Nullability refinements on private and static methods

Based on IntelliJ IDEA 2017.3 introspection results.

Issue: SPR-15756

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -25,6 +25,7 @@ import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHandlingException;
 import org.springframework.messaging.handler.annotation.DestinationVariable;
 import org.springframework.messaging.handler.annotation.ValueConstants;
+import org.springframework.util.Assert;
 
 /**
  * Resolves method parameters annotated with
@@ -52,6 +53,7 @@ public class DestinationVariableMethodArgumentResolver extends AbstractNamedValu
 	@Override
 	protected NamedValueInfo createNamedValueInfo(MethodParameter parameter) {
 		DestinationVariable annotation = parameter.getParameterAnnotation(DestinationVariable.class);
+		Assert.state(annotation != null, "No DestinationVariable annotation");
 		return new DestinationVariableNamedValueInfo(annotation);
 	}
 

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

@@ -30,6 +30,7 @@ import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHandlingException;
 import org.springframework.messaging.handler.annotation.Header;
 import org.springframework.messaging.support.NativeMessageHeaderAccessor;
+import org.springframework.util.Assert;
 
 /**
  * Resolves method parameters annotated with {@link Header @Header}.
@@ -55,6 +56,7 @@ public class HeaderMethodArgumentResolver extends AbstractNamedValueMethodArgume
 	@Override
 	protected NamedValueInfo createNamedValueInfo(MethodParameter parameter) {
 		Header annotation = parameter.getParameterAnnotation(Header.class);
+		Assert.state(annotation != null, "No Header annotation");
 		return new HeaderNamedValueInfo(annotation);
 	}
 

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

@@ -522,6 +522,7 @@ public class StompHeaderAccessor extends SimpMessageHeaderAccessor {
 		return (credentials != null ? credentials.passcode : null);
 	}
 
+	@Nullable
 	public static Integer getContentLength(Map<String, List<String>> nativeHeaders) {
 		List<String> values = nativeHeaders.get(STOMP_CONTENT_LENGTH_HEADER);
 		return (!CollectionUtils.isEmpty(values) ? Integer.valueOf(values.get(0)) : null);

spring-messaging/src/main/java/org/springframework/messaging/simp/user/DefaultUserDestinationResolver.java

@@ -161,6 +161,7 @@ public class DefaultUserDestinationResolver implements UserDestinationResolver {
 		return null;
 	}
 
+	@Nullable
 	private ParseResult parseSubscriptionMessage(Message<?> message, String sourceDestination) {
 		MessageHeaders headers = message.getHeaders();
 		String sessionId = SimpMessageHeaderAccessor.getSessionId(headers);