Add mainApplicationClass accessor to SpringApplication

Closes gh-4801

Author: mbenson

spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -914,6 +914,14 @@ private boolean isMainThread(Thread currentThread) {
 				&& "main".equals(currentThread.getThreadGroup().getName());
 	}
 
+	/**
+	 * Returns the main application class that has been deduced or explicitly configured.
+	 * @return the main application class or {@code null}
+	 */
+	public Class<?> getMainApplicationClass() {
+		return this.mainApplicationClass;
+	}
+
 	/**
 	 * Set a specific main application class that will be used as a log source and to
 	 * obtain version information. By default the main application class will be deduced.