Add support for finding package-private and parameterless main

Fixes gh-47309

Author: wilkinsona

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/MainClassFinder.java

@@ -60,6 +60,8 @@ public abstract class MainClassFinder {
 
 	private static final Type MAIN_METHOD_TYPE = Type.getMethodType(Type.VOID_TYPE, STRING_ARRAY_TYPE);
 
+	private static final Type PARAMETERLESS_MAIN_METHOD_TYPE = Type.getMethodType(Type.VOID_TYPE);
+
 	private static final String MAIN_METHOD_NAME = "main";
 
 	private static final FileFilter CLASS_FILE_FILTER = MainClassFinder::isClassFile;
@@ -286,10 +288,20 @@ private static class ClassDescriptor extends ClassVisitor {
 
 		private boolean mainMethodFound;
 
+		private boolean java25OrLater = false;
+
 		ClassDescriptor() {
 			super(SpringAsmInfo.ASM_VERSION);
 		}
 
+		@Override
+		public void visit(int version, int access, String name, String signature, String superName,
+				String[] interfaces) {
+			if (version >= 69) {
+				this.java25OrLater = true;
+			}
+		}
+
 		@Override
 		public AnnotationVisitor visitAnnotation(String desc, boolean visible) {
 			this.annotationNames.add(Type.getType(desc).getClassName());
@@ -298,13 +310,24 @@ public AnnotationVisitor visitAnnotation(String desc, boolean visible) {
 
 		@Override
 		public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
-			if (isAccess(access, Opcodes.ACC_PUBLIC, Opcodes.ACC_STATIC) && MAIN_METHOD_NAME.equals(name)
-					&& MAIN_METHOD_TYPE.getDescriptor().equals(desc)) {
-				this.mainMethodFound = true;
+			if (hasRequiredAccess(access) && MAIN_METHOD_NAME.equals(name)) {
+				if (MAIN_METHOD_TYPE.getDescriptor().equals(desc)
+						|| (this.java25OrLater && PARAMETERLESS_MAIN_METHOD_TYPE.getDescriptor().equals(desc))) {
+					this.mainMethodFound = true;
+				}
 			}
 			return null;
 		}
 
+		private boolean hasRequiredAccess(int access) {
+			if (this.java25OrLater) {
+				return !isAccess(access, Opcodes.ACC_PRIVATE) && isAccess(access, Opcodes.ACC_STATIC);
+			}
+			else {
+				return isAccess(access, Opcodes.ACC_PUBLIC, Opcodes.ACC_STATIC);
+			}
+		}
+
 		private boolean isAccess(int access, int... requiredOpsCodes) {
 			for (int requiredOpsCode : requiredOpsCodes) {
 				if ((access & requiredOpsCode) == 0) {

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/java/org/springframework/boot/loader/tools/MainClassFinderTests.java

@@ -16,11 +16,21 @@
 
 package org.springframework.boot.loader.tools;
 
+import java.io.ByteArrayInputStream;
 import java.io.File;
+import java.lang.reflect.Modifier;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.jar.JarFile;
 
+import net.bytebuddy.ByteBuddy;
+import net.bytebuddy.ClassFileVersion;
+import net.bytebuddy.description.method.MethodDescription;
+import net.bytebuddy.dynamic.scaffold.InstrumentedType;
+import net.bytebuddy.implementation.Implementation;
+import net.bytebuddy.implementation.bytecode.ByteCodeAppender;
+import net.bytebuddy.jar.asm.MethodVisitor;
+import net.bytebuddy.jar.asm.Opcodes;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
@@ -179,6 +189,88 @@ void doWithJarMainMethods() throws Exception {
 		}
 	}
 
+	@Test
+	void packagePrivateMainMethod() throws Exception {
+		this.testJarFile.addFile("a/b/c/D.class", packagePrivateMainMethod(ClassFileVersion.JAVA_V25));
+		ClassNameCollector callback = new ClassNameCollector();
+		try (JarFile jarFile = this.testJarFile.getJarFile()) {
+			MainClassFinder.doWithMainClasses(jarFile, null, callback);
+			assertThat(callback.getClassNames()).hasToString("[a.b.c.D]");
+		}
+	}
+
+	@Test
+	void packagePrivateMainMethodBeforeJava25() throws Exception {
+		this.testJarFile.addFile("a/b/c/D.class", packagePrivateMainMethod(ClassFileVersion.JAVA_V24));
+		ClassNameCollector callback = new ClassNameCollector();
+		try (JarFile jarFile = this.testJarFile.getJarFile()) {
+			MainClassFinder.doWithMainClasses(jarFile, null, callback);
+			assertThat(callback.getClassNames()).isEmpty();
+		}
+	}
+
+	@Test
+	void parameterlessMainMethod() throws Exception {
+		this.testJarFile.addFile("a/b/c/D.class", parameterlessMainMethod(ClassFileVersion.JAVA_V25));
+		ClassNameCollector callback = new ClassNameCollector();
+		try (JarFile jarFile = this.testJarFile.getJarFile()) {
+			MainClassFinder.doWithMainClasses(jarFile, null, callback);
+			assertThat(callback.getClassNames()).hasToString("[a.b.c.D]");
+		}
+	}
+
+	@Test
+	void parameterlessMainMethodBeforeJava25() throws Exception {
+		this.testJarFile.addFile("a/b/c/D.class", parameterlessMainMethod(ClassFileVersion.JAVA_V24));
+		ClassNameCollector callback = new ClassNameCollector();
+		try (JarFile jarFile = this.testJarFile.getJarFile()) {
+			MainClassFinder.doWithMainClasses(jarFile, null, callback);
+			assertThat(callback.getClassNames()).isEmpty();
+		}
+	}
+
+	private ByteArrayInputStream packagePrivateMainMethod(ClassFileVersion classFileVersion) {
+		byte[] bytecode = new ByteBuddy(classFileVersion).subclass(Object.class)
+			.defineMethod("main", void.class, Modifier.STATIC)
+			.withParameter(String[].class)
+			.intercept(new EmptyBodyImplementation())
+			.make()
+			.getBytes();
+		return new ByteArrayInputStream(bytecode);
+	}
+
+	private ByteArrayInputStream parameterlessMainMethod(ClassFileVersion classFileVersion) {
+		byte[] bytecode = new ByteBuddy(classFileVersion).subclass(Object.class)
+			.defineMethod("main", void.class, Modifier.STATIC | Modifier.PUBLIC)
+			.intercept(new EmptyBodyImplementation())
+			.make()
+			.getBytes();
+		return new ByteArrayInputStream(bytecode);
+	}
+
+	static class EmptyBodyImplementation implements Implementation {
+
+		@Override
+		public InstrumentedType prepare(InstrumentedType instrumentedType) {
+			return instrumentedType;
+		}
+
+		@Override
+		public ByteCodeAppender appender(Target implementationTarget) {
+			return new ByteCodeAppender() {
+
+				@Override
+				public Size apply(MethodVisitor methodVisitor, Context implementationContext,
+						MethodDescription instrumentedMethod) {
+					methodVisitor.visitInsn(Opcodes.RETURN);
+					return Size.ZERO;
+				}
+
+			};
+		}
+
+	}
+
 	static class ClassNameCollector implements MainClassCallback<Object> {
 
 		private final List<String> classNames = new ArrayList<>();