Logging: On The Level

Mon, 29 Jun 2026 20:18:00 +0000
A discussion of logging best practices, specifically on the correct purpose of each logging level.
When Logging Goes Bad

Admit it, you’ve definitely seen this monstrosity of a logging statement at some point during your career:
 12026-06-29T14:03:56.438+01:00 ERROR [nio-5002-exec-1] u.c.s.myproject.controllers.SearchController 
 2 : Request validation error: Parse error on request JSON.
 3java.lang.IllegalArgumentException: Parse error on request JSON
 4 at uk.co.skipoles.myproject.controllers.SearchController.search(SearchController.kt:56) ~[main/:na]
 5 at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[na:na]
 6 at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[na:na]
 7 at kotlin.reflect.jvm.internal.calls.CallerImpl$Method.callMethod(CallerImpl.kt:97) ~[kotlin-reflect-2.2.20.jar:2.2.20-release-333]
 8 at kotlin.reflect.jvm.internal.calls.CallerImpl$Method$Instance.call(CallerImpl.kt:113) ~[kotlin-reflect-2.2.20.jar:2.2.20-release-333]
 9 at kotlin.reflect.jvm.internal.KCallableImpl.callDefaultMethod$kotlin_reflection(KCallableImpl.kt:250) ~[kotlin-reflect-2.2.20.jar:2.2.20-release-333]
10 at kotlin.reflect.jvm.internal.KCallableImpl.callBy(KCallableImpl.kt:155) ~[kotlin-reflect-2.2.20.jar:2.2.20-release-333]
11 at org.springframework.web.reactive.result.method.InvocableHandlerMethod$KotlinDelegate.invokeFunction(InvocableHandlerMethod.java:374) ~[spring-webflux-6.2.10.jar:6.2.10]
12 at org.springframework.web.reactive.result.method.InvocableHandlerMethod.lambda$invoke$0(InvocableHandlerMethod.java:205) ~[spring-webflux-6.2.10.jar:6.2.10]
13 at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:132) ~[reactor-core-3.7.9.jar:3.7.9]
14 at reactor.core.publisher.MonoZip$ZipCoordinator.signal(MonoZip.java:297) ~[reactor-core-3.7.9.jar:3.7.9]
15 at reactor.core.publisher.MonoZip$ZipInner.onNext(MonoZip.java:478) ~[reactor-core-3.7.9.jar:3.7.9]
16 at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onNext(MonoPeekTerminal.java:180) ~[reactor-core-3.7.9.jar:3.7.9]
17 at reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:122) ~[reactor-core-3.7.9.jar:3.7.9]
18 at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:74) ~[reactor-core-3.7.9.jar:3.7.9]
19 at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.7.9.jar:3.7.9]
20 at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158) ~[reactor-core-3.7.9.jar:3.7.9]
21 at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onNext(FluxContextWrite.java:107) ~[reactor-core-3.7.9.jar:3.7.9]
22 at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:299) ~[reactor-core-3.7.9.jar:3.7.9]
23 at reactor.core.publisher.FluxFilterFuseable$FilterFuseableConditionalSubscriber.onNext(FluxFilterFuseable.java:337) ~[reactor-core-3.7.9.jar:3.7.9]
24 at reactor.core.publisher.Operators$BaseFluxToMonoOperator.completePossiblyEmpty(Operators.java:2096) ~[reactor-core-3.7.9.jar:3.7.9]
25 at reactor.core.publisher.MonoCollect$CollectSubscriber.onComplete(MonoCollect.java:145) ~[reactor-core-3.7.9.jar:3.7.9]
26 at reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onComplete(FluxContextWriteRestoringThreadLocals.java:149) ~[reactor-core-3.7.9.jar:3.7.9]
27 at org.springframework.http.server.reactive.AbstractListenerReadPublisher$State.onAllDataRead(AbstractListenerReadPublisher.java:501) ~[spring-web-6.2.10.jar:6.2.10]
28 at org.springframework.http.server.reactive.AbstractListenerReadPublisher.onAllDataRead(AbstractListenerReadPublisher.java:137) ~[spring-web-6.2.10.jar:6.2.10]
29 at org.springframework.http.server.reactive.ServletServerHttpRequest$RequestBodyPublisher$RequestBodyPublisherReadListener.onAllDataRead(ServletServerHttpRequest.java:382) ~[spring-web-6.2.10.jar:6.2.10]
30 at org.apache.catalina.connector.CoyoteAdapter.asyncDispatch(CoyoteAdapter.java:207) ~[tomcat-embed-core-10.1.44.jar:10.1.44]
31 at org.apache.coyote.AbstractProcessor.dispatch(AbstractProcessor.java:243) ~[tomcat-embed-core-10.1.44.jar:10.1.44]
32 at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:50) ~[tomcat-embed-core-10.1.44.jar:10.1.44]
33 at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:903) ~[tomcat-embed-core-10.1.44.jar:10.1.44]
34 at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1769) ~[tomcat-embed-core-10.1.44.jar:10.1.44]
35 at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) ~[tomcat-embed-core-10.1.44.jar:10.1.44]
36 at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1189) ~[tomcat-embed-core-10.1.44.jar:10.1.44]
37 at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:658) ~[tomcat-embed-core-10.1.44.jar:10.1.44]
38 at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:63) ~[tomcat-embed-core-10.1.44.jar:10.1.44]
39 at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
Imagine trying to support a system that’s logging like this. Makes me want to claw my eyes out, but why? Let’s look at the elements that make this a terrible logging statement:
Engineering on Software Engineering Thoughts

Logging: On The Level

When Logging Goes Bad
