Flash

A high-performance HTTP/1.1 server library for Java 21, built around virtual threads and a zero-allocation FSM router.

Modules

Module	Description
flash	Core server library — router, request parser, HTTP I/O transport
flash-extensions/flash-ext-jackson	Jackson JSON integration
flash-extensions/flash-ext-openapi	OpenAPI 3.0 spec + Swagger UI
flash-extensions/flash-ext-oidc	OIDC Authorization Code + PKCE flow
flash-extensions/flash-ext-mcp	MCP (Model Context Protocol) server — Streamable HTTP, optional OAuth2 via flash-ext-oidc
flash-extensions/flash-ext-view-core	Minimal shared SSR runtime primitives
flash-extensions/flash-ext-view-jte	Opinionated jte SSR extension
flash-extensions/flash-ext-view-thymeleaf	Opinionated Thymeleaf SSR extension
flash-bench	Demo harness (OIDC + OpenAPI + Jackson)

Requirements

• Java 21+
• Maven 3.8+

Quick start

FlashApp.create(8080)
    .get("/ping", (req, res) -> "pong")
    .start();

With full configuration:

FlashApp.create(
    FlashConfiguration.builder()
        .port(8080)
        .host("0.0.0.0")
        .maxHeaderBufferSize(65536)
        .build()
)
.get("/ping", (req, res) -> "pong")
.start();

Route registration

Lambda routes

FlashApp app = FlashApp.create(8080);

app.get("/hello", (req, res) -> "world");

app.post("/echo", (req, res) -> {
    byte[] body = req.body().bytes();
    return res.status(200).body(body);
});

app.get("/users/{id}", (req, res) -> {
    String id = req.pathParam("id");
    return "user:" + id;
});

Class-based handlers

Extend RequestHandler, annotate it, then scan its package. Dependencies are cached in onInit() after Flash has resolved its complete boot-time service graph:

@GET("/api/users")
public class ListUsers extends RequestHandler {
    private UserService users;

    @Override protected void onInit() { users = require(UserService.class); }
    @Override public Object handle(Request req, Response res) { return users.list(); }
}

app.scan("dev.example.api");

Middleware

Apply middleware at registration. Flash composes the final chain at boot:

Middleware authCheck = next -> (req, res) -> {
    if (req.header("Authorization") == null)
        return res.status(401).body("Unauthorized");
    return next.handle(req, res);
};

app.get("/secure", (req, res) -> "secret data", authCheck);

Multiple middlewares are composed outermost-first (left-to-right in the call):

app.get("/admin", handler, logging, auth, rateLimit);
// execution order: logging → auth → rateLimit → handler

Classpath scan

Scans a package for classes that extend RequestHandler and carry @Route. Each is instantiated via its public no-arg constructor:

app.scan("dev.example.handlers");

Namespace mounting

Mount a scoped sub-router under a prefix. All routes registered inside the scope get the prefix prepended automatically. The scope inherits the parent's extension context (annotation processors, services):

app.mount("/api", scope -> {
    scope.get("/health", (req, res) -> "ok");          // → GET /api/health
    scope.scan("dev.example.api");
});

Extensions

Extensions have one declarative configure method. They declare services, processors and route callbacks; Flash resolves the complete graph, materialises routes, compiles both routers, then opens listeners. Extension install order never makes a service “not ready”.

FlashApp.create(8080)
    .install(new JacksonExtension())
    .install(new OpenApiExtension("/openapi", "My API", "1.0.0"))
    .install(new OidcExtension(oidcConfig))
    .scan("dev.example.handlers")
    .start();

See extension-specific READMEs for full details:

• flash-ext-jackson
• flash-ext-openapi
• flash-ext-oidc
• flash-ext-mcp
• flash-ext-view-jte
• flash-ext-view-thymeleaf

Error handlers

app.onNotFound((req, res) -> res.status(404).body("Not found: " + req.path()));

app.onException((ex, req, res) -> {
    if (ex instanceof IllegalArgumentException)
        return res.status(400).body(ex.getMessage());
    return res.status(500).body("Internal error");
});

FlashConfiguration

Field	Default	Description
port	—	TCP port to bind
host	"0.0.0.0"	Bind address
tls	null	TLS for the default listener — see TLS
listeners	[]	Multiple bind targets (port + host + optional TLS) on one app — see TLS
maxHeaderBufferSize	65536	Max size of the header buffer (bytes)

TLS

HTTPS and WSS are a transport-layer concern only: once a listener is bound, the accepted Socket is either plain or an SSLSocket indistinguishably from HttpServer's point of view onward — the request parser, router, and WebSocket upgrade never branch on it. WSS is therefore not a separate feature; it's a WebSocket upgrade running over whatever transport it was handed.

Quick start

FlashApp.create(FlashConfiguration.builder()
        .port(443)
        .tls(TlsConfig.keystore(Path.of("cert.p12"), "changeit"))
        .build())
    .get("/ping", (req, res) -> "pong")   // HTTPS
    .ws("/live", handler)                 // WSS, same route API
    .start();

Multiple listeners

One app can bind any number of ports, each independently plain or TLS:

FlashApp.create(FlashConfiguration.builder()
        .listener(new FlashConfiguration.Listener(80))                                   // plain
        .listener(new FlashConfiguration.Listener(443, TlsConfig.keystore(cert, pass)))  // TLS
        .build());

A non-empty listeners list takes precedence over the top-level port/host/tls fields. Each listener gets its own accept threads; the router, WS router, and virtual-thread executor are shared by all of them — one app, N ports.

TlsConfig

Factory	Use
TlsConfig.keystore(Path, String)	Builds the SSLContext from a PKCS12/JKS keystore (type guessed from the extension). Pins TLSv1.2/TLSv1.3 as enabled protocols; cipher suites are left at the JDK's own curated default.
TlsConfig.ofContext(SSLContext)	Escape hatch — the given SSLContext is used exactly as built. Flash never calls setSSLParameters on this path beyond what you explicitly request via clientAuth/applicationProtocols, so anything else you configured (custom KeyManager, ALPN, cipher suites) is authoritative.

Chainable on either factory:

TlsConfig.keystore(cert, pass)
    .clientAuth(ClientAuth.REQUIRE)                    // mTLS: NONE (default) | OPTIONAL | REQUIRE
    .applicationProtocols("acme-tls/1", "http/1.1")    // ALPN, in preference order

SNI falls out of keystore() for free: a keystore holding more than one certificate entry is matched against the requested hostname by each certificate's SAN (falling back to CN) — no per-hostname config. The first entry in the keystore is the default when SNI is absent or matches nothing (same convention as nginx/HAProxy's default_server).

ALPN and custom certificate selection (e.g. TLS-ALPN-01 / RFC 8737 for on-demand ACME issuance): ALPN is resolved while consuming ClientHello/producing ServerHello, which always precedes Certificate production. A custom X509ExtendedKeyManager passed via ofContext can therefore read engine.getHandshakeApplicationProtocol() (or ((SSLSocket) socket).getHandshakeApplicationProtocol()) inside chooseEngineServerAlias/chooseServerAlias — the negotiated protocol is already resolved by then, so the certificate decision can key off it.

mTLS with a private CA: clientAuth(...) only requests/requires a client certificate; keystore() deliberately doesn't expose a way to configure which CAs are trusted for that certificate (it uses the JDK default trust store). For a private CA, build the SSLContext yourself with a TrustManagerFactory and use ofContext(...).

Reading TLS info from a request

app.get("/whoami", (req, res) -> {
    if (!req.isSecure()) return "plain";
    SSLSession session = req.sslSession();          // null iff !isSecure()
    X509Certificate peer = (X509Certificate) session.getPeerCertificates()[0]; // mTLS only
    return session.getCipherSuite() + " / " + session.getProtocol();
});

Request.isSecure() / Request.sslSession() cost nothing extra per request: the SSLSocket reference is threaded through once per connection (same mechanism as remoteAddress()), and sslSession() only calls SSLSocket#getSession() — a cached-field read once the handshake that got the request this far has already completed, never a forced handshake.

WebSocketSession mirrors this exactly (isSecure(), sslSession()) by delegating to the upgrading Request — no separate TLS state is tracked for WS.

Architecture

ServerSocket.accept()
  → RequestParser.parse()       # zero-alloc header parsing, buffer reuse across keep-alive
  → GlobalRouter.route()        # two-tier: mounted sub-routers (longest prefix) then FastPathRouterImpl
  → RequestHandler.handle()     # user handler; return value sets body
  → Request.drain()             # consume unread body for keep-alive
  → HttpServer writes response  # status line, headers, then fixed or chunked body
  → loop or close socket        # based on Connection header

• Virtual threads — each accepted socket runs on a virtual thread (Executors.newVirtualThreadPerTaskExecutor()). Java 21 required.
• Zero-allocation router — FastPathRouterImpl uses fpr-core, a byte-level FSM that matches on METHOD + path bytes with no per-request allocation.
• Keep-alive — RequestParser reuses its header buffer across requests on the same connection.
• Chunked transfer — both chunked request bodies (decoded via ChunkedInputStream) and chunked response bodies are supported.
• TLS is transport-only — see TLS. Listeners bind either a plain ServerSocket or an SSLServerSocket; nothing downstream of accept() branches on which.

Build & test

# Build all modules (skip tests)
mvn clean package -DskipTests

# Run all tests
mvn test

# Run a single test class
mvn test -pl flash -Dtest=RequestParserTest

# Run the benchmark demo server
java -jar flash-bench/target/flash-bench-1.0-SNAPSHOT.jar