Stable values

Thread-safe lazy initialization without volatile or synchronized.

Code Comparison

✕ Java 8

private volatile Logger logger;
Logger getLogger() {
    if (logger == null) {
        synchronized (this) {
            if (logger == null)
                logger = createLogger();
        }
    }
    return logger;
}

✓ Java 25 (Preview)

private final StableValue<Logger> logger =
    StableValue.of(this::createLogger);

Logger getLogger() {
    return logger.get();
}

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Why the modern way wins

🧹

Zero boilerplate

No volatile, synchronized, or null checks.

⚡

JVM-optimized

The JVM can fold the value after initialization.

🛡️

Guaranteed once

The supplier runs exactly once, even under contention.

Old Approach

Double-Checked Locking

Modern Approach

StableValue

Since JDK

Difficulty

Advanced

JDK Support

Stable values

Preview

Preview in JDK 25 (JEP 502). Requires --enable-preview.

How it works

StableValue provides a lazily initialized, immutable value with built-in thread safety. No double-checked locking, no volatile fields, no synchronized blocks. The JVM can even optimize the read path after initialization.

Virtual threads

Java 8

Thread thread = new Thread(() -> {
    System.out.println("hello");
});
thread.start();
thread.join();

Java 21+

Thread.startVirtualThread(() -> {
    IO.println("hello");
}).join();

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JDK 21+ →

Concurrency Advanced

Structured concurrency

Java 8

ExecutorService exec =
    Executors.newFixedThreadPool(2);
Future<User> u = exec.submit(this::fetchUser);
Future<Order> o = exec.submit(this::fetchOrder);
try {
    return combine(u.get(), o.get());
} finally { exec.shutdown(); }

Java 25 (Preview)

try (var scope = new StructuredTaskScope
        .ShutdownOnFailure()) {
    var u = scope.fork(this::fetchUser);
    var o = scope.fork(this::fetchOrder);
    scope.join().throwIfFailed();
    return combine(u.get(), o.get());
}

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JDK 25+ →

Concurrency Advanced

Scoped values

Java 8

static final ThreadLocal<User> CURRENT =
    new ThreadLocal<>();
void handle(Request req) {
    CURRENT.set(authenticate(req));
    try { process(); }
    finally { CURRENT.remove(); }
}

Java 25

static final ScopedValue<User> CURRENT =
    ScopedValue.newInstance();
void handle(Request req) {
    ScopedValue.where(CURRENT,
        authenticate(req)
    ).run(this::process);
}

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JDK 25+ →