Allow package-private main classes to be used with Devtools

Fixes gh-35858

spring-boot-project/spring-boot-devtools/src/intTest/java/com/example/DevToolsTestApplication.java

@@ -21,7 +21,7 @@ import org.springframework.boot.builder.SpringApplicationBuilder;
 import org.springframework.boot.web.context.WebServerPortFileWriter;
 
 @SpringBootApplication
-public class DevToolsTestApplication {
+class DevToolsTestApplication {
 
 	public static void main(String[] args) {
 		new SpringApplicationBuilder(DevToolsTestApplication.class).listeners(new WebServerPortFileWriter(args[0]))

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartLauncher.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -46,6 +46,7 @@ class RestartLauncher extends Thread {
 		try {
 			Class<?> mainClass = Class.forName(this.mainClassName, false, getContextClassLoader());
 			Method mainMethod = mainClass.getDeclaredMethod("main", String[].class);
+			mainMethod.setAccessible(true);
 			mainMethod.invoke(null, new Object[] { this.args });
 		}
 		catch (Throwable ex) {

src/checkstyle/checkstyle-suppressions.xml

@@ -59,6 +59,7 @@
 	<suppress files="DeprecatedElasticsearchRestClientProperties\.java" checks="SpringMethodVisibility" />
 	<suppress files="DeprecatedElasticsearchRestClientProperties\.java" checks="SpringMethodVisibility" />
 	<suppress files="DeprecatedReactiveElasticsearchRestClientProperties\.java" checks="SpringMethodVisibility" />
+	<suppress files="DevToolsTestApplication\.java" checks="SpringMethodVisibility" />
 	<suppress files="DevToolsR2dbcAutoConfigurationTests" checks="HideUtilityClassConstructor" />
 	<suppress files="AbstractLaunchScriptIntegrationTests" checks="IllegalImport" />
 	<suppress files="SpringBootVersion" checks="JavadocPackage" />