unable to see Undertow logs via SLF4J

[Garret Wilson]

I wrote a sample test application using Java 17. I've used embedded Tomcat, but I read high reviews for Undertow so I wanted to try it out. I don't need Java/Jakarta EE servlets; I just need to respond to requests using path templates.

I successfully started up and Undertow server and so far it's working as expected. But no logging. I see in `Undertow.start()` that I should see "starting server: …", but I don't.

```java

    public synchronized void start() {
        UndertowLogger.ROOT_LOGGER.infof("starting server: %s", Version.getFullVersionString());

```

I'm using `io.undertow:undertow-core:2.3.10.Final`. For logging I'm using SLF4J 2.0.9 and Logback 1.4.11. I have my Logback file configured to use `INFO` level logging:

```xml

<configuration>
<root level="info">
<appender-ref ref="STDOUT" />
</root>
</configuration>

```

I even tried (after a bit of research) forcing JBoss Logging to use SLF4J before starting the server:

```java

System.setProperty("org.jboss.logging.provider", "slf4j");

```

Still no logs from Undertow

I stepped through the code. Undertow's `Slf4jLocationAwareLogger` is calling `LOG_METHOD.invoke(logger, null, className, Integer.valueOf(level), text, EMPTY, thrown)` where `level` has a value of `20`. Using reflection this invokes Logback's `Logger.filterAndLog_0_Or3Plus(final String localFQCN, final Marker marker, final Level level, final String msg, final Object[] params, final Throwable t)` method the `if (effectiveLevelInt > level.levelInt)` check fails because `effectiveLevelInt` is `10000` and `level.levelInt` is `20000`, so the method returns without logging (i.e. without calling `buildLoggingEventAndAppend(localFQCN, marker, level, msg, params, t)`).

First of all, why am I not getting any logs?

Secondly, why is Undertow invoking Logback directly? The whole point of SLF4J is to provide a general abstraction layer across loggers. How can I make Undertow invoke SLF4J directly? (In fact, the latest SLF4J allows system properties to specify an actual logging implementation; see https://jira.qos.ch/browse/SLF4J-450 . If Undertow bypassing SLF4J and requires Logback or tries to guess the logging implementation and access it directly, that defeats the whole purpose of SLF4J and will break.)

Undertow looks really promising, and I think it will work for a demo I'm giving at a Java user group in two days. But I hope I can get Undertow logging working via SLF4J, as that is critical for any embedded server.

Thanks,

Garret

[Garret Wilson]

Since there were no responses here, I have raised the question on Stack Overflow.