fix inefficient use of keySet operators in messaging classes

issue: SPR-15553

spring-messaging/src/main/java/org/springframework/messaging/simp/SimpMessagingTemplate.java

@@ -258,9 +258,9 @@ public class SimpMessagingTemplate extends AbstractMessageSendingTemplate<String
 
 		SimpMessageHeaderAccessor headerAccessor = SimpMessageHeaderAccessor.create(SimpMessageType.MESSAGE);
 		initHeaders(headerAccessor);
-		for (String key : headers.keySet()) {
-			Object value = headers.get(key);
-			headerAccessor.setNativeHeader(key, (value != null ? value.toString() : null));
+		for (Map.Entry<String, Object> headerEntry : headers.entrySet()) {
+			Object value = headerEntry.getValue();
+			headerAccessor.setNativeHeader(headerEntry.getKey(), (value != null ? value.toString() : null));
 		}
 		return headerAccessor.getMessageHeaders();
 	}

spring-messaging/src/main/java/org/springframework/messaging/simp/broker/DefaultSubscriptionRegistry.java

@@ -425,8 +425,8 @@ public class DefaultSubscriptionRegistry extends AbstractSubscriptionRegistry {
 		}
 
 		public Subscription getSubscription(String subscriptionId) {
-			for (String destination : this.destinationLookup.keySet()) {
-				Set<Subscription> subs = this.destinationLookup.get(destination);
+			for (Map.Entry<String, Set<DefaultSubscriptionRegistry.Subscription>> destinationEntry : this.destinationLookup.entrySet()) {
+				Set<Subscription> subs = destinationEntry.getValue();
 				if (subs != null) {
 					for (Subscription sub : subs) {
 						if (sub.getId().equalsIgnoreCase(subscriptionId)) {
@@ -453,17 +453,17 @@ public class DefaultSubscriptionRegistry extends AbstractSubscriptionRegistry {
 		}
 
 		public String removeSubscription(String subscriptionId) {
-			for (String destination : this.destinationLookup.keySet()) {
-				Set<Subscription> subs = this.destinationLookup.get(destination);
+			for (Map.Entry<String, Set<DefaultSubscriptionRegistry.Subscription>> destinationEntry : this.destinationLookup.entrySet()) {
+				Set<Subscription> subs = destinationEntry.getValue();
 				if (subs != null) {
 					for (Subscription sub : subs) {
 						if (sub.getId().equals(subscriptionId) && subs.remove(sub)) {
 							synchronized (this.destinationLookup) {
 								if (subs.isEmpty()) {
-									this.destinationLookup.remove(destination);
+									this.destinationLookup.remove(destinationEntry.getKey());
 								}
 							}
-							return destination;
+							return destinationEntry.getKey();
 						}
 					}
 				}

spring-messaging/src/main/java/org/springframework/messaging/simp/broker/SimpleBrokerMessageHandler.java

@@ -19,6 +19,7 @@ package org.springframework.messaging.simp.broker;
 import java.security.Principal;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ScheduledFuture;
@@ -333,11 +334,11 @@ public class SimpleBrokerMessageHandler extends AbstractBrokerMessageHandler {
 			logger.debug("Broadcasting to " + subscriptions.size() + " sessions.");
 		}
 		long now = System.currentTimeMillis();
-		for (String sessionId : subscriptions.keySet()) {
-			for (String subscriptionId : subscriptions.get(sessionId)) {
+		for (Map.Entry<String, List<String>> subscriptionEntry : subscriptions.entrySet()) {
+			for (String subscriptionId : subscriptionEntry.getValue()) {
 				SimpMessageHeaderAccessor headerAccessor = SimpMessageHeaderAccessor.create(SimpMessageType.MESSAGE);
 				initHeaders(headerAccessor);
-				headerAccessor.setSessionId(sessionId);
+				headerAccessor.setSessionId(subscriptionEntry.getKey());
 				headerAccessor.setSubscriptionId(subscriptionId);
 				headerAccessor.copyHeadersIfAbsent(message.getHeaders());
 				Object payload = message.getPayload();
@@ -349,7 +350,7 @@ public class SimpleBrokerMessageHandler extends AbstractBrokerMessageHandler {
 					logger.error("Failed to send " + message, ex);
 				}
 				finally {
-					SessionInfo info = this.sessions.get(sessionId);
+					SessionInfo info = this.sessions.get(subscriptionEntry.getKey());
 					if (info != null) {
 						info.setLastWriteTime(now);
 					}

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

@@ -191,9 +191,9 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 		if (headers == null) {
 			return;
 		}
-		for (String header : headers.keySet()) {
-			for (String value : headers.get(header)) {
-				addNativeHeader(header, value);
+		for (Map.Entry<String, List<String>> headerEntry : headers.entrySet()) {
+			for (String value : headerEntry.getValue()) {
+				addNativeHeader(headerEntry.getKey(), value);
 			}
 		}
 	}