Handling Fallback for Custom LogEventFactory

[skjelmo]

Questions

• Will Log4J default to the default LogEventFactory if a custom LogEventFactory defined through the system property Log4jLogEventFactory is not available?
  (Documentation for LogEventFactory)
• Is it possible to define Log4J's built-in default LogEventFactory as a fallback?

Context

• We have configured our own LogEventFactory as described in the documentation. The intention is to split log statements for throwables with long stacktraces into separate log events to circumvent a limitation in Heroku where log lines over 10 000 bytes are split into separate chunks resulting in invalid JSON format limiting log analysis and observability.
• We use the New Relic Java Agent which bundles Log4J as a dependency of it's own. It is also affected by the system property Log4jLogEventFactory when we configured Log4J to use our custom LogEventFactory, causing a ClassNotFoundException as our custom LogEventFactory is not available to the New Relic Java Agent.
• I raised this issue first with New Relic support who believe it is up to Log4J how it handles if the LogEventFactory defined in the system property Log4jLogEventFactory is not available.

[ppkarwasz]

@skjelmo,

Yes, even if the error message fails to mention it, Log4j Core falls back to a default log event factory, if your custom factory is not available (cf. source code).

If you want to prevent the message from appearing, you can bundle your custom LogEventFactory together with a log4j2.component.properties containing:

log4j2.logEventFactory = fully_qualified_name_of_your_class

Remark: a custom LogEventFactory is not necessarily the best tool for the job, since it can not split events. You can split events with a custom Appender implementation or you can limit the size of the stacktrace (each layout has a way to limit stacktraces).

[skjelmo]

Thank you very much for the answer @ppkarwasz!

A custom Appender implementation delegating to the existing Appender worked wonders, and I hadn't considered it as a possibility before your remark.