Use flexible generic type level nullability in Converter

Allow defining converters that return non-null values by
leveraging flexible generic type level nullability in
org.springframework.core.convert.converter.Converter.

Closes gh-35947

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

@@ -29,12 +29,13 @@ import org.springframework.util.Assert;
  *
  * @author Keith Donald
  * @author Josh Cummings
+ * @author Sebastien Deleuze
  * @since 3.0
  * @param <S> the source type
- * @param <T> the target type
+ * @param <T> the target type (potentially {@code null})
  */
 @FunctionalInterface
-public interface Converter<S, T> {
+public interface Converter<S, T extends @Nullable Object> {
 
 	/**
 	 * Convert the source object of type {@code S} to target type {@code T}.
@@ -42,7 +43,7 @@ public interface Converter<S, T> {
 	 * @return the converted object, which must be an instance of {@code T} (potentially {@code null})
 	 * @throws IllegalArgumentException if the source cannot be converted to the desired target type
 	 */
-	@Nullable T convert(S source);
+	T convert(S source);
 
 	/**
 	 * Construct a composed {@link Converter} that first applies this {@link Converter}
@@ -56,7 +57,7 @@ public interface Converter<S, T> {
 	 * and then applies the {@code after} {@link Converter}
 	 * @since 5.3
 	 */
-	default <U> Converter<S, U> andThen(Converter<? super T, ? extends U> after) {
+	default <U> Converter<S, @Nullable U> andThen(Converter<? super T, ? extends @Nullable U> after) {
 		Assert.notNull(after, "'after' Converter must not be null");
 		return (S s) -> {
 			T initialResult = convert(s);

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

@@ -107,7 +107,7 @@ public class ConvertingComparator<S, T> implements Comparator<S> {
 	/**
 	 * Adapts a {@link ConversionService} and {@code targetType} to a {@link Converter}.
 	 */
-	private static class ConversionServiceConverter<S, T> implements Converter<S, T> {
+	private static class ConversionServiceConverter<S, T> implements Converter<S, @Nullable T> {
 
 		private final ConversionService conversionService;
 

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

@@ -31,7 +31,7 @@ import org.springframework.core.convert.converter.Converter;
  * @author Sam Brannen
  * @since 3.0
  */
-final class StringToBooleanConverter implements Converter<String, Boolean> {
+final class StringToBooleanConverter implements Converter<String, @Nullable Boolean> {
 
 	private static final Set<String> trueValues = Set.of("true", "on", "yes", "1");
 

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

@@ -26,7 +26,7 @@ import org.springframework.core.convert.converter.Converter;
  * @author Keith Donald
  * @since 3.0
  */
-final class StringToCharacterConverter implements Converter<String, Character> {
+final class StringToCharacterConverter implements Converter<String, @Nullable Character> {
 
 	@Override
 	public @Nullable Character convert(String source) {

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

@@ -37,7 +37,7 @@ final class StringToEnumConverterFactory implements ConverterFactory<String, Enu
 	}
 
 
-	private static class StringToEnum<T extends Enum> implements Converter<String, T> {
+	private static class StringToEnum<T extends Enum> implements Converter<String, @Nullable T> {
 
 		private final Class<T> enumType;
 

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

@@ -34,7 +34,7 @@ import org.springframework.util.StringUtils;
  * @since 3.0
  * @see StringUtils#parseLocale
  */
-final class StringToLocaleConverter implements Converter<String, Locale> {
+final class StringToLocaleConverter implements Converter<String, @Nullable Locale> {
 
 	@Override
 	public @Nullable Locale convert(String source) {

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

@@ -48,7 +48,7 @@ final class StringToNumberConverterFactory implements ConverterFactory<String, N
 	}
 
 
-	private static final class StringToNumber<T extends Number> implements Converter<String, T> {
+	private static final class StringToNumber<T extends Number> implements Converter<String, @Nullable T> {
 
 		private final Class<T> targetType;
 

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

@@ -29,7 +29,7 @@ import org.springframework.core.convert.converter.Converter;
  * @author Stephane Nicoll
  * @since 6.1
  */
-final class StringToPatternConverter implements Converter<String, Pattern> {
+final class StringToPatternConverter implements Converter<String, @Nullable Pattern> {
 
 	@Override
 	public @Nullable Pattern convert(String source) {

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

@@ -28,7 +28,7 @@ import org.springframework.core.convert.converter.Converter;
  * @author Sebastien Deleuze
  * @since 6.1
  */
-final class StringToRegexConverter implements Converter<String, Regex> {
+final class StringToRegexConverter implements Converter<String, @Nullable Regex> {
 
 	@Override
 	public @Nullable Regex convert(String source) {

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

@@ -30,7 +30,7 @@ import org.springframework.util.StringUtils;
  * @since 3.2
  * @see UUID#fromString
  */
-final class StringToUUIDConverter implements Converter<String, UUID> {
+final class StringToUUIDConverter implements Converter<String, @Nullable UUID> {
 
 	@Override
 	public @Nullable UUID convert(String source) {