Log classpath once environment has been prepared

Previously, the classpath would be logged in response to the
ApplicationStartedEvent. At this point, logging could be disabled
while the logging system is being initialized, or because the
log levels configured in the environment have not yet been applied.

This commit moves the logging to happen in response to an
ApplicationEnvironmentPreparedEvent by which point the logging
system has been initialized and its levels have been configured.

Closes gh-5313

spring-boot/src/main/java/org/springframework/boot/logging/ClasspathLoggingApplicationListener.java

@@ -22,17 +22,18 @@ import java.util.Arrays;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
+import org.springframework.boot.context.event.ApplicationEnvironmentPreparedEvent;
 import org.springframework.boot.context.event.ApplicationFailedEvent;
-import org.springframework.boot.context.event.ApplicationStartedEvent;
 import org.springframework.context.ApplicationEvent;
 import org.springframework.context.event.GenericApplicationListener;
 import org.springframework.context.event.SmartApplicationListener;
 import org.springframework.core.ResolvableType;
 
 /**
- * A {@link SmartApplicationListener} that reacts to {@link ApplicationStartedEvent start
- * events} and to {@link ApplicationFailedEvent failed events} by logging the classpath of
- * the thread context class loader (TCCL) at {@code DEBUG} level.
+ * A {@link SmartApplicationListener} that reacts to
+ * {@link ApplicationEnvironmentPreparedEvent environment prepared events} and to
+ * {@link ApplicationFailedEvent failed events} by logging the classpath of the thread
+ * context class loader (TCCL) at {@code DEBUG} level.
  *
  * @author Andy Wilkinson
  */
@@ -46,7 +47,7 @@ public final class ClasspathLoggingApplicationListener
 	@Override
 	public void onApplicationEvent(ApplicationEvent event) {
 		if (this.logger.isDebugEnabled()) {
-			if (event instanceof ApplicationStartedEvent) {
+			if (event instanceof ApplicationEnvironmentPreparedEvent) {
 				this.logger
 						.debug("Application started with classpath: " + getClasspath());
 			}
@@ -68,7 +69,7 @@ public final class ClasspathLoggingApplicationListener
 		if (type == null) {
 			return false;
 		}
-		return ApplicationStartedEvent.class.isAssignableFrom(type)
+		return ApplicationEnvironmentPreparedEvent.class.isAssignableFrom(type)
 				|| ApplicationFailedEvent.class.isAssignableFrom(type);
 	}