LinkedMultiValueMap and UnmodifiableMultiValueMap should accept null elements like MultiValueMap does

spring-core/src/main/java/org/springframework/util/LinkedMultiValueMap.java

@@ -16,6 +16,8 @@
 
 package org.springframework.util;
 
+import org.jspecify.annotations.Nullable;
+
 import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.LinkedHashMap;
@@ -31,11 +33,12 @@
  *
  * @author Arjen Poutsma
  * @author Juergen Hoeller
+ * @author Yanming Zhou
  * @since 3.0
  * @param <K> the key type
  * @param <V> the value element type
  */
-public class LinkedMultiValueMap<K, V> extends MultiValueMapAdapter<K, V> implements Serializable, Cloneable {
+public class LinkedMultiValueMap<K, V extends @Nullable Object> extends MultiValueMapAdapter<K, V> implements Serializable, Cloneable {
 
 	private static final long serialVersionUID = 3801124242820219131L;
 

spring-core/src/main/java/org/springframework/util/MultiValueMapAdapter.java

@@ -31,14 +31,15 @@
  *
  * @author Arjen Poutsma
  * @author Juergen Hoeller
+ * @author Yanming Zhou
  * @since 5.3
  * @param <K> the key type
  * @param <V> the value element type
  * @see CollectionUtils#toMultiValueMap
  * @see LinkedMultiValueMap
  */
 @SuppressWarnings("serial")
-public class MultiValueMapAdapter<K, V> implements MultiValueMap<K, V>, Serializable {
+public class MultiValueMapAdapter<K, V extends @Nullable Object> implements MultiValueMap<K, V>, Serializable {
 
 	private final Map<K, List<V>> targetMap;
 

spring-core/src/main/java/org/springframework/util/UnmodifiableMultiValueMap.java

@@ -39,11 +39,12 @@
  * Unmodifiable wrapper for {@link MultiValueMap}.
  *
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  * @since 6.0
  * @param <K> the key type
  * @param <V> the value element type
  */
-final class UnmodifiableMultiValueMap<K,V> implements MultiValueMap<K,V>, Serializable {
+final class UnmodifiableMultiValueMap<K, V extends @Nullable Object> implements MultiValueMap<K, V>, Serializable {
 
 	private static final long serialVersionUID = -8697084563854098920L;
 

spring-core/src/test/java/org/springframework/util/LinkedMultiValueMapTests.java

@@ -23,13 +23,15 @@
 import java.util.List;
 import java.util.Map;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
  * @author Arjen Poutsma
  * @author Juergen Hoeller
+ * @author Yanming Zhou
  */
 class LinkedMultiValueMapTests {
 
@@ -130,4 +132,11 @@ void equals() {
 		assertThat(map).isEqualTo(o2);
 	}
 
+	@Test
+	void nullable() {
+		LinkedMultiValueMap<String, @Nullable String> map = new LinkedMultiValueMap<>();
+		map.add("test", "test");
+		map.add("test", null);
+		assertThat(map.get("test")).containsExactly("test", null);
+	}
 }

spring-core/src/test/java/org/springframework/util/UnmodifiableMultiValueMapTests.java

@@ -24,6 +24,7 @@
 import java.util.Set;
 
 import org.assertj.core.api.ThrowableTypeAssert;
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -36,6 +37,7 @@
  * Tests for {@link UnmodifiableMultiValueMap}.
  *
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  * @since 6.0
  */
 class UnmodifiableMultiValueMapTests {
@@ -179,6 +181,15 @@ void valuesUnsupported() {
 		assertThatUnsupportedOperationException().isThrownBy(values::clear);
 	}
 
+	@Test
+	void nullable() {
+		MultiValueMap<String, @Nullable String> linkedMultiValueMap = new LinkedMultiValueMap<>();
+		linkedMultiValueMap.add("test", "test");
+		linkedMultiValueMap.add("test", null);
+		UnmodifiableMultiValueMap<String, String> map = new UnmodifiableMultiValueMap<>(linkedMultiValueMap);
+		assertThat(map.get("test")).containsExactly("test", null);
+	}
+
 	private static ThrowableTypeAssert<UnsupportedOperationException> assertThatUnsupportedOperationException() {
 		return assertThatExceptionOfType(UnsupportedOperationException.class);
 	}