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-beans/src/main/java/org/springframework/beans/TypeConverterDelegate.java

@@ -247,14 +247,14 @@ class TypeConverterDelegate {
 				// Definitely doesn't match: throw IllegalArgumentException/IllegalStateException
 				StringBuilder msg = new StringBuilder();
 				msg.append("Cannot convert value of type '").append(ClassUtils.getDescriptiveType(newValue));
-				msg.append("' to required type '").append(ClassUtils.getQualifiedName(requiredType)).append("'");
+				msg.append("' to required type '").append(ClassUtils.getQualifiedName(requiredType)).append('\'');
 				if (propertyName != null) {
-					msg.append(" for property '").append(propertyName).append("'");
+					msg.append(" for property '").append(propertyName).append('\'');
 				}
 				if (editor != null) {
 					msg.append(": PropertyEditor [").append(editor.getClass().getName()).append(
 							"] returned inappropriate value of type '").append(
-							ClassUtils.getDescriptiveType(convertedValue)).append("'");
+							ClassUtils.getDescriptiveType(convertedValue)).append('\'');
 					throw new IllegalArgumentException(msg.toString());
 				}
 				else {

spring-beans/src/main/java/org/springframework/beans/factory/annotation/RequiredAnnotationBeanPostProcessor.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.
@@ -217,13 +217,13 @@ public class RequiredAnnotationBeanPostProcessor implements SmartInstantiationAw
 					sb.append(" and");
 				}
 				else {
-					sb.append(",");
+					sb.append(',');
 				}
 			}
-			sb.append(" '").append(propertyName).append("'");
+			sb.append(" '").append(propertyName).append('\'');
 		}
 		sb.append(size == 1 ? " is" : " are");
-		sb.append(" required for bean '").append(beanName).append("'");
+		sb.append(" required for bean '").append(beanName).append('\'');
 		return sb.toString();
 	}
 

spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractBeanDefinition.java

@@ -1241,7 +1241,7 @@ public abstract class AbstractBeanDefinition extends BeanMetadataAttributeAccess
 	@Override
 	public String toString() {
 		StringBuilder sb = new StringBuilder("class [");
-		sb.append(getBeanClassName()).append("]");
+		sb.append(getBeanClassName()).append(']');
 		sb.append("; scope=").append(this.scope);
 		sb.append("; abstract=").append(this.abstractFlag);
 		sb.append("; lazyInit=").append(this.lazyInit);