Exploring Javalin: A Lightweight Java Web Framework for Fast API Development

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

Basic Example

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);

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

// Pre‑handler (runs before every request)
app.before(ctx -> {
    // code here
});
app.before("/path/*", ctx -> {
    // code for specific path
});

// Endpoint handler
app.get("/", ctx -> {
    ctx.json(object);
});
app.get("/hello/*", ctx -> {
    // handle all /hello/* sub‑paths
});

// Post‑handler (runs after every request)
app.after(ctx -> {
    // code here
});
app.after("/path/*", ctx -> {
    // code for specific path
});

The AccessManager interface can be used to implement authentication and authorization.

Deployment

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

Port 7000 Conflict on macOS

When starting Javalin on macOS, the default port 7000 may be occupied by the system ControlCenter. The conflict can be identified with:

sudo lsof -nP -i4TCP | grep 7000
sudo ps aux | grep 1578

Disabling the ControlCenter process resolves the issue (see the screenshot below).

References

Official documentation: https://javalin.io/documentation

Official tutorials: https://javalin.io/tutorials/

Running Javalin on GraalVM: https://javalin.io/2018/09/27/javalin-graalvm-example.html