Virtual Threads（正式版）顯著簡(jiǎn)化高吞吐并發(fā)編程，適合I/O密集任務(wù)；2. Structured Concurrency（正式版）提升并發(fā)代碼可讀性和安全性，避免僵尸線程；3. String Templates（預(yù)覽版）替代String.format，編譯時(shí)校驗(yàn)更安全；4. Sequenced Collections API（正式版）統(tǒng)一有序集合操作如getFirst、reversed；5. 其他新增包括Scoped Values、Record Patterns和Generational ZGC。建議盡快升級(jí)至Java 21以獲得性能提升與現(xiàn)代語(yǔ)法支持，尤其適用于I/O密集型應(yīng)用和新并發(fā)邏輯開發(fā)。

Java 21, released in September 2023, is a long-term support (LTS) version — meaning it's not just another incremental update. It brings powerful new features, performance improvements, and long-awaited language enhancements that developers have been asking for. If you're upgrading from Java 17 (also LTS), here's what's actually new and useful in Java 21.

? 1. Virtual Threads (Preview → Final)

This is the biggest game-changer in Java 21 — and it's no longer in preview! Virtual threads make writing high-throughput concurrent applications dramatically easier.

Why it matters:

• Traditional platform threads (OS threads) are expensive — limited in number and heavy on memory.
• Virtual threads are lightweight , managed by the JVM, and can scale to millions per process.
• You don't need to rewrite your code — just use Thread.startVirtualThread(Runnable) or run your existing ExecutorService tasks on a virtual thread factory.

Example:

 try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 10_000).forEach(i -> {
        executor.submit(() -> {
            Thread.sleep(Duration.ofSeconds(1)); // No blocking issues!
            System.out.println("Task " i " done");
            return null;
        });
    });
}
// Runs fast, no thread pool tuning needed

Tip: Use virtual threads for I/O-heavy tasks (web servers, DB calls, APIs) — not CPU-bound work.

? 2. Structured Concurrency (Final)

Complements virtual threads by making concurrent code easier to reason about and debug.

What it does:

• Groups related threads into a single unit of work.
• Ensures all child threads complete or fail together — no more "zombie threads".
• Catches exceptions cleanly and propagates them to the caller.

Example:

 try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Future<String> user = scope.fork(() -> fetchUser());
    Future<Integer> score = scope.fork(() -> fetchScore());

    scope.join(); // Wait for both
    scope.throwIfFailed(); // Propagate any exception

    return new Result(user.resultNow(), score.resultNow());
}

Think of it like try-with-resources for concurrency — clean, predictable, and safe.

? 3. String Templates (Preview)

A modern alternative to String.format() and concatenation — think of it as “f-strings for Java”.

Why you'll like it:

• Safer than String.format() (no runtime errors from mismatched args).
• More readable than or StringBuilder .
• Supports interpolated expressions with compile-time validation.

Example:

 String name = "Alice";
int score = 95;

// Old way
String msg = String.format("User %s scored %d", name, score);

// New way with STR template
String msg = STR."User \{name} scored \{score}";

Also supports raw templates ( FMT ) for formatting:

 double price = 19.99;
String formatted = FMT."Price: \{price,.2f}"; // "Price: 19.99"

Still in preview — but likely to become standard soon.

? 4. Sequenced Collections API (Final)

Adds consistent methods like getFirst() , getLast() , and reversed() to all ordered collections.

Before Java 21:

 List<String> list = new ArrayList<>();
String last = list.get(list.size() - 1); // Ugly

After Java 21:

 List<String> list = new ArrayList<>();
String first = list.getFirst();
String last = list.getLast();
SequencedCollection<String> reversed = list.reversed();

Works with List , Deque , and any collection that has a defined encounter order.

? 5. Other Notable Additions

• Scoped Values (Preview): Safer alternative to thread-local variables — especially useful with virtual threads.
• Record Patterns (Final): Simplify destructuring records in instanceof or switch .
• Pattern Matching for switch (Final): Match on types, records, and more inside switch expressions.
• Generational ZGC (Experimental): Improves GC pause times by separating young/old objects — great for latency-sensitive apps.

? What Should You Do Now?

• ? Upgrade to Java 21 — especially if you're on Java 17 or earlier.
• ? Start using virtual threads in I/O-heavy apps — huge perf gains with minimal code changes.
• ? Try string templates in new code — they're more readable and less error-prone.
• ? Use structured concurrency for any new async logic — it makes debugging easier.

Java 21 isn't just about new syntax — it's about making concurrency simple, safe, and scalable . These features reduce boilerplate, improve performance, and make Java feel modern again.

Basically, if you're writing Java in 2024 , Java 21 is where you want to be.

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