Vaadin 14 - The missing guide

The Vaadin documentation is a good source of initial info for Vaadin, but unfortunately in certain cases it simply doesn’t go deep enough. This is especially true with respect of the toolchain Vaadin uses, and the files it relies on. Therefore, this blog post.

Basic Vaadin concepts

All Vaadin components are generally web components, following the Web Component specification.

Web Component API is supported by all ever-green browsers (Firefox, Chrome, Safari, MS Edge).

Note: Ever-green means that the browser endlessly updates itself and you generally use the newest version.

Web Component API can be emulated, by the means of the Polyfill library - that’s how IE 11 is able to “support” Web Components even though IE 11 offers no Web Component support natively. However, Polyfills emulation is very slow and leads to 5-10 times worse performance (for example documented in Vaadin dropped IE11 support in Vaadin 15+; also see Vaadin Release Notes, section “Known Issues and Limitations” / “Performance” - very well hidden Vaadin!!)

Vaadin controls the web components from server side, by setting DOM HTML attributes, Polymer properties (kinda deprecated since Polymer will go away) and Lit Properties (Vaadin 15+); Vaadin is also able to listen on DOM events and pass them through to the server-side. The Vaadin part responsible for this is called Vaadin Flow.

Read Vaadin Overview for more details.

Note: You can still use non-webcomponent libraries: you simply create a <div> client-side, you hook the javascript library to that div and then you can control the <div> from the server-side.

Basic JavaScript concepts

  • node.js - a virtual machine able to run JavaScript code, akin to JVM
  • npm - a package manager akin to Maven
  • pnpm - a package manager akin to Maven; same as npm but much faster since it’s able to cache downloaded packages locally.
  • Bower - a package manager akin to Maven, now deprecated and superseded by npm

For best results, use pnpm. In the text below, npm will be used as a reference to both npm and pnpm.

The Compatibility/bower mode versus the npm mode

Vaadin is able to run in two modes: so-called compatibility or bower mode, and the npm mode. Vaadin 13 and lower only supported bower mode; Vaadin 14 supports both modes; Vaadin 15+ will only support npm mode.

In compatibility mode, Vaadin Servlet will use the old way of using JavaScript modules packaged as webjars (jars deployed in Maven Central, containing client-side JavaScript code). The packaging manager is Bower. Generally the webjars site is used, to create webjar out of a bower package. What the webjars page does is that it parses bower descriptor, creates a Maven pom.xml out of it (so that transitive dependencies are “working”) and that’s it: from your server-side perspective you can forget about Bower entirely and just use Maven to manage the dependencies.

Well, not entirely: Bower transitive versioning resolution algorithm works a bit differently than the Maven one (especially for version ranges such as 1.+); Vaadin had to workaround these issues by fixing versions of all webjars in the vaadin-bom but this approach is error-prone and maintenance-heavy. That’s why bower is deprecated in favor of npm/pnpm.

Bower mode is deprecated and will go away in future Vaadin versions, do not use it for new projects - always use the npm mode.

new npm mode

In npm mode, Vaadin does not use webjars at all; instead it relies on JavaScript programs to do all the work:

  1. It uses the npm/pnpm package manager to gather all packages Vaadin+your app needs, into the node_modules folder.
  2. webpack is then used to compress all modules and custom js files into one huge javascript file, both in development mode and in production mode.
  3. (In Vaadin production mode): the javascript file is then stored into the WAR file into the WEB-INF/classes/META-INF/VAADIN/build/ folder and served by VaadinServlet from there.

Note: In npm mode, Vaadin does not use webjars at all. However in Vaadin 14 webjars are transitive dependencies of the vaadin and vaadin-core Maven artifacts. While there is no harm in having webjars on classpath (Vaadin will simply ignore them), it’s best to exclude them in order to decrease the result WAR file size. See example build.gradle.kts on how to do that.

npm configures itself via the package.json and package-lock.json files, and produces the node_modules folder. The package.json and package-lock.json files are generated by Vaadin, but you are free to modify those files to include additional packages if need be. However there’s a better way to do it: by using the @NpmPackage Java annotation you declare that you need this and this npm package for your server-side component. Vaadin plugin then performs classpath scanning, gathers all @NpmPackage annotations and produces appropriate package.json/package-lock.json files.

Webpack basically takes all npm modules and the content of your frontend/ folder, and produces one huge javascript file. Webpack is configured via two files: webpack.config.js and webpack.generated.js. You can add your custom config into the webpack.config.js file, however I never needed to do that so far. Both files are generated by Vaadin.

Development vs production mode

In development mode, Vaadin doesn’t bundle the javascript files into the WAR file since that would prevent them from being hot-redeployed when modified. Instead, Vaadin Servlet will run webpack as a child process; webpack will then monitor frontend/ for changes and is able to communicate with Vaadin Servlet internally so that Vaadin Servlet can correctly serve up-to-date JavaScript code. That’s why any changes made to the frontend/ folder are immediately visible when you refresh the page.

In production mode however, Vaadin Servlet doesn’t launch webpack (since that would require having node.js and npm on your production machine); instead it relies on Vaadin Plugin to run webpack, in order to produce the one huge JavaScript file. Vaadin Plugin then packages the produced JavaScript file onto classpath, into the META-INF/VAADIN/build/ package; Vaadin Servlet is then able to serve those files to the browser, in order to initialize the client-side properly.

The META-INF/VAADIN/build/ contents should look like this (the hash is going to differ on every build):

├── vaadin-2-18d67c4ccff7e93b081a.cache.js
├── vaadin-2-18d67c4ccff7e93b081a.cache.js.gz
├── vaadin-3-b0147df339bf18eb7618.cache.js
├── vaadin-3-b0147df339bf18eb7618.cache.js.gz
├── vaadin-4-ee1d2e45569f7eca4292.cache.js
├── vaadin-4-ee1d2e45569f7eca4292.cache.js.gz
├── vaadin-5-5e9292474e82143d0a27.cache.js
├── vaadin-5-5e9292474e82143d0a27.cache.js.gz
├── vaadin-bundle-19a00eae62ad7cddd291.cache.js
├── vaadin-bundle-19a00eae62ad7cddd291.cache.js.gz
├── vaadin-bundle.es5-b1c1a3cc054c62ad7949.cache.js
├── vaadin-bundle.es5-b1c1a3cc054c62ad7949.cache.js.gz
└── webcomponentsjs
    ├── bundles
    │   ├── webcomponents-ce.js
    │   ├──
    │   ├── webcomponents-sd-ce.js
    │   ├──
    │   ├── webcomponents-sd-ce-pf.js
    │   ├──
    │   ├── webcomponents-sd.js
    │   └──
    ├── custom-elements-es5-adapter.js
    ├── package.json
    ├── src
    │   └── entrypoints
    │       ├── custom-elements-es5-adapter-index.js
    │       ├── webcomponents-bundle-index.js
    │       ├── webcomponents-ce-index.js
    │       ├── webcomponents-sd-ce-index.js
    │       ├── webcomponents-sd-ce-pf-index.js
    │       └── webcomponents-sd-index.js
    ├── webcomponents-bundle.js
    └── webcomponents-loader.js

Vaadin Configuration

The most important file is the flow-build-info.json file: Vaadin Servlet reads that file and configures itself from it at runtime. The file resides on the classpath, in the META-INF/VAADIN/config/ package. The file is produced by Vaadin Plugin. It looks like this (development mode example):

  "compatibilityMode": false,
  "productionMode": false,
  "npmFolder": "/home/mavi/work/my/vok/karibu10-helloworld-application",
  "generatedFolder": "/home/mavi/work/my/vok/karibu10-helloworld-application/target/frontend",
  "frontendFolder": "/home/mavi/work/my/vok/karibu10-helloworld-application/frontend",
  "pnpm.enable": true,
  "require.home.node": false

The most important settings are compatibilityMode and productionMode.

Production mode example:

  "compatibilityMode": false,
  "productionMode": true,
  "enableDevServer": false,
  "chunks": {
    "fallback": {
      "jsModules": [
// etc etc
      "cssImports": [

It’s very important to have this file on classpath. If it’s not, Vaadin Servlet will then try to auto-configure itself and sometimes wrongly so. For example when launching WAR in Tomcat from Intellij, the frontendFolder setting will be “auto-detected” to Tomcat’s bin/ folder, which will cause webpack to fail later on, because of missing files.

If the file is present multiple times on the classpath, then Vaadin tries to choose which one is the right one (which one is coming from the main project). That’s unfortunate since sometimes the algorithm fails and backfires unexpectedly, choosing the incorrect file (see Flow issue #8936 for more details). This usually happens when a Vaadin addon incorrectly includes the flow-build-info.json file in the jar file. That’s a bug in the addon packaging which need to be fixed and the flow-build-info.json file removed from their jar files.

For the reasons stated above, keep in mind to have the flow-build-info.json file on the classpath EXACTLY ONCE, and always coming from your main project.

The project flow-build-info.json file is generated by Vaadin plugin, in the prepare-frontend task, therefore it’s very important to always run that task before launching your app in development mode.

Checking whether Vaadin runs in development or in production mode

The most important setting is the productionMode in flow-build-info.json which configures Vaadin to run in either development or production mode.

If Vaadin runs in the development mode it will log the following to the console/logger:

[http-nio-8080-exec-1] INFO com.vaadin.flow.server.DefaultDeploymentConfiguration - 
Vaadin is running in DEBUG MODE.
When deploying application for production, remember to disable debug features. See more from

If Vaadin runs in production mode it will log the following:

[http-nio-8080-exec-1] INFO com.vaadin.flow.server.DefaultDeploymentConfiguration - Vaadin is running in production mode.

Read the Vaadin Plugin: under the hood for more info on how these files are prepared.

Written on June 24, 2020