Merge pull request #47987 from nosan

* pr/47987:
  Polish contribution
  Allow Devtools Restarter to work with a parameterless main method

Closes gh-47987

Author: snicoll

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

@@ -60,7 +60,7 @@ private boolean isLoaderClass(String className) {
 	private Method getMainMethod(StackTraceElement element) {
 		try {
 			Class<?> elementClass = Class.forName(element.getClassName());
-			Method method = elementClass.getDeclaredMethod("main", String[].class);
+			Method method = getMainMethod(elementClass);
 			if (Modifier.isStatic(method.getModifiers())) {
 				return method;
 			}
@@ -71,6 +71,15 @@ private Method getMainMethod(StackTraceElement element) {
 		return null;
 	}
 
+	private static Method getMainMethod(Class<?> clazz) throws Exception {
+		try {
+			return clazz.getDeclaredMethod("main", String[].class);
+		}
+		catch (NoSuchMethodException ex) {
+			return clazz.getDeclaredMethod("main");
+		}
+	}
+
 	/**
 	 * Returns the actual main method.
 	 * @return the main method

spring-boot-project/spring-boot-devtools/src/test/java/org/springframework/boot/devtools/restart/MainMethodTests.java

@@ -18,7 +18,6 @@
 
 import java.lang.reflect.Method;
 
-import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.boot.loader.launch.FakeJarLauncher;
@@ -37,13 +36,6 @@ class MainMethodTests {
 
 	private static final ThreadLocal<MainMethod> mainMethod = new ThreadLocal<>();
 
-	private Method actualMain;
-
-	@BeforeEach
-	void setup() throws Exception {
-		this.actualMain = Valid.class.getMethod("main", String[].class);
-	}
-
 	@Test
 	void threadMustNotBeNull() {
 		assertThatIllegalArgumentException().isThrownBy(() -> new MainMethod(null))
@@ -52,9 +44,10 @@ void threadMustNotBeNull() {
 
 	@Test
 	void validMainMethod() throws Exception {
+		Method actualMain = Valid.class.getMethod("main", String[].class);
 		MainMethod method = new TestThread(Valid::main).test();
-		assertThat(method.getMethod()).isEqualTo(this.actualMain);
-		assertThat(method.getDeclaringClassName()).isEqualTo(this.actualMain.getDeclaringClass().getName());
+		assertThat(method.getMethod()).isEqualTo(actualMain);
+		assertThat(method.getDeclaringClassName()).isEqualTo(actualMain.getDeclaringClass().getName());
 	}
 
 	@Test // gh-35214
@@ -75,14 +68,41 @@ void viaJarLauncher() throws Exception {
 	}
 
 	@Test
-	void missingArgsMainMethod() {
-		assertThatIllegalStateException().isThrownBy(() -> new TestThread(MissingArgs::main).test())
-			.withMessageContaining("Unable to find main method");
+	void detectPublicMainMethod() throws Exception {
+		Method actualMain = PublicMainMethod.class.getMethod("main", String[].class);
+		MainMethod method = new TestThread(PublicMainMethod::main).test();
+		assertThat(method.getMethod()).isEqualTo(actualMain);
+		assertThat(method.getDeclaringClassName()).isEqualTo(actualMain.getDeclaringClass().getName());
+	}
+
+	@Test
+	void detectPublicParameterlessMainMethod() throws Exception {
+		Method actualMain = PublicParameterlessMainMethod.class.getMethod("main");
+		MainMethod method = new TestThread(PublicParameterlessMainMethod::main).test();
+		assertThat(method.getMethod()).isEqualTo(actualMain);
+		assertThat(method.getDeclaringClassName()).isEqualTo(actualMain.getDeclaringClass().getName());
+	}
+
+	@Test
+	void detectPackagePrivateMainMethod() throws Exception {
+		Method actualMain = PackagePrivateMainMethod.class.getDeclaredMethod("main", String[].class);
+		MainMethod method = new TestThread(PackagePrivateMainMethod::main).test();
+		assertThat(method.getMethod()).isEqualTo(actualMain);
+		assertThat(method.getDeclaringClassName()).isEqualTo(actualMain.getDeclaringClass().getName());
+	}
+
+	@Test
+	void detectPackagePrivateParameterlessMainMethod() throws Exception {
+		Method actualMain = PackagePrivateParameterlessMainMethod.class.getDeclaredMethod("main");
+		MainMethod method = new TestThread(PackagePrivateParameterlessMainMethod::main).test();
+		assertThat(method.getMethod()).isEqualTo(actualMain);
+		assertThat(method.getDeclaringClassName()).isEqualTo(actualMain.getDeclaringClass().getName());
 	}
 
 	@Test
 	void nonStatic() {
-		assertThatIllegalStateException().isThrownBy(() -> new TestThread(() -> new NonStaticMain().main()).test())
+		assertThatIllegalStateException()
+			.isThrownBy(() -> new TestThread(() -> new NonStaticMainMethod().main()).test())
 			.withMessageContaining("Unable to find main method");
 	}
 
@@ -141,15 +161,39 @@ public static void main(String... args) {
 
 	}
 
-	public static class MissingArgs {
+	public static class PublicMainMethod {
+
+		public static void main(String... args) {
+			mainMethod.set(new MainMethod());
+		}
+
+	}
+
+	public static class PublicParameterlessMainMethod {
 
 		public static void main() {
 			mainMethod.set(new MainMethod());
 		}
 
 	}
 
-	public static class NonStaticMain {
+	public static class PackagePrivateMainMethod {
+
+		static void main(String... args) {
+			mainMethod.set(new MainMethod());
+		}
+
+	}
+
+	public static class PackagePrivateParameterlessMainMethod {
+
+		static void main() {
+			mainMethod.set(new MainMethod());
+		}
+
+	}
+
+	public static class NonStaticMainMethod {
 
 		void main(String... args) {
 			mainMethod.set(new MainMethod());