Optimize locking in AspectJProxyFactory for concurrent aspect instantiation

Closes gh-26034

spring-aop/src/main/java/org/springframework/aop/aspectj/annotation/AspectJProxyFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -160,23 +160,12 @@ public class AspectJProxyFactory extends ProxyCreatorSupport {
 	}
 
 	/**
-	 * Get the singleton aspect instance for the supplied aspect type. An instance
-	 * is created if one cannot be found in the instance cache.
+	 * Get the singleton aspect instance for the supplied aspect type.
+	 * An instance is created if one cannot be found in the instance cache.
 	 */
 	private Object getSingletonAspectInstance(Class<?> aspectClass) {
-		// Quick check without a lock...
-		Object instance = aspectCache.get(aspectClass);
-		if (instance == null) {
-			synchronized (aspectCache) {
-				// To be safe, check within full lock now...
-				instance = aspectCache.get(aspectClass);
-				if (instance == null) {
-					instance = new SimpleAspectInstanceFactory(aspectClass).getAspectInstance();
-					aspectCache.put(aspectClass, instance);
-				}
-			}
-		}
-		return instance;
+		return aspectCache.computeIfAbsent(aspectClass,
+				clazz -> new SimpleAspectInstanceFactory(clazz).getAspectInstance());
 	}