Summary

We're seeing an intermittent IllegalArgumentException during HTTP response serialization using Spring Boot 3.4.2 on Kubernetes. The exact same image sometimes works perfectly, and other times fails with this error:

java.lang.IllegalArgumentException: getWriter() has already been called for this response

The exception occurs inside AbstractJackson2HttpMessageConverter.writeInternal(...), which is triggered via the standard message conversion flow when returning a response entity. (.java#L438)

This does not appear to be a race condition in our own code, and we can't consistently reproduce it — but it happens often enough to be problematic (3–4 out of every 10 pod startups).

Stack Trace (Simplified)

The flow leading to the exception looks like this:

HttpEntityMethodProcessor.writeWithMessageConverters(...)
 → AbstractMessageConverterMethodProcessor (switching converter)
   → MappingJackson2HttpMessageConverter.write(...) [GENERIC]
     → AbstractGenericHttpMessageConverter.write(...)
       → AbstractJackson2HttpMessageConverter.writeInternal(...) ←