Fix nullness mismatch for Converter/ConverterFactory

In gh-35947, the `Converter` contract was refined to allow for nullable
return values. This created a mismatch with the `ConverterFactory`
contract.

This commit fixes this mismatch by allowing nullable return values in
`Converter` instances created by `ConverterFactory`.

Fixes gh-36063

Author: bclozel

spring-core/src/main/java/org/springframework/core/convert/converter/ConverterFactory.java

@@ -16,6 +16,8 @@
 
 package org.springframework.core.convert.converter;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * A factory for "ranged" converters that can convert objects from S to subtypes of R.
  *
@@ -36,6 +38,6 @@ public interface ConverterFactory<S, R> {
 	 * @param targetType the target type to convert to
 	 * @return a converter from S to T
 	 */
-	<T extends R> Converter<S, T> getConverter(Class<T> targetType);
+	<T extends R> Converter<S, @Nullable T> getConverter(Class<T> targetType);
 
 }

spring-core/src/main/java/org/springframework/core/convert/support/CharacterToNumberFactory.java

@@ -16,6 +16,8 @@
 
 package org.springframework.core.convert.support;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.ConverterFactory;
 import org.springframework.util.NumberUtils;
@@ -41,7 +43,7 @@
 final class CharacterToNumberFactory implements ConverterFactory<Character, Number> {
 
 	@Override
-	public <T extends Number> Converter<Character, T> getConverter(Class<T> targetType) {
+	public <T extends Number> Converter<Character, @Nullable T> getConverter(Class<T> targetType) {
 		return new CharacterToNumber<>(targetType);
 	}
 

spring-core/src/main/java/org/springframework/core/convert/support/IntegerToEnumConverterFactory.java

@@ -16,6 +16,8 @@
 
 package org.springframework.core.convert.support;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.ConverterFactory;
 
@@ -30,7 +32,7 @@
 final class IntegerToEnumConverterFactory implements ConverterFactory<Integer, Enum> {
 
 	@Override
-	public <T extends Enum> Converter<Integer, T> getConverter(Class<T> targetType) {
+	public <T extends Enum> Converter<Integer, @Nullable T> getConverter(Class<T> targetType) {
 		return new IntegerToEnum(ConversionUtils.getEnumType(targetType));
 	}
 

spring-core/src/main/java/org/springframework/core/convert/support/NumberToNumberConverterFactory.java

@@ -16,6 +16,8 @@
 
 package org.springframework.core.convert.support;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.core.convert.TypeDescriptor;
 import org.springframework.core.convert.converter.ConditionalConverter;
 import org.springframework.core.convert.converter.Converter;
@@ -43,7 +45,7 @@
 final class NumberToNumberConverterFactory implements ConverterFactory<Number, Number>, ConditionalConverter {
 
 	@Override
-	public <T extends Number> Converter<Number, T> getConverter(Class<T> targetType) {
+	public <T extends Number> Converter<Number, @Nullable T> getConverter(Class<T> targetType) {
 		return new NumberToNumber<>(targetType);
 	}
 

spring-core/src/main/java/org/springframework/core/convert/support/StringToEnumConverterFactory.java

@@ -32,7 +32,7 @@
 final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {
 
 	@Override
-	public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
+	public <T extends Enum> Converter<String, @Nullable T> getConverter(Class<T> targetType) {
 		return new StringToEnum(ConversionUtils.getEnumType(targetType));
 	}
 

spring-core/src/main/java/org/springframework/core/convert/support/StringToNumberConverterFactory.java

@@ -43,7 +43,7 @@
 final class StringToNumberConverterFactory implements ConverterFactory<String, Number> {
 
 	@Override
-	public <T extends Number> Converter<String, T> getConverter(Class<T> targetType) {
+	public <T extends Number> Converter<String, @Nullable T> getConverter(Class<T> targetType) {
 		return new StringToNumber<>(targetType);
 	}
 

spring-core/src/test/kotlin/org/springframework/core/convert/converter/ConverterFactoryNullnessTests.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2025-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.
+ */
+
+package org.springframework.core.convert.converter
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import kotlin.reflect.full.primaryConstructor
+
+/**
+ * @author Brian Clozel
+ */
+class ConverterFactoryNullnessTests {
+
+	@Test
+	fun converterFactoryWithNullableTypes() {
+		val factory = StringToIdConverterFactory
+
+		val userIdConverter = factory.getConverter(UserId::class.java)
+		assertThat(userIdConverter.convert("42")).isEqualTo(UserId("42"))
+	}
+
+	object StringToIdConverterFactory : ConverterFactory<String, Id> {
+		override fun <T : Id> getConverter(targetType: Class<T>): Converter<String, T?> {
+			val constructor = checkNotNull(targetType.kotlin.primaryConstructor)
+			return Converter { source ->
+				constructor.call(source)
+			}
+		}
+	}
+
+	abstract class Id {
+		abstract val value: String
+	}
+
+	data class UserId(override val value: String) : Id()
+
+	data class ProductId(override val value: String) : Id()
+
+}