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-context/src/main/java/org/springframework/cache/interceptor/CacheEvictOperation.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.
@@ -71,9 +71,9 @@ public class CacheEvictOperation extends CacheOperation {
 		@Override
 		protected StringBuilder getOperationDescription() {
 			StringBuilder sb = super.getOperationDescription();
-			sb.append(",");
+			sb.append(',');
 			sb.append(this.cacheWide);
-			sb.append(",");
+			sb.append(',');
 			sb.append(this.beforeInvocation);
 			return sb;
 		}

spring-context/src/main/java/org/springframework/cache/interceptor/CacheOperation.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.
@@ -216,13 +216,13 @@ public abstract class CacheOperation implements BasicOperation {
 		 */
 		protected StringBuilder getOperationDescription() {
 			StringBuilder result = new StringBuilder(getClass().getSimpleName());
-			result.append("[").append(this.name);
+			result.append('[').append(this.name);
 			result.append("] caches=").append(this.cacheNames);
 			result.append(" | key='").append(this.key);
 			result.append("' | keyGenerator='").append(this.keyGenerator);
 			result.append("' | cacheManager='").append(this.cacheManager);
 			result.append("' | cacheResolver='").append(this.cacheResolver);
-			result.append("' | condition='").append(this.condition).append("'");
+			result.append("' | condition='").append(this.condition).append('\'');
 			return result;
 		}
 

spring-context/src/main/java/org/springframework/cache/interceptor/CachePutOperation.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.
@@ -66,7 +66,7 @@ public class CachePutOperation extends CacheOperation {
 			StringBuilder sb = super.getOperationDescription();
 			sb.append(" | unless='");
 			sb.append(this.unless);
-			sb.append("'");
+			sb.append('\'');
 			return sb;
 		}
 

spring-context/src/main/java/org/springframework/cache/interceptor/CacheableOperation.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.
@@ -79,10 +79,10 @@ public class CacheableOperation extends CacheOperation {
 			StringBuilder sb = super.getOperationDescription();
 			sb.append(" | unless='");
 			sb.append(this.unless);
-			sb.append("'");
+			sb.append('\'');
 			sb.append(" | sync='");
 			sb.append(this.sync);
-			sb.append("'");
+			sb.append('\'');
 			return sb;
 		}
 

spring-context/src/main/java/org/springframework/context/event/ApplicationListenerMethodAdapter.java

@@ -396,7 +396,7 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 	 * @param message error message to append the HandlerMethod details to
 	 */
 	protected String getDetailedErrorMessage(Object bean, String message) {
-		StringBuilder sb = new StringBuilder(message).append("\n");
+		StringBuilder sb = new StringBuilder(message).append('\n');
 		sb.append("HandlerMethod details: \n");
 		sb.append("Bean [").append(bean.getClass().getName()).append("]\n");
 		sb.append("Method [").append(this.method.toGenericString()).append("]\n");
@@ -426,7 +426,7 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 		StringBuilder sb = new StringBuilder(getDetailedErrorMessage(bean, message));
 		sb.append("Resolved arguments: \n");
 		for (int i = 0; i < resolvedArgs.length; i++) {
-			sb.append("[").append(i).append("] ");
+			sb.append('[').append(i).append("] ");
 			if (resolvedArgs[i] == null) {
 				sb.append("[null] \n");
 			}

spring-context/src/main/java/org/springframework/context/support/LiveBeansView.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.
@@ -205,12 +205,12 @@ public class LiveBeansView implements LiveBeansViewMBean, ApplicationContextAwar
 				}
 			}
 			result.append("]\n");
-			result.append("}");
+			result.append('}');
 			if (it.hasNext()) {
 				result.append(",\n");
 			}
 		}
-		result.append("]");
+		result.append(']');
 		return result.toString();
 	}