[LOG4J2-3366] Fix order of property sources

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 2.10.0
• Fix Version/s: 2.18.0
• Assignee: Piotr Karwasz
• Reporter: Piotr Karwasz
• Created: 26/Jan/22 09:19
• Updated: 18/Oct/22 16:07
• Resolved: 10/May/22 06:13

Description

The current order of system property sources (cf. documentation) is rather unnatural, because:

1. It gives a higher priority to environment variables than Java system properties. Java system properties apply to a single JVM, whereas environment variables might be shared between processes.
2. It mixes up property sources accessible to system administrators and those accessible only to programmers (`log4j2.component.properties`).

IMHO Log4j should prioritize the sources accessible to system administrators over `log4j2.component.properties`, hence allowing them to easily override the defaults established by developers. It should also prioritize Java system properties over environment variables. This is what, e.g. Spring Boot does. I propose the following order:

1. System properties,
2. Environment variables,
3. `log4j2.component.properties` as failover.

While technically this would be a breaking change, the official property source order never worked and bug LOG4J2-3193 was filed only recently. This proves that almost no one configures the same property in more than one source.

Issue Links

• contains: LOG4J2-3193 ConfigurationFactory cannot find config file property normalized by EnvironmentPropertySource
• fixes: LOG4J2-3287 document log4j2.system.properties file
• relates to: LOG4J2-3621 Log4J 2.19 breaks contract of order of loading of System Properties
• links to: GitHub Pull Request #742