Schema validation

Validating params

We can use Javalin's built-in validation techniques to parse and validate params.

// UserController.java

int limit = ctx.queryParam("limit") != null
    ? Integer.parseInt(ctx.queryParam("limit")) 
    : 10; 

int offset = ctx.queryParam("offset") != null
    ? Integer.parseInt(ctx.queryParam("offset")) 
    : 0; 

int limit = ctx.queryParamAsClass("limit", Integer.class).getOrDefault(10); 
int offset = ctx.queryParamAsClass("offset", Integer.class).getOrDefault(0); 

This has a few benefits:

• it will automatically respond with a 400 if the param can't be parsed into an integer

• a reasonable error message will be generated automatically for the client.

Validating a body

It is important to ensure that users of the API are sending the correct data.

We can validate the body of a HTTP request to make sure it adheres to the correct schema.

// UserController.java

public static void createUser(Context ctx) throws Exception {
  UserRequest body = ctx.bodyValidator(UserRequest.class)
      .check(data -> data.username().length() > 2, "username must be at least 3 characters")
      .check(data -> data.username().length() < 20, "username must be less than 20 characters")
      .get();

  var user = UserRepository.create(body.username(), body.verified());

  ctx.status(201);
  ctx.json(user);