Handle default package with AOT processing

Adding generated code in the default package is not supported as we
intend to import it, most probably from another package, and that is
not supported. While this situation is hard to replicate with Java,
Kotlin is unfortunately more lenient and users can end up in that
situation if they forget to add a package statement.

This commit checks for the presence of a valid package, and throws
a dedicated exception if necessary.

Closes gh-31628

spring-core/src/main/java/org/springframework/aot/generate/GeneratedFiles.java

@@ -20,6 +20,7 @@
 import org.springframework.javapoet.JavaFile;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.StringUtils;
 import org.springframework.util.function.ThrowingConsumer;
 
 /**
@@ -43,6 +44,7 @@ public interface GeneratedFiles {
 	 * @param javaFile the java file to add
 	 */
 	default void addSourceFile(JavaFile javaFile) {
+		validatePackage(javaFile.packageName, javaFile.typeSpec.name);
 		String className = javaFile.packageName + "." + javaFile.typeSpec.name;
 		addSourceFile(className, javaFile::writeTo);
 	}
@@ -161,11 +163,20 @@ default void addFile(Kind kind, String path, ThrowingConsumer<Appendable> conten
 
 	private static String getClassNamePath(String className) {
 		Assert.hasLength(className, "'className' must not be empty");
+		validatePackage(ClassUtils.getPackageName(className), className);
 		Assert.isTrue(isJavaIdentifier(className),
 				"'className' must be a valid identifier, got '" + className + "'");
 		return ClassUtils.convertClassNameToResourcePath(className) + ".java";
 	}
 
+	private static void validatePackage(String packageName, String className) {
+		if (!StringUtils.hasLength(packageName)) {
+			throw new IllegalArgumentException("Could not add '" + className + "', "
+					+ "processing classes in the default package is not supported. "
+					+ "Did you forget to add a package statement?");
+		}
+	}
+
 	private static boolean isJavaIdentifier(String className) {
 		char[] chars = className.toCharArray();
 		for (int i = 0; i < chars.length; i++) {

spring-core/src/test/java/org/springframework/aot/generate/GeneratedFilesTests.java

@@ -60,6 +60,15 @@ void addSourceFileWithJavaFileAddsFile() throws Exception {
 				.contains("Hello, World!");
 	}
 
+	@Test
+	void addSourceFileWithJavaFileInTheDefaultPackageThrowsException() {
+		TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld").build();
+		JavaFile javaFile = JavaFile.builder("", helloWorld).build();
+		assertThatIllegalArgumentException().isThrownBy(() -> this.generatedFiles.addSourceFile(javaFile))
+				.withMessage("Could not add 'HelloWorld', processing classes in the "
+						+ "default package is not supported. Did you forget to add a package statement?");
+	}
+
 	@Test
 	void addSourceFileWithCharSequenceAddsFile() throws Exception {
 		this.generatedFiles.addSourceFile("com.example.HelloWorld", "{}");
@@ -73,6 +82,14 @@ void addSourceFileWithCharSequenceWhenClassNameIsEmptyThrowsException() {
 				.withMessage("'className' must not be empty");
 	}
 
+	@Test
+	void addSourceFileWithCharSequenceWhenClassNameIsInTheDefaultPackageThrowsException() {
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> this.generatedFiles.addSourceFile("HelloWorld", "{}"))
+				.withMessage("Could not add 'HelloWorld', processing classes in the "
+						+ "default package is not supported. Did you forget to add a package statement?");
+	}
+
 	@Test
 	void addSourceFileWithCharSequenceWhenClassNameIsInvalidThrowsException() {
 		assertThatIllegalArgumentException()