Add support for old-style exception rendering in Pattern Layout

[rschmitt]

Due to #3045, all exception messages are prefixed with a newline now:

[SIGTSTP] [1] ~/src/logging-log4j2 # rg -txml pattern
src/main/resources/log4j2.xml
6:                <pattern>%date{EEE MMM dd HH:mm:ss yyyy zzz}{UTC} [%p] (%t) %c: %m %ex%n</pattern>

[0] [1] ~/src/logging-log4j2 # ./run.sh
Darwin 842f57b044d8 24.5.0 Darwin Kernel Version 24.5.0: Tue Apr 22 19:53:27 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6041 arm64
Log4j 2.24.3:
Thu Aug 07 17:22:01 2025 UTC [ERROR] (main) org.example.App: Going to call a method now
Thu Aug 07 17:22:01 2025 UTC [ERROR] (main) org.example.App: Got exception while calling throwException, how could this have happened java.lang.RuntimeException: This is the message passed to the RuntimeException ctor
	at org.example.App.throwException(App.java:20)
	at org.example.App.main(App.java:12)

Thu Aug 07 17:22:01 2025 UTC [ERROR] (main) org.example.App: Done calling method

Log4j 2.25.0:
Thu Aug 07 17:22:02 2025 UTC [ERROR] (main) org.example.App: Going to call a method now
Thu Aug 07 17:22:02 2025 UTC [ERROR] (main) org.example.App: Got exception while calling throwException, how could this have happened
java.lang.RuntimeException: This is the message passed to the RuntimeException ctor
	at org.example.App.throwException(App.java:20)
	at org.example.App.main(App.java:12)

Thu Aug 07 17:22:02 2025 UTC [ERROR] (main) org.example.App: Done calling method

I don't see a way to reproduce the 2.24.3 style of log messages with the pattern layout.

1. Using %ex{0} to get the first line doesn't work, because a line separator is prepended.
2. I can't find anything in ThrowableFormatOptions that provides the type of the exception thrown. A pattern like %m %ex{short.message}%n doesn't fully reproduce the top line of the stack trace.
3. Using %n%ex to print the rest of the stack trace repeats the first line. In other words, there's no way to just print the frames of the stack trace.

Since this layout change affects log scans, I'd like it if there were a way to specify a pattern that reproduces the pre-2.25 layout for anyone who depends on it.

[vy]

@rschmitt, thanks so much for reaching out to us. I'm sorry to hear that your log scans are affected.

As you indicated, #3045 changed all exception messages to get prefixed with a newline. This work was a follow-up of #2691 – see this conversation. Both changes were delivered in 2.25.0.

Since this layout change affects log scans, I'd like it if there were a way to specify a pattern that reproduces the pre-2.25 layout for anyone who depends on it.

There are no configuration knobs to switch to pre-2.25.0 behavior. You can create a custom layout extending from ThrowablePatternConverter, but I guess you already know that.

As it is explicitly stated in the very first paragraph of the Pattern Layout manual

Pattern Layout is not intended for structural logging purposes. For production environments, you are strongly advised to use JSON Template Layout producing JSON output ready to be delivered to log ingestion systems ...

I understand, given this feature has been there for decades without a good offer for structured logging (i.e., JSON Template Layout is decades younger than Pattern Layout), there are tooling in the wild depending on this behavior. During 2.25.0, we had internal consensus among active maintainers to settle on the current behavior. Unless there is a pressing need to do otherwise, I don't see the current layout changing.

[vy]

@rschmitt, we're checking this with @ppkarwasz one more time to see if we can accommodate your request.

[rschmitt]

You can create a custom layout extending from ThrowablePatternConverter, but I guess you already know that.

That might be viable, actually. I just need something I can do if a lot of services turn out to be affected by this.

[rschmitt]

To give a concrete example, there are users with log-scanning regular expressions like:

(\[ERROR\]|\[TRACE\])(.*\"exception\":).*Error adding currency

It appears that multi-line regexes aren't supported. So this issue is best understood as a feature request against exception properties, which are conceptually perfect for producing these types of lines in the logs:

In this mode, extracted attributes of the Throwable are injected verbatim. That is, no newlines, suffixes, prefixes, etc. will be added.

We already have access to the message, so the only thing that's not exposed is the class name of the Throwable. It looks like this should be a simple addition to ThrowablePropertyRendererFactory, if I'm not mistaken. Maybe we could add two properties, short.throwableClassFullName (corresponding to Class::getName) and short.throwableClassShortName (corresponding to Class::getSimpleName).