Consistent not-null assertions for configured interceptors

Closes gh-25221

spring-messaging/src/main/java/org/springframework/messaging/support/AbstractMessageChannel.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -75,11 +75,13 @@ public abstract class AbstractMessageChannel implements MessageChannel, Intercep
 
 	@Override
 	public void addInterceptor(ChannelInterceptor interceptor) {
+		Assert.notNull(interceptor, "ChannelInterceptor must not be null");
 		this.interceptors.add(interceptor);
 	}
 
 	@Override
 	public void addInterceptor(int index, ChannelInterceptor interceptor) {
+		Assert.notNull(interceptor, "ChannelInterceptor must not be null");
 		this.interceptors.add(index, interceptor);
 	}
 
@@ -107,29 +109,30 @@ public abstract class AbstractMessageChannel implements MessageChannel, Intercep
 	@Override
 	public final boolean send(Message<?> message, long timeout) {
 		Assert.notNull(message, "Message must not be null");
+		Message<?> messageToUse = message;
 		ChannelInterceptorChain chain = new ChannelInterceptorChain();
 		boolean sent = false;
 		try {
-			message = chain.applyPreSend(message, this);
-			if (message == null) {
+			messageToUse = chain.applyPreSend(messageToUse, this);
+			if (messageToUse == null) {
 				return false;
 			}
-			sent = sendInternal(message, timeout);
-			chain.applyPostSend(message, this, sent);
-			chain.triggerAfterSendCompletion(message, this, sent, null);
+			sent = sendInternal(messageToUse, timeout);
+			chain.applyPostSend(messageToUse, this, sent);
+			chain.triggerAfterSendCompletion(messageToUse, this, sent, null);
 			return sent;
 		}
 		catch (Exception ex) {
-			chain.triggerAfterSendCompletion(message, this, sent, ex);
+			chain.triggerAfterSendCompletion(messageToUse, this, sent, ex);
 			if (ex instanceof MessagingException) {
 				throw (MessagingException) ex;
 			}
-			throw new MessageDeliveryException(message,"Failed to send message to " + this, ex);
+			throw new MessageDeliveryException(messageToUse,"Failed to send message to " + this, ex);
 		}
 		catch (Throwable err) {
 			MessageDeliveryException ex2 =
-					new MessageDeliveryException(message, "Failed to send message to " + this, err);
-			chain.triggerAfterSendCompletion(message, this, sent, ex2);
+					new MessageDeliveryException(messageToUse, "Failed to send message to " + this, err);
+			chain.triggerAfterSendCompletion(messageToUse, this, sent, ex2);
 			throw ex2;
 		}
 	}
@@ -200,13 +203,14 @@ public abstract class AbstractMessageChannel implements MessageChannel, Intercep
 		}
 
 		public Message<?> applyPostReceive(Message<?> message, MessageChannel channel) {
+			Message<?> messageToUse = message;
 			for (ChannelInterceptor interceptor : interceptors) {
-				message = interceptor.postReceive(message, channel);
-				if (message == null) {
+				messageToUse = interceptor.postReceive(messageToUse, channel);
+				if (messageToUse == null) {
 					return null;
 				}
 			}
-			return message;
+			return messageToUse;
 		}
 
 		public void triggerAfterReceiveCompletion(Message<?> message, MessageChannel channel, Exception ex) {