An application that uses Liquibase and spring-boot-starter-log4j2 fails to start due to Logback Classic being on the classpath

[wilkinsona]

Liquibase 3.6 moved to SLF4J for logging, but also added a compile-scoped dependency on Logback. If you try to use Log4j2 with an app that uses Liquibase, it'll fail to start:

Exception in thread "main" java.lang.IllegalArgumentException: LoggerFactory is not a Logback LoggerContext but Logback is on the classpath. Either remove Logback or the competing implementation (class org.apache.logging.slf4j.Log4jLoggerFactory loaded from file:/Users/awilkinson/.m2/repository/org/apache/logging/log4j/log4j-slf4j-impl/2.11.1/log4j-slf4j-impl-2.11.1.jar). If you are using WebLogic you will need to add 'org.slf4j' to prefer-application-packages in WEB-INF/weblogic.xml: org.apache.logging.slf4j.Log4jLoggerFactory
	at org.springframework.util.Assert.instanceCheckFailed(Assert.java:655)
	at org.springframework.util.Assert.isInstanceOf(Assert.java:555)
	at org.springframework.boot.logging.logback.LogbackLoggingSystem.getLoggerContext(LogbackLoggingSystem.java:286)
	at org.springframework.boot.logging.logback.LogbackLoggingSystem.beforeInitialize(LogbackLoggingSystem.java:102)
	at org.springframework.boot.context.logging.LoggingApplicationListener.onApplicationStartingEvent(LoggingApplicationListener.java:220)
	at org.springframework.boot.context.logging.LoggingApplicationListener.onApplicationEvent(LoggingApplicationListener.java:199)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:127)
	at org.springframework.boot.context.event.EventPublishingRunListener.starting(EventPublishingRunListener.java:69)
	at org.springframework.boot.SpringApplicationRunListeners.starting(SpringApplicationRunListeners.java:48)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:302)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248)
	at sample.liquibase.SampleLiquibaseApplication.main(SampleLiquibaseApplication.java:26)

The workaround is to exclude Logback when declaring the Liquibase dependency:

<dependency>
	<groupId>org.liquibase</groupId>
	<artifactId>liquibase-core</artifactId>
	<exclusions>
		<exclusion>
			<groupId>ch.qos.logback</groupId>
			<artifactId>logback-classic</artifactId>
		</exclusion>
	</exclusions>
</dependency>

We should exclude the dependency in our dependency management, in the same way that we exclude Commons Logging when necessary. We may be able to revert the exclusion in the future if CORE-3212 is fixed.