A Junit @rule for running tests on the JavaFX thread

JavaFXThreadingRule.java

import java.util.concurrent.CountDownLatch;

import javax.swing.SwingUtilities;

import javafx.application.Platform;
import javafx.embed.swing.JFXPanel;

import org.junit.Rule;
import org.junit.rules.TestRule;
import org.junit.runner.Description;
import org.junit.runners.model.Statement;

/**
 * A JUnit {@link Rule} for running tests on the JavaFX thread and performing
 * JavaFX initialisation.  To include in your test case, add the following code:
 * 
 * <pre>
 * {@literal @}Rule
 * public JavaFXThreadingRule jfxRule = new JavaFXThreadingRule();
 * </pre>
 * 
 * @author Andy Till
 * 
 */
public class JavaFXThreadingRule implements TestRule {
    
    /**
     * Flag for setting up the JavaFX, we only need to do this once for all tests.
     */
    private static boolean jfxIsSetup;

    @Override
    public Statement apply(Statement statement, Description description) {
        
        return new OnJFXThreadStatement(statement);
    }

    private static class OnJFXThreadStatement extends Statement {
        
        private final Statement statement;

        public OnJFXThreadStatement(Statement aStatement) {
            statement = aStatement;
        }

        private Throwable rethrownException = null;
        
        @Override
        public void evaluate() throws Throwable {
            
            if(!jfxIsSetup) {
                setupJavaFX();
                
                jfxIsSetup = true;
            }
            
            final CountDownLatch countDownLatch = new CountDownLatch(1);
            
            Platform.runLater(new Runnable() {
                @Override
                public void run() {
                    try {
                        statement.evaluate();
                    } catch (Throwable e) {
                        rethrownException = e;
                    }
                    countDownLatch.countDown();
                }});
            
            countDownLatch.await();
            
            // if an exception was thrown by the statement during evaluation,
            // then re-throw it to fail the test
            if(rethrownException != null) {
                throw rethrownException;
            }
        }

        protected void setupJavaFX() throws InterruptedException {
            
            long timeMillis = System.currentTimeMillis();
            
            final CountDownLatch latch = new CountDownLatch(1);
            
            SwingUtilities.invokeLater(new Runnable() {
                public void run() {
                    // initializes JavaFX environment
                    new JFXPanel(); 
                    
                    latch.countDown();
                }
            });
            
            System.out.println("javafx initialising...");
            latch.await();
            System.out.println("javafx is initialised in " + (System.currentTimeMillis() - timeMillis) + "ms");
        }
        
    }
}

Unfortunately does not work with @Test(timeout = ...) because JUnit apparently spans a new thread within baseStatement.evaluate(); ;-(

Cheers for this ... worked a treat.

In my opinion you should invoke countDownLatch.countDown(); in finally clause.
countDownLatch.await(); could be also dangerous. It would be better to use some timeout.

I'm curious: does this do anything which TestFX does not do? With TestFX the approach is different, in that if you need things to run in the JavaFX thread you call Platform.runLater( new Runnable(){ ... , but after calling that you can go (in the non-JavaFX test thread) WaitForAsyncUtils.waitForFxEvents(); Maybe you wrote this utility before TestFX was widely used? Also I'm never sure of the processing "cost" of TestFX, whereas I can see that your setup is clearly a lightweight thing which will take minimal time to run. Also why use SwingUtilities.invokeLater() when you have the JavaFX equivalent Platform.runLater()?

@Mrodent ha, using SwingUtilities.invokeLater() could be a bug but I can't remember now if there was a reason.