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-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/RequestMappingInfo.java

@@ -509,7 +509,7 @@ public final class RequestMappingInfo implements RequestCondition<RequestMapping
 		}
 
 		// Patterns conditions are never empty and have "" (empty path) at least.
-		builder.append(" ").append(getActivePatternsCondition());
+		builder.append(' ').append(getActivePatternsCondition());
 
 		if (!this.paramsCondition.isEmpty()) {
 			builder.append(", params ").append(this.paramsCondition);

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/SseEmitter.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.
@@ -35,6 +35,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Rossen Stoyanchev
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 4.2
  */
 public class SseEmitter extends ResponseBodyEmitter {
@@ -175,7 +176,7 @@ public class SseEmitter extends ResponseBodyEmitter {
 		SseEventBuilder data(Object object, @Nullable MediaType mediaType);
 
 		/**
-		 * Return one or more Object-MediaType  pairs to write via
+		 * Return one or more Object-MediaType pairs to write via
 		 * {@link #send(Object, MediaType)}.
 		 * @since 4.2.3
 		 */
@@ -195,25 +196,25 @@ public class SseEmitter extends ResponseBodyEmitter {
 
 		@Override
 		public SseEventBuilder id(String id) {
-			append("id:").append(id).append("\n");
+			append("id:").append(id).append('\n');
 			return this;
 		}
 
 		@Override
 		public SseEventBuilder name(String name) {
-			append("event:").append(name).append("\n");
+			append("event:").append(name).append('\n');
 			return this;
 		}
 
 		@Override
 		public SseEventBuilder reconnectTime(long reconnectTimeMillis) {
-			append("retry:").append(String.valueOf(reconnectTimeMillis)).append("\n");
+			append("retry:").append(String.valueOf(reconnectTimeMillis)).append('\n');
 			return this;
 		}
 
 		@Override
 		public SseEventBuilder comment(String comment) {
-			append(":").append(comment).append("\n");
+			append(':').append(comment).append('\n');
 			return this;
 		}
 
@@ -227,7 +228,7 @@ public class SseEmitter extends ResponseBodyEmitter {
 			append("data:");
 			saveAppendedText();
 			this.dataToSend.add(new DataWithMediaType(object, mediaType));
-			append("\n");
+			append('\n');
 			return this;
 		}
 
@@ -239,12 +240,20 @@ public class SseEmitter extends ResponseBodyEmitter {
 			return this;
 		}
 
+		SseEventBuilderImpl append(char ch) {
+			if (this.sb == null) {
+				this.sb = new StringBuilder();
+			}
+			this.sb.append(ch);
+			return this;
+		}
+
 		@Override
 		public Set<DataWithMediaType> build() {
 			if (!StringUtils.hasLength(this.sb) && this.dataToSend.isEmpty()) {
 				return Collections.emptySet();
 			}
-			append("\n");
+			append('\n');
 			saveAppendedText();
 			return this.dataToSend;
 		}

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/PathResourceResolver.java

@@ -268,7 +268,7 @@ public class PathResourceResolver extends AbstractResourceResolver {
 			while (tokenizer.hasMoreTokens()) {
 				String value = UriUtils.encode(tokenizer.nextToken(), charset);
 				sb.append(value);
-				sb.append("/");
+				sb.append('/');
 			}
 			if (!path.endsWith("/")) {
 				sb.setLength(sb.length() - 1);

spring-webmvc/src/main/java/org/springframework/web/servlet/support/BindStatus.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.
@@ -352,7 +352,7 @@ public class BindStatus {
 	public String toString() {
 		StringBuilder sb = new StringBuilder("BindStatus: ");
 		sb.append("expression=[").append(this.expression).append("]; ");
-		sb.append("value=[").append(this.value).append("]");
+		sb.append("value=[").append(this.value).append(']');
 		if (!ObjectUtils.isEmpty(this.errorCodes)) {
 			sb.append("; errorCodes=").append(Arrays.asList(this.errorCodes));
 		}

spring-webmvc/src/main/java/org/springframework/web/servlet/tags/UrlTag.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.
@@ -286,7 +286,7 @@ public class UrlTag extends HtmlEscapingAwareTag implements ParamAware {
 			}
 		}
 		if (this.type != UrlType.RELATIVE && this.type != UrlType.ABSOLUTE && !this.value.startsWith("/")) {
-			url.append("/");
+			url.append('/');
 		}
 		url.append(replaceUriTemplateParams(this.value, this.params, this.templateParams));
 		url.append(createQueryString(this.params, this.templateParams, (url.indexOf("?") == -1)));
@@ -324,15 +324,15 @@ public class UrlTag extends HtmlEscapingAwareTag implements ParamAware {
 		for (Param param : params) {
 			if (!usedParams.contains(param.getName()) && StringUtils.hasLength(param.getName())) {
 				if (includeQueryStringDelimiter && qs.length() == 0) {
-					qs.append("?");
+					qs.append('?');
 				}
 				else {
-					qs.append("&");
+					qs.append('&');
 				}
 				try {
 					qs.append(UriUtils.encodeQueryParam(param.getName(), encoding));
 					if (param.getValue() != null) {
-						qs.append("=");
+						qs.append('=');
 						qs.append(UriUtils.encodeQueryParam(param.getValue(), encoding));
 					}
 				}