add test

hutool-core/src/test/java/cn/hutool/v7/core/io/file/Issue4121Test.java

@@ -0,0 +1,29 @@
+package cn.hutool.v7.core.io.file;
+
+import cn.hutool.v7.core.io.IORuntimeException;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class Issue4121Test {
+	@Test
+	void testListFileNames_NonExistentDirectory() {
+		assertThrows(IORuntimeException.class, () -> {
+			FileUtil.listFileNames("/non/existent/path");
+		});
+	}
+
+	@Test
+	void testListFileNames_RelativePath() {
+		// 测试相对路径（相对于classpath）
+		List<String> result = FileUtil.listFileNames("META-INF");
+
+		assertEquals(3, result.size());
+		assertTrue(result.contains("MANIFEST.MF"));
+		assertTrue(result.contains("LICENSE-notice.md"));
+		assertTrue(result.contains("LICENSE.md"));
+	}
+
+}