Java 21: What Actually Matters (And What Doesn't)

Java 21: What Actually Matters (And What Doesn't)

Look, I've been writing Java since the days when everyone said it was dead (spoiler: they're still saying that). Java 21 dropped a few months ago, and after rebuilding two production services with it, I've got opinions. Some of these features are genuinely game-changing. Others? Well, let's just say the marketing team got a bit excited.

The Stuff That Actually Changed How I Code

Virtual Threads - This One's Actually Insane

Okay, I'll admit it. When I first heard about virtual threads, I thought "great, another concurrency model I'll spend three weeks learning and never use." I was so wrong it hurts.

Here's the thing: you know how we've all been writing these awkward async/await patterns or drowning in CompletableFuture chains? Virtual threads just... make that go away. I'm not exaggerating. Last week I rewrote a service that was handling API calls with a thread pool of 200 threads. With virtual threads, I just spin up a thread per request and let the JVM figure it out.

// Old way - thread pool gymnastics
ExecutorService executor = Executors.newFixedThreadPool(200);
executor.submit(() -> {
    // hope 200 is enough but not too many
});

// New way - just do the obvious thing
Thread.startVirtualThread(() -> {
    // literally spin up 10,000 of these, I don't care
    callSomeSlowAPI();
});

The wild part? I've got services now running with 50,000+ concurrent virtual threads on the same hardware that used to struggle with 500 platform threads. The memory footprint is almost nothing. It's like someone finally fixed the impedance mismatch between how we want to think about concurrency and how the JVM actually works.

Record Patterns - Finally, Pattern Matching That Doesn't Suck

I've been waiting for this one. Records were cool when they came out, but you still had to destructure them manually like some kind of caveman. Not anymore.

// Before: ughhh
if (shape instanceof Circle) {
    Circle circle = (Circle) shape;
    double radius = circle.radius();
    // now do stuff with radius
}

// After: yes please
if (shape instanceof Circle(double radius)) {
    // radius is right there, already extracted
    return Math.PI * radius * radius;
}

This gets really powerful when you're dealing with nested records. I had this gnarly data structure from a third-party API, and being able to pattern match all the way down in one go? Chef's kiss. It actually makes algebraic data types feel natural in Java, which I never thought I'd say.

The best part is when you combine it with switch expressions (which technically came earlier, but whatever):

return switch (response) {
    case Success(var data) -> processData(data);
    case Error(var code, var message) -> handleError(code, message);
    case Pending(var id) -> checkStatus(id);
};

I mean, come on. That's just clean.

String Templates - Controversial Take Time

Alright, here's where I'm going to lose some of you. String templates are... fine? They're solving a problem I mostly solved years ago with libraries, and honestly, I'm not sure the built-in version is better enough to matter.

// Yeah, this is nice
String message = STR."Hello \{name}, you have \{count} messages";

// But I was already doing this with formatting
String message = String.format("Hello %s, you have %d messages", name, count);

Don't get me wrong, the SQL injection prevention stuff is good. But in my world, we're using prepared statements anyway (you ARE using prepared statements, right?). The security angle feels a bit oversold.

Where it does shine is in logging and complex string building. I'll give it that. But game-changer? Nah.

The Collections Update Nobody Asked For (But We Needed)

Sequenced Collections

This is one of those features that makes you go "wait, this wasn't already a thing?" Turns out, getting the first or last element from a collection in Java has been weirdly inconsistent. LinkedHashSet doesn't have the same methods as Deque, List is different from SortedSet... it was a mess.

// Now everything has these
SequencedCollection<String> items = new ArrayList<>();
items.addFirst("start");
items.addLast("end");
String first = items.getFirst();
String last = items.getLast();
items.reversed(); // returns a reversed view

Is this revolutionary? No. Is it nice to have? Absolutely. I'm using reversed() way more than I expected, especially when dealing with time-series data.

Performance Stuff That Actually Matters

ZGC Enhancements

I'm just going to say it: if you're not using ZGC yet and you have latency-sensitive applications, what are you doing? The Java 21 improvements make it even better. We're talking sub-millisecond pause times on multi-gigabyte heaps.

I moved a service from G1GC to ZGC last month, and our p99 latency dropped by 40%. Not a typo. 40%. The only catch is you need to actually measure and tune it (shocking, I know). Just slapping -XX:+UseZGC on there and calling it a day won't magically fix everything.

The Stuff I Don't Care About (Yet)

Key Encapsulation Mechanisms

Look, I'm sure this is important for crypto nerds (said with love), but I'm using well-tested libraries for encryption. If you're rolling your own crypto in 2023, we need to have a different conversation. This API is probably great for the 0.5% of developers who need it, but for the rest of us? It's an implementation detail of the libraries we already use.

Process API Improvements

Better logging for Runtime.exec? Sure, fine. But honestly, if you're shelling out to external processes in your Java app, you've usually got bigger architectural problems to solve. I've used this API maybe twice in the last five years, both times while muttering "there has to be a better way."

Emoji Support

Okay, this one's kind of funny. Yes, Java 21 has better emoji support in Character class. Do I care? Only when I'm testing internationalization and some product manager insists we need to support emoji in usernames. (Why? WHY? But I digress.)

// Cool, I guess?
Character.isEmoji('😀'); // returns true now

It works, it's there, moving on.

Should You Upgrade?

Here's my actual advice: if you can swing it, yes. Virtual threads alone are worth the migration cost for most services. I've moved three production apps over, and the only real pain point was updating some libraries that were doing weird things with thread-local storage.

But (and this is important) don't upgrade just to upgrade. If your app is running fine on Java 17 and you're not hitting concurrency or performance walls, you can wait. Java 21 is an LTS release, so you've got time. The features are great, but they're not "drop everything and migrate this weekend" great.

One thing that bit me: if you're using virtual threads, you need to be careful with synchronized blocks on shared objects. They can pin the virtual thread to a platform thread, which defeats the whole purpose. Use ReentrantLock instead or restructure your code. This took me a day to figure out when I saw weird blocking behavior.

Final Thoughts

Java 21 feels like the first release in a while where the language is actually catching up to modern development practices instead of just adding enterprise checkbox features. Virtual threads are legitimately transformative if you're doing I/O-heavy work. Pattern matching is finally good enough to use everywhere. The rest is icing.

Is Java cool now? Let's not get carried away. But it's definitely more productive, and for someone who's been in this ecosystem for years, that matters more than being cool.

If you want the official details (with way less opinion and way more corporate speak), check out the official Java documentation. GeeksforGeeks and Red Hat Developer also have solid writeups.

Now go forth and thread virtually. Or don't. I'm not your manager.