Dependencies

First, we need to create a project with these dependencies: (→ Tutorial)

<dependencies>
    <dependency>
        <groupId>io.javalin</groupId>
        <artifactId>javalin</artifactId>
        <version>2.4.0</version>
    </dependency>
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-simple</artifactId>
        <version>1.7.25</version>
    </dependency>
</dependencies>

Setting up the backend

Create a Java file, for example Main.java, that has the following code:

import java.util.HashMap;
import java.util.Map;

import io.javalin.Javalin;

public class Main {

    static Map<String, String> reservations = new HashMap<String, String>() {{
        put("saturday", "No reservation");
        put("sunday", "No reservation");
    }};

    public static void main(String[] args) {

        Javalin app = Javalin.create()
            .enableStaticFiles("/public")
            .start(7777);

        app.post("/make-reservation", ctx -> {
            reservations.put(ctx.formParam("day"), ctx.formParam("time"));
            ctx.html("Your reservation has been saved");
        });

        app.get("/check-reservation", ctx -> {
            ctx.html(reservations.get(ctx.queryParam("day")));
        });

    }

}

This will create an app which listens on port 7777, and looks for static files in your /src/resources/public folder. We have two endpoints mapped, one post, which will make a reservation, and one get, which will check your reservation.

Setting up the HTML forms

Now we have to make two HTML forms for interacting with these endpoints. We can put these forms in a file /resources/public/index.html, which will be available at http://localhost:7777/.

Make reservation form

<h2>Make reservation:</h2>
<form method="post" action="/make-reservation">
    Choose day
    <select name="day">
        <option value="saturday">Saturday</option>
        <option value="sunday">Sunday</option>
    </select>
    <br>
    Choose time
    <select name="time">
        <option value="8:00 PM">8:00 PM</option>
        <option value="9:00 PM">9:00 PM</option>
    </select>
    <br>
    <button>Submit</button>
</form>

To make a reservation we need to create something on the server (in this case it’s a simple map.put(), but usually you’d have a database). When creating something on the server, you should use the POST method, which can be specified by adding method="post" to the <form> element.

In our Java code, we have a post endpoint: app.post("/make-reservation", ctx -> {...}. We need to tell our form to use this endpoint with the action attribute: action="/make-reservation". Actions are relative, so when you click submit, the browser will create a POST request to http://localhost:7777/make-reservation with the day/time values as the request-body.

Check reservation form

<h2>Check your reservation:</h2>
<form method="get" action="/check-reservation">
    Choose day
    <select name="day">
        <option value="saturday">Saturday</option>
        <option value="sunday">Sunday</option>
    </select>
    <br>
    <button>Submit</button>
</form>

To check a reservation we need to tell the server which day we’re interested in. In this case we’re not creating anything, and our action does not change the state of the server in any way, which makes it a good candidate for a GET request.

GET requests don’t have a request-body so when you click submit the browser creates a GET request to http://localhost:7777/check-reservation?day=saturday. The values of the form are added to the URL as query-parameters.

HTML form GET vs POST summary

  • POST requests should be used if the request can change the server state.
  • POST requests have their information stored in the request-body. In order to extract information from this body you have to use ctx.formParam(key) in Javalin.
  • Performing a series of GET requests should always return the same result (if no other POST request was performed in-between).
  • GET requests have no request-body, and form information is sent as query-parameters in the URL. In order to extract information from this body you have to use ctx.queryParam(key) in Javalin.

File upload example

Let’s expand our example a bit to include file uploads.

Endpoint

app.post("/upload-example", ctx -> {
    ctx.uploadedFiles("files").forEach(file -> {
        FileUtil.streamToFile(file.getContent(), "upload/" + file.getName());
    });
});

ctx.uploadedFiles("files") gives us a list of files matching the name files. We then save these files to an upload folder.

HTML form

<h1>Upload example</h1>
<form method="post" action="/upload-example" enctype="multipart/form-data">
    <input type="file" name="files" multiple>
    <button>Submit</button>
</form>

When uploading files you need to add enctype="multipart/form-data" to your <form>. If you want to upload multiple files, add the multiple attribute to your <input>.