diff --git a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/InMemoryMessageRepository.java b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/InMemoryMessageRepository.java
index 1e314b2b41b..2a6964978eb 100644
--- a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/InMemoryMessageRepository.java
+++ b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/InMemoryMessageRepository.java
@@ -20,6 +20,8 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.atomic.AtomicLong;
 
+import org.jspecify.annotations.Nullable;
+
 public class InMemoryMessageRepository implements MessageRepository {
 
 	private static final AtomicLong counter = new AtomicLong();
@@ -43,7 +45,7 @@ public class InMemoryMessageRepository implements MessageRepository {
 	}
 
 	@Override
-	public Message findMessage(Long id) {
+	public @Nullable Message findMessage(Long id) {
 		return this.messages.get(id);
 	}
 
diff --git a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/Message.java b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/Message.java
index 90fb6ca5f8a..6eaa0f71b9d 100644
--- a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/Message.java
+++ b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/Message.java
@@ -18,25 +18,26 @@ package smoketest.web.thymeleaf;
 
 import java.util.Calendar;
 
+import jakarta.annotation.Nullable;
 import jakarta.validation.constraints.NotEmpty;
 
 public class Message {
 
-	private Long id;
+	private @Nullable Long id;
 
 	@NotEmpty(message = "Text is required.")
-	private String text;
+	private @Nullable String text;
 
 	@NotEmpty(message = "Summary is required.")
-	private String summary;
+	private @Nullable String summary;
 
 	private Calendar created = Calendar.getInstance();
 
-	public Long getId() {
+	public @Nullable Long getId() {
 		return this.id;
 	}
 
-	public void setId(Long id) {
+	public void setId(@Nullable Long id) {
 		this.id = id;
 	}
 
@@ -48,19 +49,19 @@ public class Message {
 		this.created = created;
 	}
 
-	public String getText() {
+	public @Nullable String getText() {
 		return this.text;
 	}
 
-	public void setText(String text) {
+	public void setText(@Nullable String text) {
 		this.text = text;
 	}
 
-	public String getSummary() {
+	public @Nullable String getSummary() {
 		return this.summary;
 	}
 
-	public void setSummary(String summary) {
+	public void setSummary(@Nullable String summary) {
 		this.summary = summary;
 	}
 
diff --git a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/MessageRepository.java b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/MessageRepository.java
index dfb18a98688..2e29adef139 100644
--- a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/MessageRepository.java
+++ b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/MessageRepository.java
@@ -16,13 +16,15 @@
 
 package smoketest.web.thymeleaf;
 
+import org.jspecify.annotations.Nullable;
+
 public interface MessageRepository {
 
 	Iterable<Message> findAll();
 
 	Message save(Message message);
 
-	Message findMessage(Long id);
+	@Nullable Message findMessage(Long id);
 
 	void deleteMessage(Long id);
 
diff --git a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/SampleWebUiApplication.java b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/SampleWebUiApplication.java
index d6cc55ec70e..d26183ca997 100644
--- a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/SampleWebUiApplication.java
+++ b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/SampleWebUiApplication.java
@@ -16,6 +16,8 @@
 
 package smoketest.web.thymeleaf;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
 import org.springframework.context.annotation.Bean;
@@ -33,7 +35,7 @@ public class SampleWebUiApplication {
 	public Converter<String, Message> messageConverter() {
 		return new Converter<>() {
 			@Override
-			public Message convert(String id) {
+			public @Nullable Message convert(String id) {
 				return messageRepository().findMessage(Long.valueOf(id));
 			}
 		};
diff --git a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/mvc/MessageController.java b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/mvc/MessageController.java
index 215abe4ac73..0a0d0949bde 100644
--- a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/mvc/MessageController.java
+++ b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/mvc/MessageController.java
@@ -21,6 +21,7 @@ import smoketest.web.thymeleaf.Message;
 import smoketest.web.thymeleaf.MessageRepository;
 
 import org.springframework.stereotype.Controller;
+import org.springframework.util.Assert;
 import org.springframework.validation.BindingResult;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.ModelAttribute;
@@ -63,7 +64,9 @@ public class MessageController {
 		}
 		message = this.messageRepository.save(message);
 		redirect.addFlashAttribute("globalMessage", "view.success");
-		return new ModelAndView("redirect:/{message.id}", "message.id", message.getId());
+		Long id = message.getId();
+		Assert.state(id != null, "'id' must not be null");
+		return new ModelAndView("redirect:/{message.id}", "message.id", id);
 	}
 
 	@RequestMapping("foo")
diff --git a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/mvc/package-info.java b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/mvc/package-info.java
new file mode 100644
index 00000000000..4875f43d377
--- /dev/null
+++ b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/mvc/package-info.java
@@ -0,0 +1,20 @@
+/*
+ * Copyright 2012-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+@NullMarked
+package smoketest.web.thymeleaf.mvc;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/package-info.java b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/package-info.java
new file mode 100644
index 00000000000..f4a3ac0d497
--- /dev/null
+++ b/smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/package-info.java
@@ -0,0 +1,20 @@
+/*
+ * Copyright 2012-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+@NullMarked
+package smoketest.web.thymeleaf;
+
+import org.jspecify.annotations.NullMarked;