Vaadin 14 Plugins - under the hood

Neither the Vaadin Gradle Plugin nor the Vaadin Maven Plugin really document what they’re doing under the hood. When something goes wrong, it’s very hard for a server-side guy like myself to know what’s going on, especially since the entire JavaScript client-side landscape is pretty much a mystery to a Java guy. Hence this blog post.

Both Maven and Gradle Vaadin Plugins do pretty much the same thing; they both have two goals/tasks:

  • vaadin:prepare-frontend for Maven, vaadinPrepareFrontend for Gradle
  • vaadin:build-frontend for Maven, vaadinBuildFrontend for Gradle

Note: If you feel lost at any point, please read the Vaadin: the missing guide article first.

Important: that this article discusses the npm mode only: since the compatibility/bower mode is deprecated, we’re not going to discuss that anymore.


The prepare-frontend goal/task does the following things (you can follow the text below and look into the sources of the Vaadin Gradle Plugin prepare-frontend task implementation if you’d like):

First, the plugin will prepare the very important flow-build-info.json file into the classpath, into the target/ or build/ folder (depends on whether you use Maven or Gradle). By placing it into the target/ folder, the file will then later on be packaged correctly into the target artifact, be it WAR, EAR or SpringBoot jar. This way it is also visible to Vaadin when launching Vaadin with embedded Jetty.

Important: Since the flow-build-info.json file is removed when running the clean task, it’s really good to remember to run the prepare-frontend task after the clean task.

The next step is for the Vaadin Plugin to validate that the npm and node.js is of acceptable version, and will warn or fail if either one is too old. If either of the npm or node.js is missing, Vaadin plugin will automatically download both, into the $HOME/.vaadin/node folder.

Afterwards, Vaadin plugin will generate the necessary npm and webpack configuration files: webpack.config.js, webpack.generated.js and package.json. Note that the node_modules nor package-lock files are not generated at this point: they will be generated later on:

  • In development mode: Vaadin Servlet will run npm and webpack automatically, before the app is started, to populate those folders.
  • In production mode, you run build-frontend which will run npm and webpack and will produce the huge JavaScript file, containing all npm modules.

After the prepare-frontend finishes, you’re all set to run the Vaadin application in the development mode.


build-frontend builds on the outcome of the prepare-frontend task, and thus it requires the prepare-frontend task to be run beforehand.

Note: The Gradle Vaadin Plugin configures Gradle to handle this automatically, so if you run vaadinBuildFrontend task, the vaadinPrepareFrontend task is going to be run, too.

You can again follow the Vaadin Gradle plugin build-frontend task implementation.

First, the flow-build-info.json file is modified. Since we are targeting production mode here, the Vaadin Servlet will no longer start embedded webpack and will serve precompiled JavaScript files instead. That’s why we set enableDevServer to false and remove all npm-related settings like npmFolder, generatedFolder and frontendFolder. We can also remove pnpm.enable since it doesn’t matter how the precompiled JavaScript files were produced. Also we remove require.home.node since in production mode we do not need node.js, npm nor webpack anymore, since the precompiled JavaScript files will be produced by the Plugin and packaged into the WAR/EAR/JAR archive.

Next, we will run npm install to make sure all packages are available in node_modules, in order for webpack to be able to produce the precompiled JavaScript files (the runNodeUpdater() function in plugin’s sources).

Lastly, webpack is run, to package everything from frontend/ and node_modules into the precompiled JavaScript files bundle.

If this task succeeds, it should produce the following outputs:

  • node_modules and package-lock in the project root folder;
  • The META-INF/VAADIN/config/flow-build-info.json file into the folder where generated resources go;
  • The META-INF/VAADIN/config/stats.json into the folder where generated resources go;
  • The META-INF/VAADIN/build/ folder hosting generated JavaScript bundle, into the folder where generated resources go.

The placement of generated resources depends on whether the project is built by Maven (then the folder is somewhere in target/classes/) or Gradle (then it’s in build/resources/main/). It is now the job of the packaging plugin (WAR/EAR/JAR) to package those files and folders properly, so that Vaadin can use ClassLoader to load those files as resources from the classpath. For example:

  • When building JAR without Spring Boot (see vaadin14-embedded-jetty example project), those folders are simply located in the root of the jar file;
  • When building WAR (see e.g. skeleton-starter) then the folders are located in WEB-INF/classes/;
  • When building JAR+Spring Boot (e.g. skeleton-starter-flow-spring, those folders are located in BOOT-INF/classes/ in the Spring Boot jar file.

Please see Vaadin: the missing guide on the example contents of those files and folders.

See the Vaadin: Troubleshooting guide when something goes wrong.

Written on June 25, 2020