How do you unit test a JavaFX controller with JUnit

Question

What's the proper way of initializing the JavaFX runtime so you can unit test (with JUnit) controllers that make use of the concurrency facilities and Platform.runLater(Runnable)?

Calling Application.launch(...) from the @BeforeClass method results in a dead lock. If Application.launch(...) is not called then the following error is thrown:

 java.lang.IllegalStateException: Toolkit not initialized     at com.sun.javafx.application.PlatformImpl.runLater(PlatformImpl.java:121)     at com.sun.javafx.application.PlatformImpl.runLater(PlatformImpl.java:116)     at javafx.application.Platform.runLater(Platform.java:52)     at javafx.concurrent.Task.runLater(Task.java:1042)     at javafx.concurrent.Task.updateMessage(Task.java:987)     at com.xyz.AudioSegmentExtractor.call(AudioSegmentExtractor.java:64)     at com.xyz.CompletionControllerTest.setUp(CompletionControllerTest.java:69)     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)     at java.lang.reflect.Method.invoke(Method.java:601)     at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)     at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)     at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)     at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:27)     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)     at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)     at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)     at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)     at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)     at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)     at org.junit.runners.ParentRunner.run(ParentRunner.java:236)     at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)     at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197) 

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Followup: this is the motif I've been using based on recommendation by @SergeyGrinev.

... // Inside test class  public static class AsNonApp extends Application {     @Override     public void start(Stage primaryStage) throws Exception {         // noop     } }  @BeforeClass public static void initJFX() {     Thread t = new Thread("JavaFX Init Thread") {         public void run() {             Application.launch(AsNonApp.class, new String[0]);         }     };     t.setDaemon(true);     t.start(); } ... // controller tests follow... 

Answer 1

Calling launch() from @BeforeClass is a correct approach. Just note that launch() doesn't return control to calling code. So you have to wrap it into new Thread(...).start().

A 7 years later update:

Use TestFX! It will take care of launching in a proper way. E.g. you can extend your test from a TestFX's ApplicaionTest class and just use the same code:

public class MyTest extends ApplicationTest {  @Override public void start (Stage stage) throws Exception {     FXMLLoader loader = new FXMLLoader(             getClass().getResource("mypage.fxml"));     stage.setScene(scene = new Scene(loader.load(), 300, 300));     stage.show(); } 

and write tests like that:

@Test public void testBlueHasOnlyOneEntry() {     clickOn("#tfSearch").write("blue");     verifyThat("#labelCount", hasText("1")); } 

Answer 2

I found this to work,... but only after adding a Thread.sleep(500) after starting the JavaFX application thread. Presumably it takes some time to get the FX environment up and ready (about 200ms on my MacBook Pro retina)

@BeforeClass public static void setUpClass() throws InterruptedException {     // Initialise Java FX      System.out.printf("About to launch FX App\n");     Thread t = new Thread("JavaFX Init Thread") {         public void run() {             Application.launch(AsNonApp.class, new String[0]);         }     };     t.setDaemon(true);     t.start();     System.out.printf("FX App thread started\n");     Thread.sleep(500); }