Java 21, an LTS release, introduces major enhancements that modernize the platform. 1. Virtual Threads are now final, enabling efficient, lightweight concurrency ideal for I/O-bound tasks, allowing simple synchronous code to scale. 2. Structured Concurrency (preview) improves error handling and cancellation by treating related tasks as a unit, enhancing reliability and debuggability. 3. Pattern Matching for switch is finalized, supporting type checks and record deconstruction to reduce boilerplate and improve clarity. 4. Sequenced Collections API (preview) adds ordered collection operations like getFirst(), getLast(), and reversed() for more intuitive handling of ordered data. 5. Unnamed Variables and Patterns (preview) allow using _ for unused variables, reducing clutter and warnings. 6. Performance and security improvements include ZGC with concurrent class unloading, Vector API updates, Foreign Function & Memory API previews, and stronger cryptographic defaults. Developers should upgrade from older LTS versions, adopt virtual threads for scalable services, use pattern matching for cleaner code, and explore structured concurrency and sequenced collections for future readiness, making high-performance concurrent programming more accessible.

Java 21, released in September 2023, marks a major milestone in the evolution of the Java platform. With long-term support (LTS) status, it brings a host of new features, performance improvements, and language enhancements that make Java more modern, expressive, and efficient. Whether you're a seasoned Java developer or just catching up, here's what’s new and why it matters.

1. Virtual Threads (Preview → Final)

One of the most anticipated features in Java 21 is the finalization of Virtual Threads, which were introduced as a preview in Java 19 and 20.

• What are Virtual Threads?
  They are lightweight threads managed by the JVM, not the operating system. Unlike platform threads (traditional java.lang.Thread), thousands or even millions of virtual threads can be created with minimal overhead.

  

• Why it matters:
  Virtual threads revolutionize how we write concurrent applications—especially server-side ones. They allow developers to write simple, synchronous code that scales like asynchronous code.

• How to use them:

  

  try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
      IntStream.range(0, 100).forEach(i -> {
          executor.submit(() -> {
              System.out.println("Task "   i   " on thread: "   Thread.currentThread());
              return i;
          });
      });
  } // executor.close() is called automatically

  This creates 100 virtual threads that run independently but consume far fewer OS resources than traditional threads.

Tip: Use virtual threads for I/O-bound tasks (like web servers or database calls), not CPU-heavy workloads.

2. Structured Concurrency (Preview)

Structured Concurrency simplifies error handling and cancellation in concurrent code by treating groups of related tasks as a single unit of work.

• It ensures that if one thread fails, all related threads are canceled, preventing resource leaks and simplifying debugging.

• Designed to work seamlessly with virtual threads.

• Example:

  try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
      Future<String>  user  = scope.fork(() -> fetchUser());
      Future<Integer> order = scope.fork(() -> fetchOrder());
  
      scope.join();           // Wait for both tasks
      scope.throwIfFailed();  // Propagate any failure
  
      String theUser = user.resultNow();
      int theOrder = order.resultNow();
  }

This model enforces structured programming principles in concurrency—making code more readable and resilient.

3. Pattern Matching for switch (Finalized)

Pattern matching, which started with instanceof improvements, is now fully integrated into switch expressions and statements.

In Java 21, this feature is finalized, meaning it’s production-ready and no longer in preview.

• You can now deconstruct data using pattern matching:

  String result = switch (obj) {
      case null          -> "null";
      case String s      -> "String: "   s;
      case Integer i     -> "Integer: "   i;
      case Point(int x, int y) when x > 0 -> "Positive point";
      case Point(int x, int y)            -> "Point: "   x   ","   y;
      default            -> "Unknown";
  };

• Works with record patterns and type patterns.

• Reduces boilerplate and makes code more declarative.

4. Sequenced Collections API (Preview)

Java 21 introduces a new interface hierarchy to better represent ordered collections.

• New interface: SequencedCollection extends Collection and adds methods like:

  • getFirst(), getLast()
  • addFirst(), addLast()
  • removeFirst(), removeLast()

• Implementations include ArrayList, LinkedHashSet, etc.

• Also introduces SequencedMap:

  • getFirstEntry(), getLastEntry()
  • reversed() — returns a reverse-ordered view

• Example:

  SequencedCollection<String> collection = new ArrayList<>();
  collection.add("one");
  collection.add("two");
  
  System.out.println(collection.getFirst()); // "one"
  System.out.println(collection.getLast());  // "two"
  
  SequencedMap<Integer, String> map = new LinkedHashMap<>();
  map.put(1, "first");
  map.put(2, "second");
  System.out.println(map.reversed().getFirstEntry()); // 2=second

This makes working with ordered data more intuitive and consistent.

5. Unnamed Variables and Patterns (Preview)

A small but useful addition: the ability to use _ as a placeholder for unused variables or pattern components.

• Helps reduce warnings and improve code clarity when you don’t care about certain values.

• Example:

  // Instead of:
  case Point(int x, int y) when x > 0 -> System.out.println("Positive x");
  
  // You can now ignore unused parts:
  case Point(int x, _) when x > 0 -> System.out.println("Positive x");
  
  // Or in lambda:
  list.forEach((_, __) -> System.out.println("Hello"));

This avoids naming dummy variables like temp, ignored, etc.

6. Performance & Security Improvements

Beyond syntax, Java 21 includes under-the-hood enhancements:

• ZGC (Z Garbage Collector): Now has support for concurrent class unloading, reducing pause times even further.
• Vector API (4th Incubator): Enables better utilization of SIMD instructions for high-performance computing.
• Foreign Function & Memory API (2nd Preview): Simplifies calling native code and managing off-heap memory—eventually replacing JNI.
• Security: Updated cryptography algorithms and stronger defaults.

These improvements make Java faster and safer, especially for large-scale systems.

Summary: What Should You Do?

• Upgrade to Java 21 if you're on an older LTS (like Java 11 or 17)—especially for web services where virtual threads can dramatically improve throughput.
• Start using virtual threads in new concurrent applications.
• Embrace pattern matching for cleaner, safer code.
• Experiment with structured concurrency and sequenced collections—they’re likely to become standard soon.

Java isn’t standing still. With Java 21, it’s becoming simpler, faster, and more expressive—without sacrificing stability.

Basically, this release is about making high-performance, concurrent programming accessible to every developer, not just experts.

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