Move common caching class loading functionality to the separate class

[dukhlov]

Move common class-loading caching functionality into CachingClassLoader.

Update UberClassLoader to use CachingClassLoader, so now all results of loadClass() (not just findClass()) are cached.
This improves performance, especially because UberClassLoader uses a ExistenceCheckingClassLoader wrapper for the parent class loader, which introduces overhead.

By caching class-loading results for the parent as well, we avoid redundant lookups and reduce that overhead.

Testing done

Manual testing on local setup, jenkins ci gates.

Proposed changelog entries

• Improve UberClassLoader class caching

Proposed changelog category

/label internal

Proposed upgrade guidelines

N/A

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• The Jira issue, if it exists, is well-described.
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Desired reviewers

@basil, @timja

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
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[jeromepochat]

Hi! This refactor makes total sense, encouraging single-responsibility principle. Thanks!
I just left minor suggestion about Optional usage, feel free to ignore!

[jeromepochat]

suggestion: Maybe the using Optional as map value type could make its usage more natural. WDYT?

Suggested change

	private final ConcurrentHashMap<String, Object> cache = new ConcurrentHashMap<>();
	public CachingClassLoader(String name, ClassLoader parent) {
	super(name, parent);
	}
	@Override
	protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
	Object classOrEmpty = cache.computeIfAbsent(name, key -> {
	try {
	return super.loadClass(name, false);
	} catch (ClassNotFoundException e) {
	// Not found.
	return Optional.empty();
	}
	});
	if (classOrEmpty == Optional.empty()) {
	throw new ClassNotFoundException(name);
	}
	Class<?> clazz = (Class<?>) classOrEmpty;
	if (resolve) {
	resolveClass(clazz);
	}
	return clazz;
	}
	public void clearCacheMisses() {
	cache.values().removeIf(v -> v == Optional.empty());
	}
	private final ConcurrentHashMap<String, Optional<Class<?>>> cache = new ConcurrentHashMap<>();
	public CachingClassLoader(String name, ClassLoader parent) {
	super(name, parent);
	}
	@Override
	protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
	Optional<Class<?>> classOrEmpty = cache.computeIfAbsent(name, key -> {
	try {
	return Optional.of(super.loadClass(name, false));
	} catch (ClassNotFoundException e) {
	// Not found.
	return Optional.empty();
	}
	});
	if (classOrEmpty.isEmpty()) {
	throw new ClassNotFoundException(name);
	}
	Class<?> clazz = classOrEmpty.get();
	if (resolve) {
	resolveClass(clazz);
	}
	return clazz;
	}
	public void clearCacheMisses() {
	cache.values().removeIf(Optional::isEmpty);
	}

[dukhlov]

Wrapping with Optional consumes a bit more memory, so I decided not to use it. However, I agree it makes the code more natural. Let's wait for further reviews before making a final decision.

[timja]

/label ready-for-merge

---

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!