Fix WebServerPortFileWriter.getPortFile() without extension

See gh-43115

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/context/WebServerPortFileWriter.java

@@ -110,7 +110,9 @@ public class WebServerPortFileWriter implements ApplicationListener<WebServerIni
 		}
 		String name = this.file.getName();
 		String extension = StringUtils.getFilenameExtension(this.file.getName());
-		name = name.substring(0, name.length() - extension.length() - 1);
+		if (extension != null) {
+			name = name.substring(0, name.length() - extension.length() - 1);
+		}
 		if (isUpperCase(name)) {
 			name = name + "-" + namespace.toUpperCase(Locale.ENGLISH);
 		}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/context/WebServerPortFileWriterTests.java

@@ -110,6 +110,15 @@ class WebServerPortFileWriterTests {
 		assertThat(collectFileNames(file.getParentFile())).contains(managementFile);
 	}
 
+	@Test
+	void getPortFileWhenPortFileNameDoesNotHaveExtension() {
+		File file = new File(this.tempDir, "portfile");
+		WebServerPortFileWriter listener = new WebServerPortFileWriter(file);
+		WebServerApplicationContext applicationContext = mock(WebServerApplicationContext.class);
+		given(applicationContext.getServerNamespace()).willReturn("management");
+		assertThat(listener.getPortFile(applicationContext).getName()).isEqualTo("portfile-management");
+	}
+
 	private WebServerInitializedEvent mockEvent(String namespace, int port) {
 		WebServer webServer = mock(WebServer.class);
 		given(webServer.getPort()).willReturn(port);