为什么必须从自己的类中启动JavaFx应用程序？

Question

I tried the following code and was forced to launch my JavaFx application called Viewer without directly calling the method launch .

Here is the JavaFx class:

package Freelance;

public class Viewer extends Application
{
    private WebEngine myWebEngine;

    public void start(Stage stage)
    {
        stage.setTitle("Browser");

        WebView myBrowser = new WebView();
        myWebEngine = myBrowser.getEngine();
        myWebEngine.getLoadWorker().exceptionProperty().addListener(new ChangeListener<Throwable>()
        {
            @Override
            public void changed(ObservableValue<? extends Throwable> observableValue, Throwable oldException,
                    Throwable exception)
            {
                System.out.println("WebView encountered an exception loading a page: " + exception);
            }
        });
        myBrowser.setPrefSize(1600, 900);

        BorderPane root = new BorderPane();
        root.setCenter(myBrowser);
        stage.setScene(new Scene(root));
        stage.show();
        myWebEngine.load("http://www.google.com/");

    }

    public static void run()
    {
        launch("");
    }

}

Now when I try to launch this from a separate class like so:

package Freelance;

public class Test
{

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

}

I get the following error:

Exception in thread "main" java.lang.RuntimeException: Error: class Freelance.Test is not a subclass of javafx.application.Application
    at javafx.application.Application.launch(Application.java:254)
    at Freelance.Test.main(Test.java:8)

However, if I change the Test class like so:

package Freelance;

public class Test
{

    public static void main(String[] args)
    {
        Viewer.run(); // Changed from using "launch()" to "run()"
    }

}

Then it works and launches properly.

So I was just curious as to why this happens or if I am writing the code in an incorrect format.

Thank you.

Answer 1

Since launch is a static method, the application class cannot be determined by means other than inspecting the stack, since after compilation

Application.launch("");

ends up the same in the bytecode as

Viewer.launch("");

Inspecting the stack only works, if you can find the Application class somewhere in the stack, which is why launch needs to be called from the Application class.

There is an alternative however:
You could pass the Application class to the overloaded version of the launch method :

Application.launch(Viewer.class, "");

Answer 2

It doesn't have to be launched from within the Application subclass.

You can do

package Freelance;

import javafx.application.Application ;

public class Test
{

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

}

Answer 3

When you take a look at javafx.application.Apllication 's implementation of launch(String... args) , you will find this code:

public static void launch(String... args) {
    // Figure out the right class to call
    StackTraceElement[] cause = Thread.currentThread().getStackTrace();

    boolean foundThisMethod = false;
    String callingClassName = null;
    for (StackTraceElement se : cause) {
        // Skip entries until we get to the entry for this class
        String className = se.getClassName();
        String methodName = se.getMethodName();
        if (foundThisMethod) {
            callingClassName = className;
            break;
        } else if (Application.class.getName().equals(className)
                && "launch".equals(methodName)) {

            foundThisMethod = true;
        }
    }

    if (callingClassName == null) {
        throw new RuntimeException("Error: unable to determine Application class");
    }

    try {
        Class theClass = Class.forName(callingClassName, false,
                           Thread.currentThread().getContextClassLoader());
        if (Application.class.isAssignableFrom(theClass)) {
            Class<? extends Application> appClass = theClass;
            LauncherImpl.launchApplication(appClass, args);
        } else {
            throw new RuntimeException("Error: " + theClass
                    + " is not a subclass of javafx.application.Application");
        }
    } catch (RuntimeException ex) {
        throw ex;
    } catch (Exception ex) {
        throw new RuntimeException(ex);
    }
}

As you see, the foreach-loop iterates over the stack-trace. This means: your need to call the launch(String... args) method from within the class providing the start() method.

The answers of fabian and James_D provide examples on how to launch JavaFX Applications form within another class.