diff --git a/plusone-validator/pom.xml b/plusone-validator/pom.xml
index 0f999a6..41a865f 100644
--- a/plusone-validator/pom.xml
+++ b/plusone-validator/pom.xml
@@ -44,6 +44,11 @@
       <groupId>xyz.zhouxy.plusone</groupId>
       <artifactId>plusone-commons</artifactId>
     </dependency>
+    <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+      <version>1.0.0</version>
+    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter</artifactId>
diff --git a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/BasePropertyValidator.java b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/BasePropertyValidator.java
index 15ad45c..e6860e8 100644
--- a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/BasePropertyValidator.java
+++ b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/BasePropertyValidator.java
@@ -24,6 +24,8 @@ import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.function.Supplier;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * 属性校验器。包含针对属性的校验规则。
  *
@@ -135,9 +137,9 @@ public abstract class BasePropertyValidator<
     /**
      * 校验属性
      *
-     * @param obj 属性所在的对象
+     * @param obj 属性所在的对象，允许为 {@code null}
      */
-    public final void validate(T obj) {
+    public final void validate(@Nullable T obj) {
         for (Consumer<? super TProperty> consumer : consumers) {
             consumer.accept(getter.apply(obj));
         }
diff --git a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/BaseValidator.java b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/BaseValidator.java
index 13887ee..9917a5c 100644
--- a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/BaseValidator.java
+++ b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/BaseValidator.java
@@ -28,6 +28,8 @@ import java.util.function.Supplier;
 
 import xyz.zhouxy.plusone.validator.function.*;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * 校验器基类
  * <p>
@@ -311,7 +313,7 @@ public abstract class BaseValidator<T> implements IValidator<T> {
 
     /** {@inheritDoc} */
     @Override
-    public void validate(T obj) {
+    public void validate(@Nullable T obj) {
         this.rules.forEach(rule -> rule.accept(obj));
     }
 }
diff --git a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/IValidator.java b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/IValidator.java
index 5de6d64..7f22c3a 100644
--- a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/IValidator.java
+++ b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/IValidator.java
@@ -15,6 +15,8 @@
  */
 package xyz.zhouxy.plusone.validator;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * 校验器
  *
@@ -30,7 +32,7 @@ public interface IValidator<T> {
     /**
      * 校验指定对象是否符合预定义规则
      *
-     * @param obj 待校验的对象实例
+     * @param obj 待校验的对象实例，允许为 {@code null}
      */
-    void validate(T obj);
+    void validate(@Nullable T obj);
 }
diff --git a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/MapValidator.java b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/MapValidator.java
index 50a22d6..ebd5d8c 100644
--- a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/MapValidator.java
+++ b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/MapValidator.java
@@ -23,6 +23,8 @@ import java.util.Set;
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * 对 Map 进行校验的校验器
  *
@@ -59,10 +61,10 @@ public abstract class MapValidator<K, V> extends BaseValidator<Map<K, V>> {
     /**
      * 校验并拷贝，仅保留指定 key 的属性。
      *
-     * @param obj 待校验的 map
+     * @param obj 待校验的 map，允许为 {@code null}
      * @return 拷贝后的 map
      */
-    public final Map<K, V> validateAndCopy(Map<K, V> obj) {
+    public final Map<K, V> validateAndCopy(@Nullable Map<K, V> obj) {
         return validateAndCopyInternal(obj, this.keys);
     }
 
@@ -89,7 +91,7 @@ public abstract class MapValidator<K, V> extends BaseValidator<Map<K, V>> {
         return validateAndCopyInternal(obj, Arrays.asList(keys));
     }
 
-    private final Map<K, V> validateAndCopyInternal(Map<K, V> obj, Collection<K> keys) {
+    private final Map<K, V> validateAndCopyInternal(@Nullable Map<K, V> obj, Collection<K> keys) {
         validate(obj);
         return obj.entrySet().stream()
                 .filter(kv -> keys.contains(kv.getKey()))
diff --git a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/StringPropertyValidator.java b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/StringPropertyValidator.java
index e974d61..7cede95 100644
--- a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/StringPropertyValidator.java
+++ b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/StringPropertyValidator.java
@@ -547,7 +547,7 @@ public class StringPropertyValidator<T>
 
         private static Predicate<String> length(int length) {
             AssertTools.checkArgument(length >= 0,
-                "The expected length must be non-negative.");
+                    "The expected length must be non-negative.");
             return input -> input == null || input.length() == length;
         }
 
diff --git a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/ValidationException.java b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/ValidationException.java
index 29bb1ae..d6ba219 100644
--- a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/ValidationException.java
+++ b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/ValidationException.java
@@ -15,6 +15,8 @@
  */
 package xyz.zhouxy.plusone.validator;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * 校验失败异常
  *
@@ -30,11 +32,11 @@ public class ValidationException extends RuntimeException {
         super(message);
     }
 
-    private ValidationException(Throwable cause) {
+    private ValidationException(@Nullable Throwable cause) {
         super(cause);
     }
 
-    private ValidationException(String message, Throwable cause) {
+    private ValidationException(String message, @Nullable Throwable cause) {
         super(message, cause);
     }
 
@@ -71,10 +73,10 @@ public class ValidationException extends RuntimeException {
     /**
      * 创建 {@code ValidationException} 实例
      *
-     * @param cause 导致校验失败的根本异常
+     * @param cause 导致校验失败的根本异常，允许为 {@code null}
      * @return {@code ValidationException} 实例
      */
-    public static ValidationException withCause(Throwable cause) {
+    public static ValidationException withCause(@Nullable Throwable cause) {
         return new ValidationException(cause);
     }
 
@@ -82,10 +84,10 @@ public class ValidationException extends RuntimeException {
      * 创建 {@code ValidationException} 实例
      *
      * @param message 异常信息
-     * @param cause 导致校验失败的根本异常
+     * @param cause 导致校验失败的根本异常，允许为 {@code null}
      * @return {@code ValidationException} 实例
      */
-    public static ValidationException withMessageAndCause(String message, Throwable cause) {
+    public static ValidationException withMessageAndCause(String message, @Nullable Throwable cause) {
         return new ValidationException(message, cause);
     }
 }
diff --git a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/function/package-info.java b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/function/package-info.java
new file mode 100644
index 0000000..dff0381
--- /dev/null
+++ b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/function/package-info.java
@@ -0,0 +1,37 @@
+/*
+ * Copyright 2025-present ZhouXY
+ *
+ * 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.
+ */
+
+/**
+ * 函数式接口包，提供带序列化能力的特殊 {@code Function} 子类型。
+ *
+ * <p>
+ * 本包中的接口均继承自 {@link java.util.function.Function}，同时实现
+ * {@link java.io.Serializable}，使 lambda 表达式或方法引用可以被序列化。
+ * 主要用作 {@link xyz.zhouxy.plusone.validator.BaseValidator} 中
+ * {@code ruleFor} 方法族的参数类型，以支持特定类型的属性取值。
+ *
+ * <ul>
+ *   <li>{@link xyz.zhouxy.plusone.validator.function.ToStringFunction ToStringFunction} — Function&lt;T, String&gt;</li>
+ *   <li>{@link xyz.zhouxy.plusone.validator.function.ToIntegerFunction ToIntegerFunction} — Function&lt;T, Integer&gt;</li>
+ *   <li>{@link xyz.zhouxy.plusone.validator.function.ToLongObjectFunction ToLongObjectFunction} — Function&lt;T, Long&gt;</li>
+ *   <li>{@link xyz.zhouxy.plusone.validator.function.ToDoubleObjectFunction ToDoubleObjectFunction} — Function&lt;T, Double&gt;</li>
+ *   <li>{@link xyz.zhouxy.plusone.validator.function.ToBoolObjectFunction ToBoolObjectFunction} — Function&lt;T, Boolean&gt;</li>
+ * </ul>
+ */
+@NullMarked
+package xyz.zhouxy.plusone.validator.function;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/package-info.java b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/package-info.java
new file mode 100644
index 0000000..92c0404
--- /dev/null
+++ b/plusone-validator/src/main/java/xyz/zhouxy/plusone/validator/package-info.java
@@ -0,0 +1,52 @@
+/*
+ * Copyright 2025-present ZhouXY
+ *
+ * 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.
+ */
+
+/**
+ * Plusone Validator 核心包，提供流式校验 API。
+ *
+ * <p>
+ * 使用 lambda 表达式（包括方法引用）和流式 API 构建校验规则，对对象进行校验。
+ * 支持校验普通对象（POJO）和 {@code Map} 对象。
+ *
+ * <h3>快速开始</h3>
+ * <pre>{@code
+ * public class PersonValidator extends BaseValidator<Person> {
+ *     public PersonValidator() {
+ *         ruleFor(Person::getName)
+ *             .notNull()
+ *             .notBlank();
+ *         ruleForInt(Person::getAge)
+ *             .gt(0);
+ *     }
+ * }
+ * }</pre>
+ *
+ * <h3>核心概念</h3>
+ * <ul>
+ *   <li>{@link xyz.zhouxy.plusone.validator.IValidator IValidator} — 校验器接口</li>
+ *   <li>{@link xyz.zhouxy.plusone.validator.BaseValidator BaseValidator} — 校验器基类，通过继承并添加规则构建校验器</li>
+ *   <li>{@link xyz.zhouxy.plusone.validator.BasePropertyValidator BasePropertyValidator} — 属性校验器基类</li>
+ *   <li>{@link xyz.zhouxy.plusone.validator.MapValidator MapValidator} — Map 校验器</li>
+ *   <li>{@link xyz.zhouxy.plusone.validator.ValidationException ValidationException} — 校验失败异常</li>
+ * </ul>
+ *
+ * @see xyz.zhouxy.plusone.validator.BaseValidator
+ * @see xyz.zhouxy.plusone.validator.MapValidator
+ */
+@NullMarked
+package xyz.zhouxy.plusone.validator;
+
+import org.jspecify.annotations.NullMarked;