Refine changes for PR

Closes gh-23882

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/support/AbstractNamedValueMethodArgumentResolver.java

@@ -73,10 +73,16 @@ public abstract class AbstractNamedValueMethodArgumentResolver implements Handle
 	 * @param beanFactory a bean factory for resolving {@code ${...}}
 	 * placeholders and {@code #{...}} SpEL expressions in default values
 	 */
-	protected AbstractNamedValueMethodArgumentResolver(@Nullable ConversionService conversionService,
+	protected AbstractNamedValueMethodArgumentResolver(ConversionService conversionService,
 			@Nullable ConfigurableBeanFactory beanFactory) {
 
-		this.conversionService = conversionService != null ? conversionService : DefaultConversionService.getSharedInstance();
+		// Fallback on shared ConversionService for now for historic reasons.
+		// Possibly remove after discussion in gh-23882.
+
+		//noinspection ConstantConditions
+		this.conversionService = conversionService != null ?
+				conversionService : DefaultConversionService.getSharedInstance();
+
 		this.configurableBeanFactory = beanFactory;
 		this.expressionContext = (beanFactory != null ? new BeanExpressionContext(beanFactory, null) : null);
 	}

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/support/DestinationVariableMethodArgumentResolver.java

@@ -41,7 +41,7 @@ public class DestinationVariableMethodArgumentResolver extends AbstractNamedValu
 			DestinationVariableMethodArgumentResolver.class.getSimpleName() + ".templateVariables";
 
 
-	public DestinationVariableMethodArgumentResolver(@Nullable ConversionService conversionService) {
+	public DestinationVariableMethodArgumentResolver(ConversionService conversionService) {
 		super(conversionService, null);
 	}
 

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/support/HeaderMethodArgumentResolver.java

@@ -49,7 +49,7 @@ public class HeaderMethodArgumentResolver extends AbstractNamedValueMethodArgume
 
 
 	public HeaderMethodArgumentResolver(
-			@Nullable ConversionService conversionService, @Nullable ConfigurableBeanFactory beanFactory) {
+			ConversionService conversionService, @Nullable ConfigurableBeanFactory beanFactory) {
 
 		super(conversionService, beanFactory);
 	}

spring-messaging/src/test/java/org/springframework/messaging/handler/annotation/support/HeaderMethodArgumentResolverTests.java

@@ -145,14 +145,6 @@ public void resolveOptionalHeaderWithValue() throws Exception {
 		assertThat(result).isEqualTo(Optional.of("bar"));
 	}
 
-	@Test
-	public void resolveOptionalHeaderWithValueFromNullConversionServiceInput() throws Exception {
-		GenericApplicationContext context = new GenericApplicationContext();
-		context.refresh();
-		resolver = new HeaderMethodArgumentResolver(null, context.getBeanFactory());
-		resolveOptionalHeaderWithValue();
-	}
-
 	@Test
 	public void resolveOptionalHeaderAsEmpty() throws Exception {
 		Message<String> message = MessageBuilder.withPayload("foo").build();