Java 21: The Features That Matter for Backend Engineers

Java 21: The Features That Matter for Backend Engineers

Java 21 is a Long-Term Support (LTS) release — the first since Java 17. It finalises several features that have been previewing for years.

Virtual Threads (Project Loom)

Virtual threads are lightweight, JVM-managed threads that cost ~1KB of heap instead of ~1MB for a platform thread. You can have millions.

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 100_000).forEach(i ->
        executor.submit(() -> {
            Thread.sleep(Duration.ofSeconds(1));
            return i;
        })
    );
}
// All 100,000 tasks complete in ~1 second

For Spring Boot, one property enables them everywhere:

spring:
  threads:
    virtual:
      enabled: true

Interview takeaway: Virtual threads shine for I/O-bound workloads (REST calls, DB queries). For CPU-bound work, they offer no benefit.

Records

Immutable data carriers without boilerplate:

record Point(int x, int y) {}
record User(String name, String email) {
    // Compact constructor for validation
    User {
        Objects.requireNonNull(name, "name must not be null");
    }
}

Pattern Matching for switch

String format(Object obj) {
    return switch (obj) {
        case Integer i -> "Integer: " + i;
        case String s  -> "String: " + s;
        case null      -> "null";
        default        -> "Unknown: " + obj;
    };
}

Sequenced Collections

New SequencedCollection interface standardises first/last access across all ordered collections:

List<String> list = new ArrayList<>(List.of("a", "b", "c"));
list.getFirst(); // "a"
list.getLast();  // "c"
list.reversed(); // ["c", "b", "a"] (view, not copy)