Refine null-safety in the spring-messaging module

Closes gh-34158

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/AbstractMethodMessageHandler.java

@@ -514,12 +514,13 @@ public abstract class AbstractMethodMessageHandler<T>
 		handleMatch(bestMatch.mapping, bestMatch.handlerMethod, lookupDestination, message);
 	}
 
-	@SuppressWarnings("NullAway")
 	private void addMatchesToCollection(Collection<T> mappingsToCheck, Message<?> message, List<Match> matches) {
 		for (T mapping : mappingsToCheck) {
 			T match = getMatchingMapping(mapping, message);
 			if (match != null) {
-				matches.add(new Match(match, this.handlerMethods.get(mapping)));
+				HandlerMethod handlerMethod = this.handlerMethods.get(mapping);
+				Assert.state(handlerMethod != null, "HandlerMethod must not be null");
+				matches.add(new Match(match, handlerMethod));
 			}
 		}
 	}

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/reactive/AbstractMethodMessageHandler.java

@@ -496,14 +496,15 @@ public abstract class AbstractMethodMessageHandler<T>
 	 */
 	protected abstract RouteMatcher.@Nullable Route getDestination(Message<?> message);
 
-	@SuppressWarnings("NullAway")
 	private void addMatchesToCollection(
 			Collection<T> mappingsToCheck, Message<?> message, List<Match<T>> matches) {
 
 		for (T mapping : mappingsToCheck) {
 			T match = getMatchingMapping(mapping, message);
 			if (match != null) {
-				matches.add(new Match<>(match, this.handlerMethods.get(mapping)));
+				HandlerMethod handlerMethod = this.handlerMethods.get(mapping);
+				Assert.state(handlerMethod != null, "HandlerMethod must not be null");
+				matches.add(new Match<>(match, handlerMethod));
 			}
 		}
 	}

spring-messaging/src/main/java/org/springframework/messaging/rsocket/service/RSocketServiceMethod.java

@@ -90,7 +90,7 @@ final class RSocketServiceMethod {
 		return parameters;
 	}
 
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Dataflow analysis limitation
 	private static @Nullable String initRoute(
 			Method method, Class<?> containingClass, RSocketStrategies strategies,
 			@Nullable StringValueResolver embeddedValueResolver) {

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompDecoder.java

@@ -127,7 +127,7 @@ public class StompDecoder {
 	/**
 	 * Decode a single STOMP frame from the given {@code byteBuffer} into a {@link Message}.
 	 */
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Dataflow analysis limitation
 	private @Nullable Message<byte[]> decodeMessage(ByteBuffer byteBuffer, @Nullable MultiValueMap<String, String> headers) {
 		Message<byte[]> decodedMessage = null;
 		skipEol(byteBuffer);

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompHeaderAccessor.java

@@ -236,7 +236,7 @@ public class StompHeaderAccessor extends SimpMessageHeaderAccessor {
 		return (SimpMessageType.HEARTBEAT == getMessageType());
 	}
 
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Dataflow analysis limitation
 	public long[] getHeartbeat() {
 		String rawValue = getFirstNativeHeader(STOMP_HEARTBEAT_HEADER);
 		int pos = (rawValue != null ? rawValue.indexOf(',') : -1);

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompHeaders.java

@@ -272,7 +272,7 @@ public class StompHeaders implements MultiValueMap<String, String>, Serializable
 	/**
 	 * Get the heartbeat header.
 	 */
-	@SuppressWarnings("NullAway")
+	@SuppressWarnings("NullAway") // Dataflow analysis limitation
 	public long @Nullable [] getHeartbeat() {
 		String rawValue = getFirst(HEARTBEAT);
 		int pos = (rawValue != null ? rawValue.indexOf(',') : -1);

spring-messaging/src/test/java/org/springframework/messaging/handler/invocation/MethodMessageHandlerTests.java

@@ -25,6 +25,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -236,7 +237,7 @@ class MethodMessageHandlerTests {
 		}
 
 		@Override
-		protected String getMatchingMapping(String mapping, Message<?> message) {
+		protected @Nullable String getMatchingMapping(String mapping, Message<?> message) {
 			String destination = getLookupDestination(getDestination(message));
 			Assert.notNull(destination, "No destination");
 			return mapping.equals(destination) || this.pathMatcher.match(mapping, destination) ? mapping : null;