ppkarwasz opened a new issue, #3832:
URL: https://github.com/apache/logging-log4j2/issues/3832
The current `log4j-core-test` module includes a variety of test fixtures
primarily designed to test Log4j Core itself and its internal plugins. These
fixtures assume that:
* Static loggers (other than `StatusLogger`) are absent from the code under
test.
* A new `LoggerContext` can be created per test case.
* Loggers are created by the test cases, not the code under test.
This model works well for testing Log4j itself but breaks down in the
context of third-party libraries and applications, where:
* Loggers are typically declared as `static final` fields.
* Logger initialization might happen before the test setup, making it
impractical to substitute a different `LoggerContext` or logger instance at
runtime.
As a result, most test fixtures in `log4j-core-test` are not reusable
outside of Log4j’s own testing. The only widely useful component is
`ListAppender`, which depends solely on Log4j Core. However, to use just
`ListAppender`, developers must still depend on the entire `log4j-core-test`
module—bringing in unnecessary dependencies and potential conflicts that often
need to be manually excluded.
### Proposal
To improve the usability and modularity of Log4j test support for
third-party projects:
1. **Extract `ListAppender` into a new, standalone artifact**
This artifact would:
* Be explicitly intended for use by external libraries and applications
in testing scenarios.
* Only include minimal and broadly useful test utilities like
`ListAppender`.
* Have the same backward compatibility guarantees as Log4j Core itself.
2. **Evaluate and incorporate improvements from past community feedback**
In particular, we should revisit insights from this [dev@logging
thread](https://lists.apache.org/thread/3n9xqc132pm49w3m67zm6wnjfw6yphkq)
Specifically, we should:
* Address concerns around using `ListAppender` in parallel test
executions.
* Improve thread-safety and isolation for better test reliability.
### Benefits
Although testing log output in unit tests is sometimes seen as excessive or
even bad practice, there are specific cases where it is both justified and
valuable:
* **Audit Logging Compliance**
Verifying that audit logs are emitted correctly is crucial in systems
where traceability and accountability are required. This aligns with one of the
core purposes of Log4j Core itself.
* **Capturing Operational Details Not Exposed to Users**
In many systems, user-facing error messages are intentionally vague for
security or UX reasons. Logs often carry the detailed context needed for
debugging, so it's useful to ensure those details are reliably recorded.
* **Monitoring Failures in Background Threads**
Failures in asynchronous or background threads often can't propagate to
the main execution flow. Logging may be the *only* way these failures surface,
making it essential to validate they’re captured appropriately.
To support these use cases effectively, it's important to provide users with
a lightweight, purpose-built artifact focused solely on general logging test
utilities. This avoids the complexity and overhead of `log4j-core-test`, which
includes many features intended only for internal testing of Log4j Core itself.
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