vok

Writing full-stack statically-typed web apps on JVM can't get any simpler

This project is maintained by mvysny

Index Getting Started Guides

Getting started with Vaadin-on-Kotlin Vaadin 8

VoK is an opinionated web application framework which uses Vaadin for its front-end and Kotlin for its back-end. While Vaadin lets you forget the web and program user interfaces and let’s you program much like when creating a desktop application with conventional Java toolkits such as AWT, Swing, or SWT, Kotlin allows you to write clean understandable and save code.

This guide teaches you how to get Vaadin On Kotlin (VoK) up and running. It introduces the basic concepts needed to create your own applications.

Tribute: I got heavily inspired by the excellent Ruby on Rails tutorial.

After reading this guide, you will be able to:

Note: To skip the introduction and dive straight into the code, go to Chapter 3.2

Guide Assumptions

I assume you have no prior knowledge of VoK and your goal is to create a VoK application from scratch. Though you don’t need to be a seasoned programmer to follow through, I assume a basic working knowledge of Kotlin and Vaadin.

If you have no prior experience with Kotlin or Vaadin, you might get overwhelmed by the sheer amount of new information. Therefore, I recommend to take it slowly and get familiar with both Vaadin and Kotlin first:

I encourage you to experiment with the Karibu-DSL Hello World Example at any point since VoK uses Karibu-DSL under the hood, therefore the lessons learned in the Karibu-DSL Hello World example will come in handy later on in all VoK-based apps.

What is Vaadin-on-Kotlin?

VoK is a glue between Vaadin, Kotlin and other frameworks which allows you to write web apps smoothly. It is designed to make the art of programming of web applications easier by making assumptions about what every developer needs to get started. It allows you to write less code while accomplishing more than many other languages and frameworks.

VoK is opinionated software. It makes the assumption that there is a “best” way to do things, and it’s designed to encourage that way - and in some cases to discourage alternatives.

The VoK philosophy includes three major guiding principles:

Notes For Java Programmers

Traditionally both JavaEE and Spring acted as this “glue” which held various frameworks together. But, with the advent of the Kotlin programming language, we believe that the features of the Kotlin programming language alone are all that’s necessary in the modern programming. We believe that the era of traditional approach of using the Dependency Injection, Annotations and auto-discovery magic is over and the Kotlin programming language itself is all that’s needed to glue stuff together.

While the Dependency Injection (DI) itself is not hard to understand, it comes with unfortunate consequences:

Therefore, VoK itself is not using DI; you can of course use Spring or JavaEE in your project alongside VoK if necessary.

Note on MVC: The Model-View-Controller pattern is very popular with page-oriented frameworks such as Ruby on Rails, Groovy on Grails and Python Django. There is typically lot of things going on in the page, and the MVC pattern helps keeping the code separated into smaller, more easily digestable packages.

However, since Components are a much smaller unit of reuse than pages, employing MVC with Component-oriented frameworks does not make that much sense: for example it will usually leave you with nearly empty Views. We thus believe that using MVC does more harm than good since it adds unnecessary complexity. Therefore this tutorial will not use MVC.

Creating a New VoK Project

The best way to read this guide is to follow it step by step. All steps are essential to run this example application and no additional code or steps are needed.

By following along with this guide, you’ll create a VoK project called blog, a (very) simple weblog. Before you can start building the application, you need to make sure that you have Java 8 JDK installed.

Prerequisites

Make sure you have Java 8 JDK installed.

The example application has Gradle bundled in; Gradle will then download everything else (Vaadin, Kotlin, libraries, the Jetty server which is used to run the app from the command line). This makes VoK applications really portable since they work flawlessly on any OS and CPU which supports Java 8 - be it Windows, Linux or Mac, on x86, ARM or others.

The example application also uses an embedded Java database called H2, so there is no need for you to set up any database.

While it is possible to edit the project files using any text editor, we recommend to install Intellij IDEA which provides awesome Kotlin support including auto-completion. You can use IDEA Community edition, which is free and allows you to run gradle tasks to run the app, or you can purchase the Ultimate edition which also supports debugging/hot-redeployment of the web app on Tomcat and other servers, and also offers awesome database integration tools.

Creating the Blog Application

Getting the example application is really easy. If you have Git installed, just open a command line and run the following command:

$ git clone https://github.com/mvysny/vok-helloworld-app

If you don’t have Git, you can simply download the application as a zip file from GitHub: https://github.com/mvysny/vok-helloworld-app/archive/master.zip.

After you create the blog application, switch to its folder:

$ cd vok-helloworld-app
$ ./gradlew

This will download everything necessary and will compile the example application’s WAR file.

Note: WAR (Web ARchive) is Java standard for packaging web applications. WAR file is a zip file which can be deployed to all Java Servlet Servers, including Tomcat, JBoss etc.

The vok-helloworld-app directory has a number of files and folders that make up the structure of a VoK application. Most of the work in this tutorial will happen in the web/src/main/kotlin folder, but here’s a basic rundown on the function of each of the files and folders:

File/Folder Purpose
web/src/main/kotlin Contains the source files of all of your Views, Servlets, REST endpoints, async jobs for your application. You’ll focus on this folder for the remainder of this guide.
web/src/main/webapp Contains the Vaadin Theme (a SCSS-based theme which gets compiled to CSS). All Themes inherit from the Valo Theme. JavaScript files, additional CSS files and images are also placed here.
web/src/main/resources Contains the logger configuration file (logback.xml)
build.gradle This file defines tasks that can be run from the command line. You should add your own tasks by adding code to this file. There is much you can do with Gradle - you can for example use the ssh plugin to deploy the WAR to your production environment.
README.md This is a brief instruction manual for your application. You should edit this file to tell others what your application does, how to set it up, and so on.
.gitignore This file tells git which files (or patterns) it should ignore. See Github - Ignoring files for more info about ignoring files.

Hello, Vaadin-on-Kotlin!

To begin with, let’s get some text up on screen quickly. To do this, you need to get a web server running.

Starting up the Web Server

You actually have a functional VoK application already. To see it, you need to start a web server on your development machine. You can do this by running the following in the vok-helloworld-app directory:

$ ./gradlew clean web:appRun

This will fire up Jetty, an embeddable Java web server. To see your application in action, open a browser window and navigate to http://localhost:8080. You should see the Vaadin-on-Kotlin default information page:

Welcome VoK

Note: To stop the web server, hit Ctrl+C in the terminal window where it’s running. To verify the server has stopped you should see your command prompt cursor again. For most UNIX-like systems including macOS this will be a dollar sign $.

Note: changes in theme files will only be propagated when you are running ./gradlew clean web:appRun and there is no styles.css file. If there is, your changes will be ignored until you compile the theme again, by running ./gradlew vaadinThemeCompile. Just delete the styles.css file, to apply changes to your styles immediately as you edit them.

Changes made in your Kotlin files will be propagated to the running server only after you compile them, by running ./gradlew build.

The “Welcome aboard” page is the smoke test for a new VoK application: it makes sure that you have your software configured correctly enough to serve a page.

Say “Hello”, Vaadin

To get VoK saying “Hello”, you need to create a View.

A View’s purpose is to provide a Vaadin Component (usually a Layout containing other components), which then interacts with the user. The Navigator decides which View receives which requests. There is exactly one route to a View. You can collect the data to be displayed right in the View itself (for small applications), or you can define so-called Service layer (a group of regular Kotlin classes which define a clear API and are responsible for fetching of the data). VoK however does not enforce this, and we will not use this pattern in the tutorial.

All Vaadin Components have three parts:

For example, a Button client-side (VButton) contains the logic to send the notification about the mouse click server-side; server-side Button allows you to register listeners which listen for button clicks.

Another example: VGrid shows a list of data in tabular fashion; it performs scrolling and fetching of the data as the user scrolls, via the Connector. Server-side Grid allows you to set the DataProvider which will actually fetch the data, from the database or from anywhere, depending on how you implement it.

To create a new View, all that’s needed is to create a Kotlin class which implements the View interface and extends some Vaadin Component.

Create the web/src/main/kotlin/com/example/vok/MyWelcomeView.kt file and make sure it looks like follows:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.vaadin.navigator.View
import com.vaadin.ui.VerticalLayout
import com.vaadin.ui.themes.ValoTheme

@AutoView("")
class MyWelcomeView: VerticalLayout(), View {
    init {
        label("Hello, Vaadin-on-Kotlin!") {
            styleName = ValoTheme.LABEL_H1
        }
    }
}

Setting the Application Home Page

Now that we have made the view, we need to tell VoK when we want “Hello, Vaadin-on-Kotlin!” to show up. In our case, we want it to show up when we navigate to the root URL of our site, http://localhost:8080. At the moment, “Welcome aboard” is occupying that spot.

Open up the WelcomeView.kt file and change the @AutoView("") annotation to the following: @AutoView("old-welcome"). This will map the original “Welcome aboard” page to http://localhost:8080/old-welcome , making space for our new Hello page.

Having the @AutoView("") on MyWelcomeView will tell the VoK Navigator to map requests to the root of the application to the MyWelcomeView view.

Launch the web server again and navigate to http://localhost:8080 in your browser. You’ll see the “Hello, Vaadin-on-Kotlin!” message you put into the web/src/main/kotlin/com/example/vok/MyWelcomeView.kt, indicating that this new Navigator route is indeed going to MyWelcomeView and is rendering the view correctly.

Getting Up and Running

Now that you’ve seen how to create a view, let’s create something with a bit more substance.

In the Blog application, you will now create a new database table, or in REST terms, a resource. A resource is the term used for a collection of similar objects, such as articles, people or animals. You can create, read, update and destroy items for a resource and these operations are referred to as CRUD operations.

VoK provides a resources method which can be used to declare a standard REST resource. But first, let us define the article. Create the web/src/main/kotlin/com/example/vok/Article.kt file with the following contents:

package com.example.vok

import com.github.vokorm.*

data class Article(
        override var id: Long? = null,
        var title: String? = null,
        var text: String? = null
) : Entity<Long> {
    companion object : Dao<Article>
}

This will define a so-called entity class, which basically represents a row in the “Article” database table.

We can now implement the REST endpoint for REST clients to access the article resource.

Note: This step is completely optional and is actually not used by Vaadin, since Vaadin connectors use its internal JSON protocol (called UIDL) to communicate with components. Having REST may come handy though, since we can use it to examine the state of the database (using the curl or wget tools).

Just create a file web/src/main/kotlin/com/example/vok/ArticleRest.kt which will look as follows:

package com.example.vok

import com.github.mvysny.karibudsl.v8.getAll
import com.github.vokorm.*
import io.javalin.Javalin
import io.javalin.NotFoundResponse

fun Javalin.articleRest() {
    get("/rest/articles/:id") { ctx ->
        val id = ctx.pathParam("id").toLong()
        ctx.json(Article.findById(id) ?: throw NotFoundResponse("No article with id $id"))
    }
    get("/rest/articles") { ctx -> ctx.json(Article.findAll()) }
}

In order to take these REST endpoints into use, in the Bootstrap.kt, edit the configureRest() function at the end of the file and make sure it calls our articleRest() function:

fun Javalin.configureRest(): Javalin {
    val gson = GsonBuilder().create()
    gson.configureToJavalin()
    articleRest()
    return this
}

This will add the possibility to retrieve the articles via a REST call. Just restart the server and try

$ curl localhost:8080/rest/articles

You will get Internal server error; the server log will show a long stacktrace, with the most interesting part being

Caused by: org.h2.jdbc.JdbcSQLException: Table "ARTICLE" not found; SQL statement:
select * from Article [42102-196]
	at org.h2.message.DbException.getJdbcSQLException(DbException.java:345)
	at org.h2.message.DbException.get(DbException.java:179)
	at org.h2.message.DbException.get(DbException.java:155)

That is to be expected since we haven’t yet created the table for Articles. We’ll do that in a minute. In the next section, you will add the ability to create new articles in your application and be able to view them. This is the “C” and the “R” from CRUD: create and read. The form for doing this will look like this:

Create Article Screenshot

It will look a little basic for now, but that’s ok. We’ll look at improving the styling for it afterwards.

Laying down the groundwork

Firstly, you need a place within the application to create a new article. A great place for that would be at create-article. Navigate to http://localhost:8080/create-article and you’ll see a general error:

Navigator Error

This happens because there is no View yet, mapped to the create-article route.

The first form

The solution to this particular problem is simple: create a Kotlin file named web/src/main/kotlin/com/example/vok/CreateArticleView.kt as follows:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.vaadin.navigator.*
import com.vaadin.ui.VerticalLayout
import com.vaadin.ui.themes.ValoTheme

@AutoView
class CreateArticleView: VerticalLayout(), View {
    private val binder = beanValidationBinder<Article>()
    init {
        label("New Article") {
            styleName = ValoTheme.LABEL_H1
        }
        textField("Title") {
            bind(binder).bind(Article::title)
        }
        textArea("Text") {
            bind(binder).bind(Article::text)
        }
        button("Save Article") 
    }
}

If you restart the server and refresh the page now, you’ll see the exact same form from our example above. Building forms in VoK is really just that easy!

There is a problem with the form though - when you click the “Save Article” button, nothing will happen. Currently, the click listener is empty, we will need to add the database code to save the article.

Creating articles

To make the “Save Article” button do something, just change the class as follows:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.vaadin.navigator.*
import com.vaadin.ui.VerticalLayout
import com.vaadin.ui.themes.ValoTheme

@AutoView
class CreateArticleView: VerticalLayout(), View {
    private val binder = beanValidationBinder<Article>()
    init {
        label("New Article") {
            styleName = ValoTheme.LABEL_H1
        }
        textField("Title") {
            bind(binder).bind(Article::title)
        }
        textArea("Text") {
            bind(binder).bind(Article::text)
        }
        button("Save Article") {
            onLeftClick {
                val article = Article()
                if (binder.writeBeanIfValid(article)) {
                    article.save()
                }
            }
        }
    }
}

Now when you click the “Save Article” button, you’ll see the good old Oops error - it’s because we haven’t created the database table for Article yet.

Creating the Article model

Luckily, we have already created the model - it’s the Article entity class. We will use VoK-ORM which will map the Article object to a relational database. By default it will map to the “Article” table. To create the table, we will have to create the migration.

Note: Sql2o is smart enough to automatically map column names to the Article class properties, which means you don’t have to provide the database name for every property inside entities, as that will be done automatically by Sql2o.

To create the migration, create a file named V01__CreateArticle.sql in the web/src/main/resources/db/migration directory, with the following contents:

create table Article(
  id bigint auto_increment PRIMARY KEY,
  title varchar(200) NOT NULL,
  text varchar(4000) NOT NULL
);

This is a SQL data definition (DDL) script which will create a table named Article with three columns. We are using the H2 database SQL dialect here.

Running a Migration

As we’ve just seen, migrations are simple SQL scripts which create and modify database tables. The database migration is done automatically, on the web app startup, by the Bootstrap class. You can also reconfigure your app to do the migrations manually instead, simply by commenting out relevant part of the Bootstrap class and altering the build.gradle file as stated in the Flyway Gradle documentation.

If you look in the V01__CreateArticle.sql file, you’ll see the V01 prefix, followed by two underscores, and then the name. The name may be arbitrary, and it doesn’t affect the migration outcome. The number defines the ordering - migration scripts will be run sorted by the version number, and exactly once. You can read more about the exact numbering rules in the Flyway Versioned Migrations Guide.

When Flyway runs this migration it will create an articles table with one string column and a text column.

At this point, you can simply kill and restart the server, to automatically run all migrations. Since we are currently using an in-memory H2 database, its contents are gone when the server is killed, and since we are starting with a fresh database, all migrations will run. When we’ll use a persistent database, Flyway will make sure that only a newly defined migrations are run.

Saving data in the CreateArticleView

Back in CreateArticleView view, everything is ready. Try clicking the “Save Article” button - seemingly nothing will happen, but the article will be saved into the database. To actually see the article, we will redirect to an ArticleView which we’ll define later.

Note: You might be wondering why the A in Article is capitalized above, whereas most other references to articles in this guide have used lowercase. In this context, we are referring to the class named Article that is defined in web/src/main/kotlin/com/example/vok/Article.kt. Class names in Kotlin must begin with a capital letter.

Note: As we’ll see later, binder.writeBeanIfValid() returns a boolean indicating whether the article was saved or not.

Showing Articles

If you submit the form again now, VoK will just stay on the form. That’s not very useful though, so let’s add the show action before proceeding.

Vaadin Navigator supports adding parameters after the view name. This way, we can pass the Article ID to the ArticleView as follows: http://localhost:8080/article/12. As we did before, we need to add the web/src/main/kotlin/com/example/vok/ArticleView.kt file:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.github.vokorm.getById
import com.vaadin.navigator.*
import com.vaadin.ui.*

@AutoView
class ArticleView: FormLayout(), View {
    private val title: Label
    private val text: Label
    init {
        title = label {
            caption = "Title:"
        }
        text = label {
            caption = "Text:"
        }
    }
    override fun enter(event: ViewChangeListener.ViewChangeEvent) {
        val articleId = event.parameterList[0]?.toLong() ?: throw RuntimeException("Article ID is missing")
        val article = Article.getById(articleId)
        title.value = article.title
        text.value = article.text
    }

    companion object {
        fun navigateTo(articleId: Long) = navigateToView<ArticleView>(articleId.toString())
    }
}

A couple of things to note. We use Article.getById(id) to find the article we’re interested in, passing in event.parameterList[0] to get the first parameter from the request. In Vaadin, parameters are slash-separated and are not named. The parameter is passed to the navigateToView<>() function which takes the view class and a list of string parameters as its input, constructs the target URL and redirects the browser to the URL. In this case, http://localhost:8080/article/12.

The Navigator then detects that the URL has been changed, it parses the view name out of the URL and invokes the view.enter() method. You can retrieve the parameter list using event.parameterList map.

To navigate to the Article View, just add ArticleView.navigateTo(article.id!!) to your CreateArticleView.kt file, right below the article.save() call as follows:

...
        button("Save Article") {
            onLeftClick {
                val article = Article()
                if (binder.writeBeanIfValid(article)) {
                    article.save()
                    ArticleView.navigateTo(article.id!!)
                }
            }
        }
...

With this change, you should finally be able to create new articles. Visit http://localhost:8080/create-article and give it a try!

Show Article

Listing all articles

We still need a way to list all our articles, so let’s do that. We’ll create the web/src/main/kotlin/com/examples/vok/ArticlesView.kt with the following contents:

package com.example.vok

import eu.vaadinonkotlin.vaadin8.sql2o.dataProvider
import com.github.mvysny.karibudsl.v8.*
import com.vaadin.navigator.*
import com.vaadin.ui.*
import com.vaadin.ui.themes.ValoTheme

@AutoView
class ArticlesView: VerticalLayout(), View {
    private val grid: Grid<Article>
    init {
        setSizeFull()
        label("Listing Articles") {
            styleName = ValoTheme.LABEL_H1
        }
        grid = grid(dataProvider = Article.dataProvider) {
            expandRatio = 1f; setSizeFull()
            addColumnFor(Article::id)
            addColumnFor(Article::title)
            addColumnFor(Article::text)
        }
    }
    override fun enter(event: ViewChangeListener.ViewChangeEvent?) {
        grid.refresh()
    }
}

Now if you go to http://localhost:8080/articles you will see a list of all the articles that you have created. Note that we have used the Grid component. Grid is a powerful tabular component which supports paging and lazy-loading of the data, including sorting and filtering.

You can now create, show, and list articles. Now let’s add some links to navigate through pages.

Open web/src/main/kotlin/com/example/vok/MyWelcomeView.kt and modify its init {} contents as follows:

    init {
        verticalLayout {
            label("Hello, Vaadin-on-Kotlin!") {
                styleName = ValoTheme.LABEL_H1
            }
            button("My Blog") {
                styleName = ValoTheme.BUTTON_LINK
                onLeftClick { navigateToView<ArticlesView>() }
            }
        }
    }

The Vaadin Button component can also act as a link. It thus creates a hyperlink based on text to display and where to go - in this case, to the path for articles.

Let’s add links to the other views as well, starting with adding this “New Article” link to ArticlesView class, placing it above the grid declaration:

        button("New Article") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { navigateToView<CreateArticleView>() }
        }

This link will allow you to bring up the form that lets you create a new article.

Now, add another link in CreateArticleView, underneath the form’s “Save Article” button, as the last line of the init{} block, to go back to the index action:

        button("Back") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { navigateToView<ArticlesView>() }
        }

Finally, add a link to the ArticleView view to go back to the index action as well (into the init{} block under those labels), so that people who are viewing a single article can go back and view the whole list again:

        button("Back") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { navigateToView<ArticlesView>() }
        }

Note: remember, when you are running the server via ./gradlew web:appRun, you will either need to kill the server and re-run again, or you’ll need to run ./gradlew build in another terminal, to actually see the outcome of your changes.

Adding Some Validation

The entity file, Article.kt is about as simple as it can get.

There isn’t much to this file - but both Sql2o and Vaadin-on-Kotlin supplies a great deal of functionality to your entities for free, including basic database CRUD (Create, Read, Update, Destroy) operations, data validation, as well as sophisticated search support and the ability to relate multiple models to one another.

Vaadin-on-Kotlin includes methods to help you validate the data that you send to models. Open the Article.kt file and edit it:

package com.example.vok

import com.github.vokorm.*
import eu.vaadinonkotlin.vaadin8.sql2o.*
import org.hibernate.validator.constraints.Length
import javax.validation.constraints.NotNull

data class Article(
        override var id: Long? = null,

        @field:NotNull
        @field:Length(min = 5)
        var title: String? = null,

        var text: String? = null
) : Entity<Long> {
    companion object : Dao<Article>
}

These changes will ensure that all articles have a title that is at least five characters long. VoK can validate a variety of conditions in an entity, including the presence or uniqueness of columns, their format, and the existence of associated objects. The Hibernate Validator is used to provide validation support; validations are covered in detail in the Hibernate Validator documentation.

With the validation now in place, when you call binder.writeBeanIfValid(article) on an invalid article, it will return false. If you open CreateArticleView.kt again, you’ll notice that we actually check the result of calling binder.writeBeanIfValid(article) inside the create action. However, if writeBeanIfValid() fails, we need to show the form back to the user, and mark all invalid fields. To do this, change the button definition as follows:

        button("Save Article") {
            onLeftClick { event ->
                val article = Article()
                if (binder.validate().isOk && binder.writeBeanIfValid(article)) {
                    article.save()
                    ArticleView.navigateTo(article.id!!)
                } else {
                    event.button.componentError = UserError("There are invalid fields")
                }
            }
        }

If you reload http://localhost:8080/create-article and try to save an article without a title, VoK will send you back to the form, with the invalid fields marked red; also the “Save Article” button will be marked red. The binder.validate().isOk call will mark invalid fields, while binder.writeBeanIfValid(article) will write the values to the article entity, but only if everything is valid.

Updating Articles

We’ve covered the “CR” part of CRUD. Now let’s focus on the “U” part, updating articles.

The first step we’ll take is adding the web/src/main/kotlin/com/example/vok/EditArticleView.kt:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.github.vokorm.getById
import com.vaadin.navigator.*
import com.vaadin.server.UserError
import com.vaadin.ui.VerticalLayout
import com.vaadin.ui.themes.ValoTheme

@AutoView
class EditArticleView: VerticalLayout(), View {
    private val binder = beanValidationBinder<Article>()
    private var article: Article? = null
    init {
        label("Edit Article") {
            styleName = ValoTheme.LABEL_H1
        }
        textField("Title") {
            bind(binder).bind(Article::title)
        }
        textArea("Text") {
            bind(binder).bind(Article::text)
        }
        button("Save Article") {
            onLeftClick { event ->
                val article = article!!
                if (binder.validate().isOk && binder.writeBeanIfValid(article)) {
                    article.save()
                    ArticleView.navigateTo(article.id!!)
                } else {
                    event.button.componentError = UserError("There are invalid fields")
                }
            }
        }
        button("Back") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { navigateToView<ArticlesView>() }
        }
    }
    override fun enter(event: ViewChangeListener.ViewChangeEvent) {
        val articleId = event.parameterList[0]!!.toLong()
        edit(Article.getById(articleId))
    }

    private fun edit(article: Article) {
        this.article = article
        binder.readBean(article)
    }

    companion object {
        fun navigateTo(articleId: Long) = navigateToView<EditArticleView>(articleId.toString())
    }
}

The view will contain a form similar to the one we used when creating new articles. The only difference is that when the view is entered (that is, navigated to), it looks up the article ID, loads the article and binds it with the components.

Finally, we want to show a link to the edit action in the list of all the articles, so let’s add that now to ArticlesView.kt to make it appear next to the “Show” link. Just change the grid {} block as follows:

        grid = grid(dataProvider = Article.dataProvider) {
            expandRatio = 1f; setSizeFull()
            addColumnFor(Article::id)
            addColumnFor(Article::title)
            addColumnFor(Article::text)
            addColumn({ "Show" }, ButtonRenderer<Article>({ event -> ArticleView.navigateTo(event.item.id!!) }))
            addColumn({ "Edit" }, ButtonRenderer<Article>({ event -> EditArticleView.navigateTo(event.item.id!!) }))
        }

Note: The ButtonRenderer will be marked red; you will need to import the class. You can do that simply by pressing Alt+Enter and choosing Import from the menu.

And we’ll also add one to the ArticleView.kt template as well, so that there’s also an “Edit” link on an article’s page. Modify the class to look as follows:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.github.vokorm.getById
import com.vaadin.navigator.*
import com.vaadin.ui.*
import com.vaadin.ui.themes.ValoTheme

@AutoView
class ArticleView: FormLayout(), View {
    private lateinit var article: Article
    private val title: Label
    private val text: Label
    init {
        title = label {
            caption = "Title:"
        }
        text = label {
            caption = "Text:"
        }
        button("Edit") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { EditArticleView.navigateTo(article.id!!) }
        }
        button("Back") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { navigateToView<ArticlesView>() }
        }
    }
    override fun enter(event: ViewChangeListener.ViewChangeEvent) {
        val articleId = event.parameterList[0]?.toLong() ?: throw RuntimeException("Article ID is missing")
        article = Article.getById(articleId)
        title.value = article.title
        text.value = article.text
    }

    companion object {
        fun navigateTo(articleId: Long) = navigateToView<ArticleView>(articleId.toString())
    }
}

And here’s how our app looks so far:

Article List View

Creating components to clean up duplication in views

Our EditArticleView view looks very similar to the CreateArticleView view; in fact, they both share the same code for displaying the form. Let’s remove this duplication by using a common component.

Create a new file web/src/main/kotlin/com/example/vok/ArticleEditor.kt with the following content:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.vaadin.server.UserError
import com.vaadin.ui.*
import com.vaadin.ui.themes.ValoTheme

class ArticleEditor : VerticalLayout() {
    private val binder = beanValidationBinder<Article>()
    var article: Article? = null
        set(value) {
            field = value
            if (value != null) binder.readBean(value)
        }

    init {
        isMargin = false
        textField("Title") {
            bind(binder).bind(Article::title)
        }
        textArea("Text") {
            bind(binder).bind(Article::text)
        }
        button("Save Article") {
            onLeftClick { event ->
                val article = article!!
                if (binder.validate().isOk && binder.writeBeanIfValid(article)) {
                    article.save()
                    ArticleView.navigateTo(article.id!!)
                } else {
                    event.button.componentError = UserError("There are invalid fields")
                }
            }
        }
        button("Back") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { navigateToView<ArticlesView>() }
        }
    }
}

fun HasComponents.articleEditor(block: ArticleEditor.()->Unit = {}) = init(ArticleEditor(), block)

This is just an ordinary Vaadin component which you can insert anywhere into your View. Its API consists of a single public property named article. When you assign this property, the ArticleEditor will populate the fields and the user will be able to edit the article. When you press the “Save Article” button, the ArticleEditor component will either create a new article, or update an existing one.

The function HasComponents.articleEditor() looks interesting. The function has been specially crafted in a way that allows us to build Vaadin UIs in a structured way, using purely Kotlin code. This technique is called DSL (Domain Specific Language). The name fits - in a way we have constructed a ‘language’ used to create Vaadin UIs. You can find more information about the DSLs at the DSLs: Explained article.

Using type-safe builders or DSL has the advantage that the Kotlin compiler will check for typos, and the Kotlin IDEA plugin will help us with the auto-completion.

Note: The Karibu-DSL library actually defines such builder functions for every Vaadin component. You can check the Basic.kt file for the definitions of the sources for the button and textField builder methods.

Now, let’s update the CreateArticleView.kt view to use this new component, rewriting it completely:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.vaadin.navigator.*
import com.vaadin.ui.VerticalLayout
import com.vaadin.ui.themes.ValoTheme

@AutoView
class CreateArticleView: VerticalLayout(), View {
    private val editor: ArticleEditor
    init {
        label("New Article") {
            styleName = ValoTheme.LABEL_H1
        }
        editor = articleEditor {
            article = Article()
        }
    }
}

Then do the same for the EditArticleView.kt view:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.github.vokorm.getById
import com.vaadin.navigator.*
import com.vaadin.ui.VerticalLayout
import com.vaadin.ui.themes.ValoTheme

@AutoView
class EditArticleView : VerticalLayout(), View {
    private val editor: ArticleEditor
    init {
        label("Edit Article") {
            styleName = ValoTheme.LABEL_H1
        }
        editor = articleEditor()
    }
    override fun enter(event: ViewChangeListener.ViewChangeEvent) {
        val articleId = event.parameterList[0]!!.toLong()
        editor.article = Article.getById(articleId)
    }

    companion object {
        fun navigateTo(articleId: Long) = navigateToView<EditArticleView>(articleId.toString())
    }
}

Deleting Articles

We’re now ready to cover the “D” part of CRUD, deleting articles from the database. To delete the article, all that’s needed is to call delete() in the article from appropriate place.

We will add a ‘Destroy’ link to the ArticlesView.kt file, to wrap everything together:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.vaadin.navigator.*
import com.vaadin.ui.*
import com.vaadin.ui.renderers.ButtonRenderer
import com.vaadin.ui.themes.ValoTheme
import eu.vaadinonkotlin.vaadin8.sql2o.dataProvider

@AutoView
class ArticlesView: VerticalLayout(), View {
    private val grid: Grid<Article>
    init {
        setSizeFull()
        label("Listing Articles") {
            styleName = ValoTheme.LABEL_H1
        }
        button("New Article") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { navigateToView<CreateArticleView>() }
        }
        grid = grid(dataProvider = Article.dataProvider) {
            expandRatio = 1f; setSizeFull()
            addColumnFor(Article::id)
            addColumnFor(Article::title)
            addColumnFor(Article::text)
            addColumn({ "Show" }, ButtonRenderer<Article>({ event -> ArticleView.navigateTo(event.item.id!!) }))
            addColumn({ "Edit" }, ButtonRenderer<Article>({ event -> EditArticleView.navigateTo(event.item.id!!) }))
            addColumn({ "Destroy" }, ButtonRenderer<Article>({ event ->
                confirmDialog {
                    event.item.delete()
                    this@grid.refresh()
                }
            }))
        }
    }
    override fun enter(event: ViewChangeListener.ViewChangeEvent?) {
        grid.refresh()
    }
}

The “Destroy” button calls the confirmDialog which shows a simple Vaadin dialog. The function is implemented in a way that it will call the follow-up block when the “Yes” button is clicked. The block will just delete the article and refresh the Grid, to display the new data. To get rid of the confirmation dialog, just delete the confirmDialog line:

            addColumn({ "Destroy" }, ButtonRenderer<Article>({ event ->
                event.item.delete()
                this@grid.refresh()
            }))

Note: To see the definition of the function, just open up Intellij IDEA and click your mouse on the confirmDialog function name while holding the Control key.

Delete Article

Congratulations, you can now create, show, list, update and destroy articles.

Adding a Second Database Entity

It’s time to add a second database table to the application. The second database table will handle comments on articles.

Creating the ‘Comments’ Entity

We’ll create a Comment entity to hold comments for an article. Create the following file: web/src/main/kotlin/com/example/vok/Comment.kt with the following contents:

package com.example.vok

import com.github.vokorm.*
import org.hibernate.validator.constraints.Length
import javax.validation.constraints.NotNull

data class Comment(
        override var id: Long? = null,

        var article_id: Long? = null,

        @field:NotNull
        @field:Length(min = 3)
        var commenter: String? = null,

        @field:NotNull
        @field:Length(min = 3)
        var body: String? = null
) : Entity<Long> {
    companion object : Dao<Comment>

    val article: Article? get() = if (article_id == null) null else Article.findById(article_id!!)
}

This is very similar to the Article entity that you saw earlier. The difference is the property article which sets up an association. You’ll learn a little about associations in the next section of this guide.

Note the article_id column - it tells which Article the comment belongs to. You can get a better understanding after analyzing the appropriate migration script. Just create web/src/main/resources/db/migration/V02__CreateComment.sql file with the following contents:

create TABLE Comment(
  id bigint auto_increment PRIMARY KEY,
  commenter varchar(200) NOT NULL,
  body VARCHAR(4000) NOT NULL,
  article_id bigint not null REFERENCES Article(id)
);

The article_id line creates an integer column called article_id, an index for it, and a foreign key constraint that points to the id column of the articles table. Go ahead and run the project.

Since we are running an embedded database which starts in a clear state, all migrations will run:

15:43:44.532 [RMI TCP Connection(2)-127.0.0.1] INFO  com.example.vok.Bootstrap - Running DB migrations
15:43:44.553 [RMI TCP Connection(2)-127.0.0.1] INFO  o.f.c.internal.util.VersionPrinter - Flyway 4.2.0 by Boxfuse
15:43:44.676 [RMI TCP Connection(2)-127.0.0.1] INFO  o.f.c.i.dbsupport.DbSupportFactory - Database: jdbc:h2:mem:test (H2 1.4)
15:43:44.754 [RMI TCP Connection(2)-127.0.0.1] INFO  o.f.core.internal.command.DbValidate - Successfully validated 2 migrations (execution time 00:00.026s)
15:43:44.767 [RMI TCP Connection(2)-127.0.0.1] INFO  o.f.c.i.m.MetaDataTableImpl - Creating Metadata table: "PUBLIC"."schema_version"
15:43:44.792 [RMI TCP Connection(2)-127.0.0.1] INFO  o.f.core.internal.command.DbMigrate - Current version of schema "PUBLIC": << Empty Schema >>
15:43:44.794 [RMI TCP Connection(2)-127.0.0.1] INFO  o.f.core.internal.command.DbMigrate - Migrating schema "PUBLIC" to version 01 - CreateArticle
15:43:44.813 [RMI TCP Connection(2)-127.0.0.1] INFO  o.f.core.internal.command.DbMigrate - Migrating schema "PUBLIC" to version 02 - CreateComment
15:43:44.823 [RMI TCP Connection(2)-127.0.0.1] INFO  o.f.core.internal.command.DbMigrate - Successfully applied 2 migrations to schema "PUBLIC" (execution time 00:00.057s).

However, if we were to use a persistent database, FlyWay would be smart enough to only execute the migrations that have not already been run against the current database.

Associating Models

Vaadin on Kotlin associations let you easily declare the relationship between two entities. In the case of comments and articles, you could write out the relationships this way:

You’ve already seen the line of code inside the Comment entity (Comment.kt) that makes each comment belong to an Article:

    val article: Article? get() = if (article_id == null) null else Article.findById(article_id!!)

You’ll need to edit Article.kt to add the other side of the association:

package com.example.vok

import com.github.vokorm.*
import eu.vaadinonkotlin.vaadin8.*
import eu.vaadinonkotlin.vaadin8.sql2o.*
import org.hibernate.validator.constraints.Length
import javax.validation.constraints.NotNull

data class Article(
        override var id: Long? = null,

        @field:NotNull
        @field:Length(min = 5)
        var title: String? = null,

        var text: String? = null
) : Entity<Long> {
    companion object : Dao<Article>

    val comments: VokDataProvider<Comment> get() = Comment.dataProvider.withFilter { Comment::article_id eq id }
}

These two declarations enable a good bit of automatic behavior. For example, if you have a variable article containing an article, you can retrieve all the comments belonging to that article as an array using article.comments.getAll().

Note: Note that the comments field is outside of the data class constructor. This is intentional, since the comments field is not really a field but a computed property and thus can not stand as a constructor parameter. comments is hence lazy - it is evaluated every time it is read; reading it causes a database select to be run. That’s why the comments property shouldn’t appear in Article.toString(), so that logging a newly created article (which calls toString()) won’t run a select. Computed properties also do not appear in the JSON output as returned by the REST services - this way we can prevent polluting of the REST JSON article output with all comments.

Exposing Comments via REST

You can expose the comments via the REST interface. This is completely optional and is not used by Vaadin in any way, it may just be handy to check your database status via the curl tool. Edit ArticleRest.kt:

package com.example.vok

import com.github.mvysny.karibudsl.v8.getAll
import com.github.vokorm.*
import io.javalin.Javalin
import io.javalin.NotFoundResponse

fun Javalin.articleRest() {
    get("/rest/articles/:id") { ctx ->
        val id = ctx.pathParam("id").toLong()
        ctx.json(Article.findById(id) ?: throw NotFoundResponse("No article with id $id"))
    }
    get("/rest/articles") { ctx -> ctx.json(Article.findAll()) }
    get("/rest/articles/:id/comments") { ctx ->
        val id = ctx.pathParam("id").toLong()
        val article = Article.findById(id) ?: throw NotFoundResponse("No article with id $id")
        ctx.json(article.comments.getAll())
    }
}

Now, you can run curl in your terminal:

$ curl localhost:8080/rest/articles/1/comments
[{"id":1,"commenter":"A buddy programmer","body":"I like Vaadin-on-Kotlin, too!"}]

Writing a View

Like with any blog, our readers will create their comments directly after reading the article, and once they have added their comment, will be sent back to the article show page to see their comment now listed.

So first, we’ll wire up the ArticleView.kt view to let us make a new comment:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.github.vokorm.getById
import com.vaadin.navigator.*
import com.vaadin.server.UserError
import com.vaadin.ui.*
import com.vaadin.ui.themes.ValoTheme

@AutoView
class ArticleView: FormLayout(), View {
    private lateinit var article: Article
    private val title: Label
    private val text: Label
    private val comments: Label
    private val commentBinder = beanValidationBinder<Comment>()
    private lateinit var createComment: Button
    init {
        title = label {
            caption = "Title:"
        }
        text = label {
            caption = "Text:"
        }
        comments = label { caption = "Comments" }
        formLayout {
            caption = "Add a comment:"
            textField("Commenter:") {
                bind(commentBinder).bind(Comment::commenter)
            }
            textField("Body:") {
                bind(commentBinder).bind(Comment::body)
            }
            createComment = button("Create") {
                onLeftClick { createComment() }
            }
        }
        button("Edit") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { EditArticleView.navigateTo(article.id!!) }
        }
        button("Back") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { navigateToView<ArticlesView>() }
        }
    }
    override fun enter(event: ViewChangeListener.ViewChangeEvent) {
        val articleId = event.parameterList[0]?.toLong() ?: throw RuntimeException("Article ID is missing")
        article = Article.getById(articleId)
        title.value = article.title
        text.value = article.text
    }
    private fun createComment() {
        val comment = Comment()
        if (!commentBinder.validate().isOk || !commentBinder.writeBeanIfValid(comment)) {
            createComment.componentError = UserError("There are invalid fields")
        } else {
            createComment.componentError = null
            comment.article_id = article.id
            comment.save()
            refreshComments()
            commentBinder.readBean(Comment())  // this clears the comment fields
        }
    }
    private fun refreshComments() {
        comments.html(
            // force-update the comments list.
            article.comments.getAll().joinToString("") { comment ->
                "<p><strong>Commenter:</strong>${comment.commenter}</p><p><strong>Comment:</strong>${comment.body}</p>"
            }
        )
    }
    companion object {
        fun navigateTo(articleId: Long) = navigateToView<ArticleView>(articleId.toString())
    }
}

This adds a form on the Article show page that creates a new comment, by calling the comment.save() code.

Once we have made the new comment, we need to stay on the page of the original article. That’s why there is no navigate call in the createComment() function. However, since the page does not reload (remember we use the single-page-framework), we need to refresh the comments ourselves. See the comments label? We will populate this label with a html-formatted list of all comments. This exactly is done by the refreshComments() function.

Now you can add articles and comments to your blog and have them show up in the right places.

Create and List Comments

Note: in the refreshComments() the getAll() function will re-fetch the fresh list of comments; the comments are not cached in the Article.comments field. If you need to access the comment list multiple times, it is best to store the list of comments into a variable.

Refactoring

Now that we have articles and comments working, take a look at the web/src/main/kotlin/com/example/vok/ArticleView.kt view. It is getting long and awkward. We can create reusable components to clean it up.

The Comments Component

First, we will extract a component which will show comments for given article. Since we will need to add a ‘delete’ link in the future, the Label component will no longer suffice. Create the web/src/main/kotlin/com/example/vok/CommentsComponent.kt file:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.github.vokorm.getById
import com.vaadin.ui.*

class CommentsComponent : VerticalLayout() {
    var articleId: Long = 0L
        set(value) { field = value; refresh() }
    init {
        caption = "Comments"; isMargin = false
    }

    fun refresh() {
        removeAllComponents()
        Article.getById(articleId).comments.getAll().forEach { comment ->
            label {
                html("<p><strong>Commenter:</strong>${comment.commenter}</p><p><strong>Comment:</strong>${comment.body}</p>")
            }
        }
    }
}
// the extension function which will allow us to use CommentsComponent inside a DSL
fun HasComponents.commentsComponent(block: CommentsComponent.()->Unit = {}) = init(CommentsComponent(), block)

The component has a handy property articleId which, upon setting, will populate itself with comments for that particular article.

We also need the means to include the CommentsComponent component into the DSL, and thus we have also introduced the DSL extension method as well. The extension function simply calls the init() function, which performs the following things:

You can learn more about how DSL works, from the Writing Vaadin Apps In Kotlin Part 4 tutorial.

Converting the Comments Form to a component

Let us also move that new comment section out to its own component. Create the file named web/src/main/kotlin/com/example/vok/NewCommentForm.kt with the following contents:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.vaadin.server.UserError
import com.vaadin.ui.*

class NewCommentForm : FormLayout() {
    var commentCreatedListener: ()->Unit = {}
    lateinit var article: Article
    private val commentBinder = beanValidationBinder<Comment>()
    private val createComment: Button
    init {
        caption = "Add a comment:"
        textField("Commenter:") {
            bind(commentBinder).bind(Comment::commenter)
        }
        textField("Body:") {
            bind(commentBinder).bind(Comment::body)
        }
        createComment = button("Create") {
            onLeftClick { createComment() }
        }
    }

    private fun createComment() {
        val comment = Comment()
        if (!commentBinder.validate().isOk || !commentBinder.writeBeanIfValid(comment)) {
            createComment.componentError = UserError("There are invalid fields")
        } else {
            createComment.componentError = null
            comment.article_id = article.id
            comment.save()
            commentBinder.readBean(Comment())  // this clears the comment fields
            commentCreatedListener()
        }
    }
}
fun HasComponents.newCommentForm(block: NewCommentForm.()->Unit = {}) = init(NewCommentForm(), block)

The component handles the comment creation now. In order for the component to work properly, the article for which the comment is to be created must be set. Also, when the component creates the comment, it needs to notify the ArticleView about this fact. This is done by the means of the commentCreatedListener listener which the component calls upon comment creation. Now, let’s refactor the ArticleView.kt to make use of the NewCommentForm component, and register itself to NewCommentForm as a listener:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.github.vokorm.getById
import com.vaadin.navigator.*
import com.vaadin.ui.*
import com.vaadin.ui.themes.ValoTheme

@AutoView
class ArticleView: VerticalLayout(), View {
    private lateinit var article: Article
    private lateinit var title: Label
    private lateinit var text: Label
    private val comments: CommentsComponent
    private val newComment: NewCommentForm
    init {
        formLayout {
            title = label {
                caption = "Title:"
            }
            text = label {
                caption = "Text:"
            }
        }
        comments = commentsComponent()
        newComment = newCommentForm {
            commentCreatedListener = { comments.refresh() }
        }
        button("Edit") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { EditArticleView.navigateTo(article.id!!) }
        }
        button("Back") {
            styleName = ValoTheme.BUTTON_LINK
            onLeftClick { navigateToView<ArticlesView>() }
        }
    }
    override fun enter(event: ViewChangeListener.ViewChangeEvent) {
        val articleId = event.parameterList[0]?.toLong() ?: throw RuntimeException("Article ID is missing")
        article = Article.getById(articleId)
        title.value = article.title
        text.value = article.text
        comments.articleId = article.id!!
        newComment.article = article
    }

    companion object {
        fun navigateTo(articleId: Long) = navigateToView<ArticleView>(articleId.toString())
    }
}

Deleting Comments

Another important feature of a blog is being able to delete spam comments. To do this, we need to implement a link of some sort in the CommentsComponent.

Let’s add a link button to the CommentsComponent.kt file:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.github.vokorm.getById
import com.vaadin.ui.*
import com.vaadin.ui.themes.ValoTheme

class CommentsComponent : VerticalLayout() {
    var articleId: Long = 0L
        set(value) { field = value; refresh() }
    init {
        caption = "Comments"; isMargin = false
    }

    fun refresh() {
        removeAllComponents()
        Article.getById(articleId).comments.getAll().forEach { comment ->
            label {
                html("<p><strong>Commenter:</strong>${comment.commenter}</p><p><strong>Comment:</strong>${comment.body}</p>")
            }
            button("Delete comment") {
                styleName = ValoTheme.BUTTON_LINK
                onLeftClick { comment.delete(); refresh() }
            }
        }
    }
}
// the extension function which will allow us to use CommentsComponent inside a DSL
fun HasComponents.commentsComponent(block: CommentsComponent.()->Unit = {}) = init(CommentsComponent(), block)

Clicking the “Delete comment” button will delete the comment and refresh the component, to show the rest of the comments.

Deleting Associated Objects

If you delete an article, its associated comments will also need to be deleted, otherwise they would simply occupy space in the database. Or even worse, since we have the foreign constraint set up, the database would fail to delete the article. We will need to modify the Article.delete() method to do that for us. Modify the Article.kt file and add the delete function right below the comments val, as follows:

    override fun delete() = db {
        Comment.deleteBy { Comment::article_id eq id }
        super.delete()
    }

Security

If you were to publish your blog online, anyone would be able to add, edit and delete articles or delete comments.

9.1 The Login Dialog

Adding security to Java WAR apps is usually done by letting the web server (e.g. Tomcat) handle the username/password storage and verification, while our web app provides the login dialog. To keep this guide web server agnostic, we’ll do the verification ourselves.

We will implement a login service and a login form. Just create the web/src/main/kotlin/com/example/vok/LoginService.kt file:

package com.example.vok

import com.github.mvysny.karibudsl.v8.*
import com.vaadin.server.*
import com.vaadin.ui.*
import eu.vaadinonkotlin.vaadin8.Session
import eu.vaadinonkotlin.vaadin8.loginForm
import java.io.Serializable

data class User(val name: String) : Serializable

class LoginService : Serializable {
    fun login(username: String, password: String): Boolean {
        currentUser = User(username)
        Page.getCurrent().reload()
        return true
    }
    var currentUser: User? = null
    private set

    fun logout() {
        VaadinSession.getCurrent().close()
        Page.getCurrent().reload()
    }

    val isLoggedIn get() = currentUser != null
}

val Session.loginService: LoginService get() = getOrPut { LoginService() }

class LoginView : VerticalLayout() {
    init {
        setSizeFull()
        loginForm("Vaadin-on-Kotlin Sample App") {
            alignment = Alignment.MIDDLE_CENTER
            onLogin { username, password ->
                if (!Session.loginService.login(username, password)) {
                    usernameField.componentError = UserError("The user does not exist or invalid password")
                }
            }
        }
    }
}

The code is a bit longer, but the result is worth it. This is how the LoginView component looks like:

Login View

The LoginService class handles the process of login/logout. Upon login, we will store the information about the currently logged-in user into the session. This will serve as a marker that there is someone logged in. We will also tell the browser to reload the page - this will reinstantiate the MyUI. We will now configure MyUI to show a login form if there’s nobody logged in yet. Just edit MyUI.kt and change the init() method as follows:

    override fun init(request: VaadinRequest?) {
        if (!Session.loginService.isLoggedIn) {
            setContent(LoginView())
            return
        }
        setContent(content)
        navigator = Navigator(this, content as ViewDisplay)
        navigator.addProvider(autoViewProvider)
        ...  // etc, the rest of the init() method
    }

Other Security Considerations

Security, especially in web applications, is a broad and detailed area. You can decide not to use the login dialog at all, and instead use the HTTP Basic Auth, thus letting the web server handle the security completely. You can also employ other security options. This is however out of scope of this tutorial.

Please see the Vaadin-on-Kotlin Security Demo sample project for more details.

What’s Next?

Now that you’ve seen your first VoK application, you should feel free to update it and experiment on your own.

Remember you don’t have to do everything without help. As you need assistance getting up and running with VoK, feel free to consult these support resources:

Feedback

You’re encouraged to help improve the quality of this guide.

Please contribute if you see any typos or factual errors. To get started, you can read our documentation contributions section.

You may also find incomplete content, or stuff that is not up to date. Please do add any missing documentation for master. Check the Vaadin On Kotlin Guides Guidelines for style and conventions.

If for whatever reason you spot something to fix but cannot patch it yourself, please open an issue.

And last but not least, any kind of discussion regarding Vaadin on Kotlin documentation is very welcome in the Vaadin Forums.

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License