enabled=true
management.endpoints.web.path-mapping.prometheus=metrics

This is the config that had been working before the version update (albeit we used the now deprecated 

management.endpoint.prometheus.enabled=true

instead of 

management.endpoint.prometheus.access=read_only

but switching back to that makes no difference).

When accessing the Prometheus endpoint (whether we let Prometheus scape itself or just curl the endpoint), we get a status 500, the response when using plain

curl <host>:<port>/metrics

is 

 {"timestamp":1758895051901,"status":500,"error":"Internal Server Error","path":"/metrics"}

At that moment, the Springboot app logs this line

DefaultHandlerExceptionResolver - Resolved [org.springframework.http.converter.HttpMessageNotWritableException: No converter for [class [B] with preset Content-Type 'text/plain;version=0.0.4;charset=utf-8']

I debugged into this a bit. Apparently PrometheusScrapeEndpoint provides the result as a bytearray (which I found somewhat surprising).

When trying to convert this to an actual HTTP response org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor has two available message converters: - StringHttpMessageConverter - this can produce text/plain responses, but only handles String result objects. - MappingJackson2HttpMessageConverter - this can support any class of result objects, but produces only application/json content types.

I am not sure what the actual constellation should be here - a converter that supports bytearrays and can produce text/plain responses? Or should the PrometheusScrapeEndpoint (or rather some equivalent) actually produce a String result (I find the latter more naturally, obviously).

Any hints appreciated.

Regards

PPF

Comment From: wilkinsona

I cannot reproduce the described behavior. If you would like us to spend some more time investigating, please spend some time providing a complete yet minimal sample that reproduces the problem. You can share it with us by pushing it to a separate repository on GitHub or by zipping it up and attaching it to this issue.