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-aop/src/main/java/org/springframework/aop/aspectj/AspectJAdviceParameterNameDiscoverer.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.
@@ -625,7 +625,7 @@ public class AspectJAdviceParameterNameDiscoverer implements ParameterNameDiscov
 			StringBuilder sb = new StringBuilder();
 			if (bodyStart >= 0 && bodyStart != (currentToken.length() - 1)) {
 				sb.append(currentToken.substring(bodyStart + 1));
-				sb.append(" ");
+				sb.append(' ');
 			}
 			numTokensConsumed++;
 			int currentIndex = startIndex + numTokensConsumed;
@@ -645,7 +645,7 @@ public class AspectJAdviceParameterNameDiscoverer implements ParameterNameDiscov
 					toAppend = toAppend.substring(1);
 				}
 				sb.append(toAppend);
-				sb.append(" ");
+				sb.append(' ');
 				currentIndex++;
 				numTokensConsumed++;
 			}

spring-aop/src/main/java/org/springframework/aop/aspectj/AspectJExpressionPointcut.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.
@@ -547,7 +547,7 @@ public class AspectJExpressionPointcut extends AbstractExpressionPointcut
 		StringBuilder sb = new StringBuilder("AspectJExpressionPointcut: (");
 		for (int i = 0; i < this.pointcutParameterTypes.length; i++) {
 			sb.append(this.pointcutParameterTypes[i].getName());
-			sb.append(" ");
+			sb.append(' ');
 			sb.append(this.pointcutParameterNames[i]);
 			if ((i+1) < this.pointcutParameterTypes.length) {
 				sb.append(", ");

spring-aop/src/main/java/org/springframework/aop/aspectj/MethodInvocationProceedingJoinPoint.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.
@@ -255,19 +255,19 @@ public class MethodInvocationProceedingJoinPoint implements ProceedingJoinPoint,
 			StringBuilder sb = new StringBuilder();
 			if (includeModifier) {
 				sb.append(Modifier.toString(getModifiers()));
-				sb.append(" ");
+				sb.append(' ');
 			}
 			if (includeReturnTypeAndArgs) {
 				appendType(sb, getReturnType(), useLongReturnAndArgumentTypeName);
-				sb.append(" ");
+				sb.append(' ');
 			}
 			appendType(sb, getDeclaringType(), useLongTypeName);
-			sb.append(".");
+			sb.append('.');
 			sb.append(getMethod().getName());
-			sb.append("(");
+			sb.append('(');
 			Class<?>[] parametersTypes = getParameterTypes();
 			appendTypes(sb, parametersTypes, includeReturnTypeAndArgs, useLongReturnAndArgumentTypeName);
-			sb.append(")");
+			sb.append(')');
 			return sb.toString();
 		}
 
@@ -278,7 +278,7 @@ public class MethodInvocationProceedingJoinPoint implements ProceedingJoinPoint,
 				for (int size = types.length, i = 0; i < size; i++) {
 					appendType(sb, types[i], useLongReturnAndArgumentTypeName);
 					if (i < size - 1) {
-						sb.append(",");
+						sb.append(',');
 					}
 				}
 			}

spring-aop/src/main/java/org/springframework/aop/support/AbstractBeanFactoryPointcutAdvisor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 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.
@@ -137,7 +137,7 @@ public abstract class AbstractBeanFactoryPointcutAdvisor extends AbstractPointcu
 		StringBuilder sb = new StringBuilder(getClass().getName());
 		sb.append(": advice ");
 		if (this.adviceBeanName != null) {
-			sb.append("bean '").append(this.adviceBeanName).append("'");
+			sb.append("bean '").append(this.adviceBeanName).append('\'');
 		}
 		else {
 			sb.append(this.advice);

spring-aop/src/main/java/org/springframework/aop/target/AbstractBeanFactoryBasedTargetSource.java

@@ -191,9 +191,9 @@ public abstract class AbstractBeanFactoryBasedTargetSource implements TargetSour
 	@Override
 	public String toString() {
 		StringBuilder sb = new StringBuilder(getClass().getSimpleName());
-		sb.append(" for target bean '").append(this.targetBeanName).append("'");
+		sb.append(" for target bean '").append(this.targetBeanName).append('\'');
 		if (this.targetClass != null) {
-			sb.append(" of type [").append(this.targetClass.getName()).append("]");
+			sb.append(" of type [").append(this.targetClass.getName()).append(']');
 		}
 		return sb.toString();
 	}