Explore Javalin: A Lightweight Java Backend Framework with WebSocket & HTTP2

Javalin is a lightweight Java web framework. It supports WebSocket, HTTP/2 and asynchronous requests. It originated as a fork of SparkJava and was later influenced by the JavaScript framework koa.js.

Example: a more complex Hello World

var app = Javalin.create(config -> {
    config.defaultContentType = "application/json";
    config.autogenerateEtags = true;
    config.addStaticFiles("/public");
    config.asyncRequestTimeout = 10_000L;
    config.dynamicGzip = true;
    config.enforceSsl = true;
}).routes(() -> {
    path("users", () -> {
        get(UserController::getAll);
        post(UserController::create);
        path(":user-id", () -> {
            get(UserController::getOne);
            patch(UserController::update);
            delete(UserController::delete);
        });
        ws("events", userController::webSocketEvents);
    });
}).start(port);

Path parameter validation

var myQpStr = ctx.queryParam("my-qp"); // returns string or null
var myQpInt = ctx.pathParam("my-qp", Integer.class).get(); // returns int or throws
var myQpInt = ctx.formParam("my-qp", Integer.class).check(i -> i > 4).get(); // int > 4

// validate dependent query parameters
var fromDate = ctx.queryParam("from", Instant.class).get();
var toDate = ctx.queryParam("to", Instant.class)
        .check(it -> it.isAfter(fromDate), "'to' has to be after 'from'")
        .get();

// validate JSON body
var myObject = ctx.bodyValidator(MyObject.class)
        .check(obj -> obj.myObjectProperty == someValue)
        .get();

Handlers

// before handler
app.before(ctx -> {
    // runs before all requests
});
app.before("/path/*", ctx -> {
    // runs before /path/* requests
});

// endpoint handler
app.get("/", ctx -> {
    // some code
    ctx.json(object);
});
app.get("/hello/*", ctx -> {
    // captures all requests under /hello/
});

// after handler
app.after(ctx -> {
    // runs after all requests
});
app.after("/path/*", ctx -> {
    // runs after /path/* requests
});

AccessManager can be used for authentication and authorization.

To deploy a Javalin application, package it into a jar (e.g., using the maven-assembly-plugin) and run java -jar filename.jar. Javalin includes an embedded Jetty server, so no external application server is required.

Javalin provides educational pages highlighting its benefits for students, and a series of tutorials such as “Running on GraalVM” and “Kotlin CRUD REST API”. Documentation and downloads are available via Maven Central.

Deployment execution

Running mvn package creates a jar that can be started with java -jar xxx.jar.

Port 7000 conflict on macOS

When starting Javalin on macOS, the default port 7000 may be occupied. Use lsof -nP -i4TCP | grep 7000 to identify the process, then ps aux | grep 1578 to find the owning process. In this case, the system's ControlCenter was using the port; disabling it resolves the issue.

References: official documentation (https://javalin.io/documentation), tutorials (https://javalin.io/tutorials/), and the GraalVM example.