Jetty implements its own log factory class: org.eclipse.jetty.util.log.Log. When this class is loaded by the classloader, it will look for an available log interface in the classpath (the default is slf4j, you can specify the log interface by configuring the startup parameter -Dorg.eclipse.jetty.util.log.class=xxx), if you can’t find it Then use the default standard output log (org.eclipse.jetty.util.log.StdErrLog).
Jetty6 itself does not have slf4j. When it starts, it will try to load the slf4j class. If it succeeds, it will use org.mortbay.log.Slf4jLog to record the log. Send log information to the underlying log system. If it fails, use org.mortbay.log.StdErrLog to record the log, that is, output the log information to the stderr file of the operating system.