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In this blog, I will show you how I got started with Vaadin Flow. With Vaadin Flow, you can create web apps without HTML or JavaScript, but entirely with Java. Let’s get started! 1. Introduction For some time, I have been curious about Vaadin Flow. I am not very good in frontend developement but sometimes I try to get myself together and then I take a look at a frontend framework. In a previous post, I looked at React for example. Although the React introduction was a nice experience, I still did not think I could create a frontend with React before investing a serious amount of time. At Twitter, I often saw posts coming by about Vaadin and this took my interest. So, I put Vaadin on my blog todo list. A few weeks ago, I started to take a closer look at Vaadin, and more specifically at Vaadin Flow. Reading some blogs and examples, it reminded me about Java Swing but where Java Swing would create a traditional desktop application, Vaadin will create a web application for me. That sounds cool, because now I probably do not have a very steep learning curve and I can continue working in my Java IDE. Besides that, it also seems to integrate quite well with Spring, which I also already know. Enough prerequisites which should lead to a succes story for me. During my research I came up to the following interesting articles: Why I (still) love Vaadin, by Nicolas Fränkel; 5 Reasons Why Enterprises Use Vaadin For Their Web Application UI Needs, by Ankurman Shrestha Vaadin also takes care of the communication between frontend and backend. But when you are using a First API approach, you can call the API from the Vaadin backend part preventing duplication in your code. 2. Vaadin Flow Tutorial The easiest way to get started, is by following the Vaadin Flow Tutorial. Beware that you select the correct version of Vaadin Flow. At the time of writing, Vaadin 14 is the LTS version and is the one you should use when running in production, Vaadin 22 is the latest non-LTS version with new features to experiment with. I am most interested in the current production status and follow the Vaadin 14 Tutorial. The tutorial contains a basic project which you will enhance each time with code snippets which are provided. Also, the concepts and code snippets are clearly explained. Some basic components are used, login is added and it is also shown how you can write tests for the application. The latter is a great advantage because automated testing of a frontend application is always difficult to execute. It takes about 3-4 hours to complete the tutorial, but after that, you already have a good basis for creating your own project. The only disadvantage for me is that the tutorial is mainly copying of code snippets. This way, you miss certain details when trying to use Vaadin Flow yourself. 3. Own Project Let’s see whether I am able to create a frontend for a simple application. The application I create is quite basic and contains of a list of Shows for which ShowEvents can be created. A Show consists out of a title and a ShowEvent consists out of a date and the Show the event is for. The sources for this project can be found at GitHub. I will not explain the entire application, but I will mention some highlights how I built the application. 3.1 Get Started In order to get a quick start, I navigated to start.vaadin.com. Here you can configure the basic setup for the application. I chose an Empty template and added two views: one for the Shows and one for the ShowEvents. In the Settings tab, I changed the Project, the Group ID and decided to use Java 17. When ready configuring, I clicked the Download button and opened the project in my favorite IDE. In the pom, I changed the vaadin dependency to vaadin-core. Reason for this is that I only want to use free components and this should be more than enough for the basic application I want to build. XML com.vaadin vaadin-core ... Next, I wanted to run the application by running the main method of the Application class. Remember, Vaadin integrates well with Spring, so we are talking about a Spring Boot application here. However, this gave the following exception: Shell java.lang.IllegalStateException: The compatibility mode is explicitly set to 'false', but there are neither 'flow-build-info.json' nor 'webpack.config.js' file available in the project/work I was a little too enthusiastic, I had to build the application first. Shell $ mvn clean package After this, the application started successfully. 3.2 Creating the Application I started with creating the data package and copying the AbstractEntity of the tutorial into it. This required adding the spring-boot-starter-data-jpa dependency to the pom. OK, I know I wrote that by copying you miss some details and now I do it myself, but there is nothing wrong with a smart copy now and then. XML org.springframework.boot spring-boot-starter-data-jpa Next, I created the Show and ShowEvent entities and added the JPA repositories. Vaadin also provides a dependency for generating example data. This way, you can generate data into your application for testing purposes. Therefore, I added the exampledata dependency and created the DataGenerator class. XML com.vaadin exampledata 4.1.0 In order to get this working, I had to remove the NotEmpty annotation because of the following exception. I did not investigate this exception any further. Shell No validator could be found for constraint 'javax.validation.constraints.NotEmpty' validating type 'java.time.LocalDate' Next, I created the ShowService and added the H2 database as a dependency to the pom. XML com.h2database h2 Building and starting the application was successfull again and after that, I implemented the ShowsView, ShowEventsView and the ShowEventForm. The first two are for displaying the Shows and ShowEvents into a grid and the ShowEventForm will allow us to change and delete a ShowEvent. To give you an impression, some screenshots are shown here. The Shows page. The ShowEvents page. The ShowEvent form. 3.3 Lazy Loading In the DataGenerator the number of ShowEvents was initially set to 50, but when I increase this to 1000, then a problem occurs in the ShowEvents page. The whole list of ShowEvents is retrieved and loaded into memory. This will get worse when the list grows even further. The solution is provided in the documentation. First, I needed to change to ShowEventRepository by extending it from PagingAndSortingRepository instead of JpaRepository. Java public interface ShowEventRepository extends PagingAndSortingRepository { } Following the instructions of the documentation worked just fine when I scrolled slowly through the list, but when I scrolled a bit faster an IndexOutOfBoundsException was thrown. Shell java.lang.IndexOutOfBoundsException: Index 50 out of bounds for length 50 In order to explain why this exception occurs, we need to take a look at the code for fetching the results. Java private void updateList() { DataProvider dataProvider = DataProvider.fromCallbacks( // First callback fetches items based on a query query -> { // The index of the first item to load int offset = query.getOffset(); // The number of items to load int limit = query.getLimit(); List showEvents = showService.fetchShowEvents(offset, limit); return showEvents.stream(); }, // Second callback fetches the total number of items currently in the Grid. // The grid can then use it to properly adjust the scrollbars. query -> showService.countShowEvents()); grid.setDataProvider(dataProvider); } The offset and the limit (number of items to load) are retrieved from the query provided by Vaadin. However, JPA pagination needs a page and a limit. Therefore, the page needs to be calculated by dividing the offset by the limit. Initially, I just used the offset directly for the page and this caused the error. Java public List fetchShowEvents(int offset, int limit) { return showEventRepository.findAll(PageRequest.of(offset / limit, limit)).stream().toList(); } 4. Conclusion My first experience with Vaadin is quite positive. In most cases, I just need a frontend for an administrator or a limited set of users. With Vaadin I can make use of my Java experience and still provide a fancy web application. Also, the idea that I am able to create tests appeals me a lot. The free components will be most of the time enough for these kind of applications and if not, also some more advanced components are provided by means of a license.

In this post, follow an overview of resources with information and examples of Java projects to control electronic components on the Raspberry Pi.