Writing QUIC in Pure Java

I maintain gumdrop , an async, non-blocking Java server framework. Last year I wanted to add HTTP/3 support, and ran into a wall: the Java ecosystem essentially doesn't have QUIC. The JDK's own experimental support (JEP 517) is client-only. Netty gets HTTP/3 by shelling out to quiche + BoringSSL over JNI — which works, but you're back to native builds, platform-specific binaries, and a C library sitting underneath your "pure Java" framework. I used that approach first. It was clumsy enough that I went looking for a pure-Java alternative. There's exactly one: Kwik. But Kwik is blocking per connection — one thread per QUIC connection. That's a non-starter for a framework built around single-threaded selector loops handling tens of thousands of concurrent connections. So I wrote a QUIC implementation from scratch: packet protection, loss detection and NewReno congestion control, connection migration, 0-RTT, QPACK, an HTTP/3 client and server — all driven by the same non-blocking event loop as everything else in gumdrop. Collaboration note: TLS 1.3 comes from Agent15 — also from Kwik's author, Peter Doornbosch, but just the handshake layer, not the connection model. We're currently working together on making PQC — hybrid key exchange and signatures — the default there. Why the thread model matters The reason this mattered beyond HTTP/3: gumdrop isn't a web framework with QUIC bolted on, it's a general async I/O framework, and QUIC is just a transport. One thread per connection is exactly the model gumdrop exists to avoid — it caps concurrency at your thread pool, not your file descriptors, and it's the reason a "just use Kwik" fix was never really on the table. The same QUIC stack backs DNS-over-QUIC (DoQ) as a first-class DNS transport alongside DoT, DoH, UDP, and TCP — and the DNS resolver itself is fully async, with no blocking InetAddress.getByName() anywhere in the I/O path, which is its own small miracle in Java. HTTP, SMTP, IMAP, POP3, FTP, MQTT, SOCKS — it's the