Avoid lock contention in CaffeineCacheManager

Prior to this commit, using a dynamic `CaffeineCacheManager` would rely
on `ConcurrentHashMap#computeIfAbsent` for retrieving and creating cache
instances as needed. It turns out that using this method concurrently
can cause lock contention even when all known cache instances are
instantiated.

This commit avoids using this method if the cache instance already
exists and avoid storing `null` entries in the map. This change reduces
lock contention and the overall HashMap size in the non-dynamic case.

Fixes gh-30066

Author: bclozel

spring-context-support/src/main/java/org/springframework/cache/caffeine/CaffeineCacheManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -51,6 +51,7 @@
  * @author Juergen Hoeller
  * @author Stephane Nicoll
  * @author Sam Brannen
+ * @author Brian Clozel
  * @since 4.3
  * @see CaffeineCache
  */
@@ -188,8 +189,13 @@ public Collection<String> getCacheNames() {
 	@Override
 	@Nullable
 	public Cache getCache(String name) {
-		return this.cacheMap.computeIfAbsent(name, cacheName ->
-				this.dynamic ? createCaffeineCache(cacheName) : null);
+		if (this.dynamic) {
+			Cache cache = this.cacheMap.get(name);
+			return (cache != null) ? cache : this.cacheMap.computeIfAbsent(name, this::createCaffeineCache);
+		}
+		else {
+			return this.cacheMap.get(name);
+		}
 	}