How to fix JavaFX IllegalArgumentError: cannot access class

Question

I'm trying to learn to use JavaFX, which I'm using in a maven based project. However, when trying to run a basic code example online, I'm getting errors and exceptions: there might be problems with how I imported maven dependencies in pom.xml, but I couldn't find them. How do I fix this?

Here is my code:

    package pom;

    import javafx.application.Application;
    import javafx.scene.Scene;
    import javafx.scene.control.Button;
    import javafx.scene.layout.StackPane;
    import javafx.stage.Stage;

    public class App extends Application {

        public static void main(String[] args) {
            launch(args);
        }

        @Override
        public void start(Stage primaryStage) throws Exception {
            primaryStage.setTitle("DANNI PUZZAAAA");
            Button button = new Button();
            button.setText("click me");
            StackPane layout = new StackPane();
            layout.getChildren().add(button);
            Scene scene = new Scene(layout, 300, 250);
            primaryStage.setScene(scene);
            primaryStage.show();
        }
    }

Here is the error log:

Exception in Application start method
java.lang.reflect.InvocationTargetException
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:567)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:464)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:363)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:567)
at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1051)
Caused by: java.lang.RuntimeException: Exception in Application start method
at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:900)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
at java.base/java.lang.Thread.run(Thread.java:835)
Caused by: java.lang.IllegalAccessError: superclass access check failed: class com.sun.javafx.scene.control.ControlHelper (in unnamed module @0x6dc32247) cannot access class com.sun.javafx.scene.layout.RegionHelper (in module javafx.graphics) because module javafx.graphics does not export com.sun.javafx.scene.layout to unnamed module @0x6dc32247
at java.base/java.lang.ClassLoader.defineClass1(Native Method)
at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1016)
at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:151)
at java.base/jdk.internal.loader.BuiltinClassLoader.defineClass(BuiltinClassLoader.java:802)
at java.base/jdk.internal.loader.BuiltinClassLoader.findClassOnClassPathOrNull(BuiltinClassLoader.java:700)
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:623)
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
at javafx.scene.control.Control.<clinit>(Control.java:86)
at pom.App.start(App.java:18)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:389)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
Exception running application pom.App

And here are my pom.xml dependencies and plugins:

<dependencies>
<dependency>
  <groupId>junit</groupId>
  <artifactId>junit</artifactId>
  <version>4.11</version>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.openjfx</groupId>
  <artifactId>javafx-controls</artifactId>
  <version>12.0.1</version>
</dependency>
<dependency>
  <groupId>org.openjfx</groupId>
  <artifactId>javafx-fxml</artifactId>
  <version>12.0.1</version>
</dependency>

<plugins>
    <!-- clean lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#clean_Lifecycle -->
    <plugin>
      <artifactId>maven-clean-plugin</artifactId>
      <version>3.1.0</version>
    </plugin>
    <!-- default lifecycle, jar packaging: see https://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
    <plugin>
      <artifactId>maven-resources-plugin</artifactId>
      <version>3.0.2</version>
    </plugin>
    <plugin>
      <artifactId>maven-compiler-plugin</artifactId>
      <version>3.8.0</version>
    </plugin>
    <plugin>
      <artifactId>maven-surefire-plugin</artifactId>
      <version>2.22.1</version>
    </plugin>
    <plugin>
      <artifactId>maven-jar-plugin</artifactId>
      <version>3.0.2</version>
    </plugin>
    <plugin>
      <artifactId>maven-install-plugin</artifactId>
      <version>2.5.2</version>
    </plugin>
    <plugin>
      <artifactId>maven-deploy-plugin</artifactId>
      <version>2.8.2</version>
    </plugin>
    <!-- site lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
    <plugin>
      <artifactId>maven-site-plugin</artifactId>
      <version>3.7.1</version>
    </plugin>
    <plugin>
      <artifactId>maven-project-info-reports-plugin</artifactId>
      <version>3.0.0</version>
    </plugin>
    <!--JAVA FX-->
    <plugin>
      <groupId>org.openjfx</groupId>
      <artifactId>javafx-maven-plugin</artifactId>
      <version>0.0.2</version>
      <configuration>
        <mainClass>org.openjfx.App</mainClass>
      </configuration>
    </plugin>
    <!--end of JAVA FX-->
  </plugins>

Thanks in advance.

'Caused by: java.lang.RuntimeException: Exception in Application start method', indicates that there is some exception in the 'start' method, may be a NullPointer. Surround the code within start with try catch and try to get a more specific exception. — Ironluca, May 19 '19 at 13:38
Your package `pom` doesn't match the `mainClass` you have set in your pom: `org.openjfx.App`. See a proper tutorial here: https://openjfx.io/openjfx-docs/#maven — José Pereda, May 19 '19 at 13:40
@JoséPereda you're right. I tried fixing with both `App` and `pom.App`, but I still get the same errors as before. — giozom1, May 19 '19 at 13:56
Though I would expect the [javafx-maven-plugin](https://github.com/openjfx/javafx-maven-plugin) to properly handle this for you. — Slaw, May 19 '19 at 14:47
@Slaw That would be the case, should the project have been properly setup, but I don't think that is the case. — José Pereda, May 19 '19 at 15:00
@giozom Make sure you have the proper project structure, and see this [sample](https://github.com/openjfx/samples/tree/master/CommandLine/Non-modular/Maven/hellofx). — José Pereda, May 19 '19 at 15:00

How to fix JavaFX IllegalArgumentError: cannot access class

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