Console output may be lost when using Log4j2 with something that replaces System.out

[micopiira]

When upgrading to Spring Boot 3.4.2 our logging stopped completely working.

After narrowing down I figured the problem happens with Spring Boot 3.4.2 when using Log4j2 together with BlockHound.

Here is a minimum reproducer repo: https://github.com/micopiira/spring-boot-log4j2-issue

Spring Boot 3.4.1 with exact same Log4j2 version and BlockHound version works.

Spring Boot 3.4.1:

Spring Boot 3.4.2:

[wilkinsona]

Thanks for the report and for the minimal sample. I assume that something has changed in Spring Boot 3.4.2 that is causing BlockHound to break Log4j2 in some way. Unfortunately, given that this works fine without BlockHound and the way in which BlockHound works, I don't think there's anything we can do about this in Spring Boot. Please raise a BlockHound issue so that they can investigate further.

[wilkinsona]

Re-opening. Thanks to @violetagg's analysis, this has been tracked down to an assumption in StatusConsoleListener#closeNonSystemStream that is faulty when BlockHound is involved. It's only started causing a problem due to #43578.

[nosan]

Thanks for the sample, @micopiira.

StandardOutputIntegration.applyTo(...) replaces the standard OUT and ERR streams with PrintStreamDelegate. During initialization, Log4J2LoggingSystem attempts to reset the StatusLogger.FallbackListener stream (which was previously replaced by BlockHound). This results in PrintStreamDelegate being closed, which in turn delegates the close call to the standard OUT and ERR streams.

One of the options is to revert b6b9237 and fix SampleLog4j2StructuredLoggingApplicationTests.
main...nosan:spring-boot:43963

Meanwhile, you can use this solution reactor/BlockHound#469 (comment)