Avoid repeated instantiation of AutowiredArgumentMarker

Prior to this commit, the AutowiredArgumentMarker type was repeatedly
instantiated in ConstructorResolver.

This commit replaces the AutowiredArgumentMarker type with a simple
Object instance.

Closes gh-23469

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

@@ -66,13 +66,14 @@ import org.springframework.util.StringUtils;
 
 /**
  * Delegate for resolving constructors and factory methods.
- * Performs constructor resolution through argument matching.
+ * <p>Performs constructor resolution through argument matching.
  *
  * @author Juergen Hoeller
  * @author Rob Harrop
  * @author Mark Fisher
  * @author Costin Leau
  * @author Sebastien Deleuze
+ * @author Sam Brannen
  * @since 2.0
  * @see #autowireConstructor
  * @see #instantiateUsingFactoryMethod
@@ -82,9 +83,16 @@ class ConstructorResolver {
 
 	private static final Object[] EMPTY_ARGS = new Object[0];
 
+	/**
+	 * Marker for autowired arguments in a cached argument array, to be later replaced
+	 * by a {@linkplain #resolveAutowiredArgument resolved autowired argument}.
+	 */
+	private static final Object autowiredArgumentMarker = new Object();
+
 	private static final NamedThreadLocal<InjectionPoint> currentInjectionPoint =
 			new NamedThreadLocal<>("Current injection point");
 
+
 	private final AbstractAutowireCapableBeanFactory beanFactory;
 
 	private final Log logger;
@@ -771,7 +779,7 @@ class ConstructorResolver {
 							methodParam, beanName, autowiredBeanNames, converter, fallback);
 					args.rawArguments[paramIndex] = autowiredArgument;
 					args.arguments[paramIndex] = autowiredArgument;
-					args.preparedArguments[paramIndex] = new AutowiredArgumentMarker();
+					args.preparedArguments[paramIndex] = autowiredArgumentMarker;
 					args.resolveNecessary = true;
 				}
 				catch (BeansException ex) {
@@ -809,7 +817,7 @@ class ConstructorResolver {
 		for (int argIndex = 0; argIndex < argsToResolve.length; argIndex++) {
 			Object argValue = argsToResolve[argIndex];
 			MethodParameter methodParam = MethodParameter.forExecutable(executable, argIndex);
-			if (argValue instanceof AutowiredArgumentMarker) {
+			if (argValue == autowiredArgumentMarker) {
 				argValue = resolveAutowiredArgument(methodParam, beanName, null, converter, fallback);
 			}
 			else if (argValue instanceof BeanMetadataElement) {
@@ -963,13 +971,6 @@ class ConstructorResolver {
 	}
 
 
-	/**
-	 * Marker for autowired arguments in a cached argument array.
- 	 */
-	private static class AutowiredArgumentMarker {
-	}
-
-
 	/**
 	 * Delegate for checking Java 6's {@link ConstructorProperties} annotation.
 	 */