Executing a new JavaFX application on a separate JVM

Question

I'm trying to launch a new process on a separate JVM via code following the method illustrated here: Executing a Java application in a separate process

The code I'm using is the following (taken from the question above):

public static int exec(Class klass) throws IOException, InterruptedException {
        String javaHome = System.getProperty("java.home");
        String javaBin = javaHome +
                File.separator + "bin" +
                File.separator + "java";
        String classpath = System.getProperty("java.class.path");
        String className = klass.getName();

        ProcessBuilder builder = new ProcessBuilder(javaBin,"-cp",classpath,className);
        Process process = builder.inheritIO().start();
        process.waitFor();
        return process.exitValue();
    }

...in which klass is the class I want to launch. This would work for a normal Java process, but the problem is that I'm trying to launch a JavaFX application, and the code above generates the following error:

Error: JavaFX runtime components are missing, and are required to run this application

So, to add the JavaFX modules, I tried including the --module-path and --add-modules commands in the declaration of builder, I even attempted copying and pasting the entire execution command, and I kept getting this other error:

Unrecognized option: (command string with modules)
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

How could I solve this?

Let me know if details are needed.

Answer 1

Since the advent of modules, there are at least three different ways a JavaFX application can be configured:

1. Put everything on the module-path, including the JavaFX modules and your module.

   • This is the ideal situation but not always possible/viable (e.g. because of incompatible dependencies).

2. Put the JavaFX modules on the module-path and your own code on the class-path.

   • This configuration requires the use of --add-modules.

3. Put everything on the class-path, including the JavaFX modules and your own code.

   • With this configuration your main class cannot be a subtype of Application. Otherwise you get the error you mentioned in your question: "Error: JavaFX runtime components are missing, and are required to run this application".
   • This configuration allows for easy use of so-called fat/uber JARs.
   • Warning: This approach is explicitly unsupported.

The command line used with ProcessBuilder will depend on which configuration your application uses. You also have to take into account any other options passed the command line, such as the default encoding or locale. Unfortunately, your question doesn't provide enough information to tell what exactly is going wrong. The error you mention makes me think you're using the third configuration, but I can't be sure.

That said, I'll give some examples of launching the same application from within the application; you should be able to modify things to fit your needs. Note I used Java/JavaFX 13.0.1 when testing the below code.

---

Configuration #1

Put everything on the module-path.

module-info.java:

module app {
  requires javafx.controls;

  exports com.example.app to
      javafx.graphics;
}

Main.java:

package com.example.app;

import java.io.IOException;
import java.io.UncheckedIOException;
import java.nio.file.Path;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

import static java.lang.System.getProperty;

public class Main extends Application {

  private static void launchProcess() {
    try {
      new ProcessBuilder(
              Path.of(getProperty("java.home"), "bin", "java").toString(),
              "--module-path",
              getProperty("jdk.module.path"),
              "--module",
              getProperty("jdk.module.main") + "/" + getProperty("jdk.module.main.class"))
          .inheritIO()
          .start();
    } catch (IOException ex) {
      throw new UncheckedIOException(ex);
    }
  }

  @Override
  public void start(Stage primaryStage) {
    Button launchBtn = new Button("Launch process");
    launchBtn.setOnAction(
        event -> {
          event.consume();
          launchProcess();
        });
    primaryStage.setScene(new Scene(new StackPane(launchBtn), 500, 300));
    primaryStage.setTitle("Multi-Process Example");
    primaryStage.show();
  }
}

Command line:

java --module-path <PATH> --module app/com.example.app.Main

Replace "<PATH>" with a path containing both the JavaFX modules and the above module.

---

Configuration #2

Put JavaFX modules on the module-path and your code on the class-path.

Main.java:

package com.example.app;

import java.io.IOException;
import java.io.UncheckedIOException;
import java.nio.file.Path;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

import static java.lang.System.getProperty;

public class Main extends Application {

  private static void launchProcess() {
    try {
      new ProcessBuilder(
              Path.of(getProperty("java.home"), "bin", "java").toString(),
              "--module-path",
              getProperty("jdk.module.path"),
              "--add-modules",
              "javafx.controls",
              "--class-path",
              getProperty("java.class.path"),
              Main.class.getName())
          .inheritIO()
          .start();
    } catch (IOException ex) {
      throw new UncheckedIOException(ex);
    }
  }

  @Override
  public void start(Stage primaryStage) {
    Button launchBtn = new Button("Launch process");
    launchBtn.setOnAction(
        event -> {
          event.consume();
          launchProcess();
        });
    primaryStage.setScene(new Scene(new StackPane(launchBtn), 500, 300));
    primaryStage.setTitle("Multi-Process Example");
    primaryStage.show();
  }
}

Command line:

java --module-path <M_PATH> --add-modules javafx.controls --class-path <C_PATH> com.example.app.Main

Replace "<M_PATH>" with a path containing the JavaFX modules and replace "<C_PATH>" with a path containing the above code.

---

Configuration #3

Put everything on the class-path. Note the main class (now Launcher) is not a subclass of Application.

Launcher.java:

package com.example.app;

import javafx.application.Application;

public class Launcher {

  public static void main(String[] args) {
    Application.launch(Main.class, args);
  }
}

Main.java:

package com.example.app;

import java.io.IOException;
import java.io.UncheckedIOException;
import java.nio.file.Path;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

import static java.lang.System.getProperty;

public class Main extends Application {

  private static void launchProcess() {
    try {
      new ProcessBuilder(
              Path.of(getProperty("java.home"), "bin", "java").toString(),
              "--class-path",
              getProperty("java.class.path"),
              Launcher.class.getName())
          .inheritIO()
          .start();
    } catch (IOException ex) {
      throw new UncheckedIOException(ex);
    }
  }

  @Override
  public void start(Stage primaryStage) {
    Button launchBtn = new Button("Launch process");
    launchBtn.setOnAction(
        event -> {
          event.consume();
          launchProcess();
        });
    primaryStage.setScene(new Scene(new StackPane(launchBtn), 500, 300));
    primaryStage.setTitle("Multi-Process Example");
    primaryStage.show();
  }
}

Command line:

java --class-path <PATH> com.example.app.Launcher

Replace "<PATH>" with a path containing the JavaFX JARs and the above code.