Can someone help me understand runtime exception of ClassNotFoundException for org.slf4j.event.LoggingEvent?

[ayaohsu]

Hi, I am using logstash-logback-encode 6.6, and my application throws the following runtime exception:

Caused by: java.lang.ClassNotFoundException: org.slf4j.event.LoggingEvent
	at java.net.URLClassLoader.findClass(URLClassLoader.java:387)

I know it has to do with maven dependencies, and the exception goes away after doing the following upgrade:
"org.slf4j" & "slf4j-api" & "1.7.7" -> "org.slf4j" & "slf4j-api" & "1.7.36"

But I would like to know why this is a runtime exception, instead of just failing at the compile time?

From my understanding, ClassNotFoundException is thrown when the classloader cannot find it from the classpath.
However, wouldn't this be something that a compiler checks, during some sort of linking process?

I am pretty new to Java and would appreciate any guidance / insights provided.

Thank you for your help!

[philsttr]

But I would like to know why this is a runtime exception, instead of just failing at the compile time?

When compiling your application's source code, the compiler only checks things directly referenced by your application's source code. Your application's source code most likely does not directly reference org.slf4j.event.LoggingEvent, so the compiler does not check for its existence.

org.slf4j.event.LoggingEvent was added in slf4j 1.7.15

Your application is using logstash-logback-encoder 6.6, which was compiled against logback-core 1.2.3, which was compiled against slf4j 1.7.25, which defines org.slf4j.event.LoggingEvent. Since logback-core directly uses org.slf4j.event.LoggingEvent, the compiler checked for its existence when compiling logback-core.

Later, when the compiler compiles your application's source code against logstash-logback-encoder 6.6, logback-core 1.2.3, and slf4j 1.7.7, the compiler did not report any problems, because your application's source code did not reference org.slf4j.event.LoggingEvent.

However, at runtime, when your application runs with logstash-logback-encoder 6.6, logback-core 1.2.3, and slf4j 1.7.7 on the classpath, the jvm will throw the ClassNotFoundException because it loaded the bytecode for a class from logback-core 1.2.3 which requires org.slf4j.event.LoggingEvent from slf4j.

[ayaohsu]

Thank you so much for the clear explanation!
You are right about my application code not referring to org.slf4j.event.LoggingEvent directly so the compiler didn't check for that.