Merge pull request #4831 from mrumpf/gh-4822

* pr/4831:
  Fixed the check whether Log4j2 is available on the classpath

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/log4j2/Log4J2RestartListener.java

@@ -38,7 +38,7 @@ public class Log4J2RestartListener implements RestartListener {
 
 	@Override
 	public void beforeRestart() {
-		if (ClassUtils.isPresent("org.apache.logging.log4j.LogManager",
+		if (ClassUtils.isPresent("org.apache.logging.log4j.core.impl.Log4jContextFactory",
 				getClass().getClassLoader())) {
 			prepareLog4J2ForRestart();
 		}

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

@@ -43,7 +43,7 @@ public abstract class LoggingSystem {
 		Map<String, String> systems = new LinkedHashMap<String, String>();
 		systems.put("ch.qos.logback.core.Appender",
 				"org.springframework.boot.logging.logback.LogbackLoggingSystem");
-		systems.put("org.apache.logging.log4j.LogManager",
+		systems.put("org.apache.logging.log4j.core.impl.Log4jContextFactory",
 				"org.springframework.boot.logging.log4j2.Log4J2LoggingSystem");
 		systems.put("org.apache.log4j.PropertyConfigurator",
 				"org.springframework.boot.logging.log4j.Log4JLoggingSystem");