URL params

URL structure

To make parts of the url dynamic in Javalin, we can use curly braces like so:

app.get("/users/{x}/{y}/{z}", ctx -> {
  var x = ctx.pathParam("x");
  var y = ctx.pathParam("y");
  var z = ctx.pathParam("z");

  System.out.println(x);
  System.out.println(y);
  System.out.println(z);
});

Now, a GET request to /users/3/awesome/true would make the server log:

3
awesome
true

WARNING

Note that these are all strings. We need to parse them to use them as their intended data type, e.g.

var z = Boolean.parseBoolean(ctx.pathParam("z"));

Using URL params

A practical application of URL params is to create dynamic endpoints, where users of the API can request particular rows from a table.

server

app.get("/users/{userId}", ctx -> {
  var id = Integer.parseInt(ctx.pathParam("userId"));
  var user = UserRepository.findById(id);
  ctx.json(user);
})

client

> GET /users/13 HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/8.5.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Date: Thu, 18 Jul 2024 10:52:45 GMT
< Content-Type: application/json
< Content-Length: 212
<
{
  "id": 13,
  "username": "EdShearing",
  "verified": 1
}

Note that we could respond with a 404 Not Found in case there is no such user:

if (user == null) {
  ctx.status(404);
  return;
}