A list of frequently asked questions regarding the Gradle plugin.

Q: I’m getting java.io.FileNotFoundException: JAR entry during vaadinBuildFrontend

The full stack trace is

java.io.FileNotFoundException: JAR entry org/x/y/Z.class not found in /your/project/folder/build/libs/yourproject.jar
        at com.vaadin.flow.server.frontend.scanner.FrontendDependencies.visitClass(FrontendDependencies.java:490)
        at com.vaadin.flow.server.frontend.scanner.FrontendDependencies.collectEndpoints(FrontendDependencies.java:250)
        at com.vaadin.flow.server.frontend.scanner.FrontendDependencies.computeEndpoints(FrontendDependencies.java:228)
        at com.vaadin.flow.server.frontend.scanner.FrontendDependencies.<init>(FrontendDependencies.java:98)

Java 8 JDK may fail with a cryptic java.io.FileNotFoundException: JAR entry org/x/y/Z.class not found in /your/project/folder/build/libs/yourproject.jar when it’s running out of file handles on Linux. It could be that the Gradle daemon has been running in the background for some time, amassing lots of opened files in the meantime. Try killing it with killall java then restart the build, or run gradle with the --no-daemon switch.

You can also increase the file open limit. Try printing the current limit of open files via ulimit -n; if it’s 1024 or lower try to increase it:

$ ulimit -n 200000
$ killall java

Kill all java processes to make sure the Gradle daemon is dead and is started anew next time, using the new file limit.

Note that the new limit only applies to this shell process (and all processes launched by it), so make sure to run Gradle from this shell.

Reported as flow #12489

Q: I’m getting IllegalArgumentException: wrong number of arguments

The full stack trace is

Caused by: java.lang.IllegalArgumentException: wrong number of arguments
        at com.vaadin.flow.server.webcomponent.WebComponentModulesWriter$DirectoryWriter.generateWebComponentsToDirectory(WebComponentModulesWriter.java:246)
        at com.vaadin.flow.server.frontend.FrontendWebComponentGenerator.generateWebComponents(FrontendWebComponentGenerator.java:95)
        at com.vaadin.flow.server.frontend.NodeTasks.<init>(NodeTasks.java:451)

and it’s caused by your project using different Vaadin version than the Vaadin Gradle Plugin was intended for. For example, your project is using Vaadin 14.2 while you’re trying to use Gradle plugin 0.14.6.0. Try to change the Vaadin Gradle plugin version in your build.gradle to better match your Vaadin version. Alternatively, try to upgrade Vaadin version in your project.

Please see the Compatibility chart at the Vaadin gradle plugin page.

Q: My @CssImport is not picked up

Perhaps the component the import is attached to is not visible on classpath. More precisely:

• In development mode, Vaadin relies on servlet container to find all classes with Vaadin annotations. Make sure your servlet container properly discover all jars in runtime.
• In production mode, the Vaadin Maven/Gradle plugin is responsible for figuring out the classpath and finding all classes. Make sure the runtime classpath is properly populated at compile time.