Embedded Broker does not work with Log4j2

[john2home]

Expected behavior

Log4j 2 SLF4J Binding can be used for SLF4J API

Actual behavior

if you exclude logback-classic in gradle because you want to use log4j2 you get an exception on start

2021-11-26 08:55:04.694 UTC|${sys:mea.location}|ERROR| c.h.u.UncaughtExceptionHandler:32  Uncaught exception in thread 'embedded-hivemq-state-change-executor'.
java.lang.NoClassDefFoundError: ch/qos/logback/core/joran/spi/JoranException
	at com.hivemq.HiveMQServer.bootstrap(HiveMQServer.java:109) ~[hivemq-community-edition-embedded-2021.3.jar:2021.3]
	at com.hivemq.embedded.internal.EmbeddedHiveMQImpl.stateChange(EmbeddedHiveMQImpl.java:147) ~[hivemq-community-edition-embedded-2021.3.jar:2021.3]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[?:?]
	at java.lang.Thread.run(Thread.java:829) [?:?]
Caused by: java.lang.ClassNotFoundException: ch.qos.logback.core.joran.spi.JoranException
	at jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581) ~[?:?]
	at jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178) ~[?:?]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:522) ~[?:?]
	... 5 more

if you use logback-classic and log4j2 you get

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/D:/repo/.gradle/caches/modules-2/files-2.1/org.apache.logging.log4j/log4j-slf4j-impl/2.14.1/9a40554b8dab7ac9606089c87ae8a5ba914ec932/log4j-slf4j-impl-2.14.1.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/D:/repo/.gradle/caches/modules-2/files-2.1/ch.qos.logback/logback-classic/1.2.3/7c4f3c474fb2c041d8028740440937705ebb473a/logback-classic-1.2.3.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.apache.logging.slf4j.Log4jLoggerFactory]
2021-11-26 09:00:02.833 UTC|${sys:mea.location}|INFO | c.h.e.i.EmbeddedHiveMQImpl:75  Setting default authentication behavior to ALLOW ALL
2021-11-26 09:00:08.360 UTC|${sys:mea.location}|INFO | c.h.e.i.EmbeddedHiveMQImpl:141  Starting EmbeddedHiveMQ.
2021-11-26 09:00:08.374 UTC|${sys:mea.location}|INFO | c.h.c.i.SystemInformationImpl:146  HiveMQ version: 2021.3
Exception in thread "main" java.util.concurrent.CompletionException: java.lang.ClassCastException: class org.apache.logging.slf4j.Log4jLoggerFactory cannot be cast to class ch.qos.logback.classic.LoggerContext (org.apache.logging.slf4j.Log4jLoggerFactory and ch.qos.logback.classic.LoggerContext are in unnamed module of loader 'app')
	at java.base/java.util.concurrent.CompletableFuture.reportJoin(CompletableFuture.java:412)
	at java.base/java.util.concurrent.CompletableFuture.join(CompletableFuture.java:2044)
	at hivemq.EmbeddedBroker.main(EmbeddedBroker.java:33)
Caused by: java.lang.ClassCastException: class org.apache.logging.slf4j.Log4jLoggerFactory cannot be cast to class ch.qos.logback.classic.LoggerContext (org.apache.logging.slf4j.Log4jLoggerFactory and ch.qos.logback.classic.LoggerContext are in unnamed module of loader 'app')
	at com.hivemq.bootstrap.LoggingBootstrap.getRootLogger(LoggingBootstrap.java:145)
	at com.hivemq.bootstrap.LoggingBootstrap.prepareLogging(LoggingBootstrap.java:67)
	at com.hivemq.HiveMQServer.bootstrap(HiveMQServer.java:109)
	at com.hivemq.embedded.internal.EmbeddedHiveMQImpl.stateChange(EmbeddedHiveMQImpl.java:147)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:829)

To Reproduce

Steps

Use embedded broker example found on this site

Reproducer code

build.gradle

def log4jVersion = '2.14.1'
dependencies {
	implementation("com.hivemq:hivemq-community-edition-embedded:2021.3") {
		exclude group: 'ch.qos.logback', module: 'logback-classic'
	}
	implementation("org.apache.logging.log4j:log4j-api:$log4jVersion")
	implementation("org.apache.logging.log4j:log4j-core:$log4jVersion")
	runtimeOnly("org.apache.logging.log4j:log4j-slf4j-impl:$log4jVersion")
}

Details

• Affected HiveMQ CE version(s): 2021.3
• Used JVM version: Adoptium 11.0.12+7 64bit

[DC2-DanielKrueger]

We do not support LOG4j2. I'll close this ticket.

[Zomis]

I think closing this ticket and only saying "We do not support Log4j2" is the wrong way to go. The solution is to depend on slf4j-api and not a specific implementation of slf4j (e.g. Logback or Log4j2).

Solving #366 should solve this issue as well.

[pglombardo]

Hi @Zomis - that's a fair point. Now that the request is filed in #366 and linked here - we'll make sure to update both issues once any updates happen.

Apologies for the brevity in closing this issue previously without a proper explanation.