Java 25 — Non-Obvious Deltas

Baseline Java fluency is assumed. This skill holds only facts commonly stated wrong because of stale (2023-era) advice.

Feature status on Java 25 (LTS)

Virtual-thread fan-out has NO cap

newVirtualThreadPerTaskExecutor() spawns one VT per submit, unbounded. For large batches against a finite downstream (HTTP connection pool, rate-limited upstream, DB pool), bound the resource — not the threads — or the pool fails with e.g. reactor-netty Pending acquire queue has reached its maximum size:

Semaphore gate = new Semaphore(props.concurrency());
var futures = items.stream()
    .map(i -> CompletableFuture.supplyAsync(() -> {
        gate.acquireUninterruptibly();
        try { return fetchOne(i); } finally { gate.release(); }
    }, virtualThreadExecutor))
    .toList();

• A *-concurrency config property bounds nothing by itself — before adding one, grep for its getter to confirm a real Semaphore consumer exists.
• Cap ≤ the client's ConnectionProvider.maxConnections; validate empirically with a representative batch.
• Small fan-outs (a handful of parallel calls) need no gate — don't add one.