Use StringBuilder.append(char) where possible

To slightly improve performance, this commit switches to
StringBuilder.append(char) instead of StringBuilder.append(String)
whenever we append a single character to a StringBuilder.

Closes gh-27098

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -70,7 +70,7 @@ public class MethodArgumentNotValidException extends MethodArgumentResolutionExc
 		StringBuilder sb = new StringBuilder();
 		sb.append(bindingResult.getErrorCount()).append(" error(s): ");
 		for (ObjectError error : bindingResult.getAllErrors()) {
-			sb.append("[").append(error).append("] ");
+			sb.append('[').append(error).append("] ");
 		}
 		return sb.toString();
 	}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -208,7 +208,7 @@ public class SimpMessageHeaderAccessor extends NativeMessageHeaderAccessor {
 		}
 		StringBuilder sb = getBaseLogMessage();
 		if (!CollectionUtils.isEmpty(getSessionAttributes())) {
-			sb.append(" attributes[").append(getSessionAttributes().size()).append("]");
+			sb.append(" attributes[").append(getSessionAttributes().size()).append(']');
 		}
 		sb.append(getShortPayloadLogMessage(payload));
 		return sb.toString();

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -452,7 +452,7 @@ public class StompHeaderAccessor extends SimpMessageHeaderAccessor {
 			return super.getDetailedLogMessage(payload);
 		}
 		StringBuilder sb = new StringBuilder();
-		sb.append(command.name()).append(" ");
+		sb.append(command.name()).append(' ');
 		Map<String, List<String>> nativeHeaders = getNativeHeaders();
 		if (nativeHeaders != null) {
 			sb.append(nativeHeaders);

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -112,12 +112,12 @@ public class GenericMessage<T> implements Message<T>, Serializable {
 		StringBuilder sb = new StringBuilder(getClass().getSimpleName());
 		sb.append(" [payload=");
 		if (this.payload instanceof byte[]) {
-			sb.append("byte[").append(((byte[]) this.payload).length).append("]");
+			sb.append("byte[").append(((byte[]) this.payload).length).append(']');
 		}
 		else {
 			sb.append(this.payload);
 		}
-		sb.append(", headers=").append(this.headers).append("]");
+		sb.append(", headers=").append(this.headers).append(']');
 		return sb.toString();
 	}
 

spring-messaging/src/test/java/org/springframework/messaging/simp/stomp/StompHeaderAccessorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -241,7 +241,7 @@ public class StompHeaderAccessorTests {
 
 		StringBuilder sb = new StringBuilder();
 		for (int i = 0; i < 80; i++) {
-			sb.append("a");
+			sb.append('a');
 		}
 		final String payload = sb.toString() + " > 80";
 		actual = accessor.getShortLogMessage(payload.getBytes(StandardCharsets.UTF_8));

spring-messaging/src/test/java/org/springframework/messaging/support/MessageHeaderAccessorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -320,7 +320,7 @@ public class MessageHeaderAccessorTests {
 
 		StringBuilder sb = new StringBuilder();
 		for (int i = 0; i < 80; i++) {
-			sb.append("a");
+			sb.append('a');
 		}
 		final String payload = sb.toString() + " > 80";
 
@@ -356,7 +356,7 @@ public class MessageHeaderAccessorTests {
 
 		StringBuilder sb = new StringBuilder();
 		for (int i = 0; i < 80; i++) {
-			sb.append("a");
+			sb.append('a');
 		}
 		final String payload = sb.toString() + " > 80";