diff --git a/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/FindMainClass.java b/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/FindMainClass.java
index 1b231ff66c3..039f8867f31 100644
--- a/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/FindMainClass.java
+++ b/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/FindMainClass.java
@@ -23,6 +23,7 @@ import java.util.jar.JarFile;
 import org.apache.tools.ant.BuildException;
 import org.apache.tools.ant.Project;
 import org.apache.tools.ant.Task;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.boot.loader.tools.MainClassFinder;
 import org.springframework.util.StringUtils;
@@ -37,11 +38,11 @@ public class FindMainClass extends Task {
 
 	private static final String SPRING_BOOT_APPLICATION_CLASS_NAME = "org.springframework.boot.autoconfigure.SpringBootApplication";
 
-	private String mainClass;
+	private @Nullable String mainClass;
 
-	private File classesRoot;
+	private @Nullable File classesRoot;
 
-	private String property;
+	private @Nullable String property;
 
 	public FindMainClass(Project project) {
 		setProject(project);
diff --git a/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/ShareAntlibLoader.java b/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/ShareAntlibLoader.java
index 224d2cd66fb..ad739764c75 100644
--- a/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/ShareAntlibLoader.java
+++ b/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/ShareAntlibLoader.java
@@ -19,6 +19,7 @@ package org.springframework.boot.ant;
 import org.apache.tools.ant.BuildException;
 import org.apache.tools.ant.Project;
 import org.apache.tools.ant.Task;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.util.StringUtils;
 
@@ -30,7 +31,7 @@ import org.springframework.util.StringUtils;
  */
 public class ShareAntlibLoader extends Task {
 
-	private String refid;
+	private @Nullable String refid;
 
 	public ShareAntlibLoader(Project project) {
 		setProject(project);
@@ -44,7 +45,7 @@ public class ShareAntlibLoader extends Task {
 		getProject().addReference(this.refid, getClass().getClassLoader());
 	}
 
-	public void setRefid(String refid) {
+	public void setRefid(@Nullable String refid) {
 		this.refid = refid;
 	}
 
diff --git a/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/package-info.java b/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/package-info.java
index 641d7df3f8b..2ac1dbb84e4 100644
--- a/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/package-info.java
+++ b/build-plugin/spring-boot-antlib/src/main/java/org/springframework/boot/ant/package-info.java
@@ -17,4 +17,7 @@
 /**
  * Support for building Spring Boot applications using Ant.
  */
+@NullMarked
 package org.springframework.boot.ant;
+
+import org.jspecify.annotations.NullMarked;