Can I specify an instance of a test class when using a JUnit5 launcher

Question

I would like to programmatically launch JUnit5 tests not on a class but on a specific instance of the class so I can perform dependency injection through the class's constructor.

I can launch execution on a class with:

launcher.execute(request().selectors(selectClass(MyTest.class)).build());

Is there a way to instead launch it on, for example, new MyTest(someParameter)?

Answer 1

Use a Jupiter extension for this purpose. Either TestInstanceFactory or TestInstancePostProcessor look like good candidates:

class CreateMyTestExtension implements TestInstanceFactory {

    @Override
    public Object createTestInstance(TestInstanceFactoryContext factoryContext, ExtensionContext extensionContext) throws TestInstantiationException {
        String param = "a string"; // Get it from wherever
        return new MyTest(param);
    }
}


@ExtendWith(CreateMyTestExtension.class)
class MyTest {

    private final String param;

    MyTest(String param) {
        this.param = param;
    }

    @Test
    void aTest() {
        System.out.println("param = " + param);
    }
}

You can then run your test through the IDE or Gradle or Maven or use the standard code for launcher as suggested in the question:

launcher.execute(request().selectors(selectClass(MyTest.class)).build());

The open question is: Where does param come from? Depending on the answer to that you may have to use additional configuration parameters. Those can be set for a discovery request during launching and queried from the extension context: extensionContext.getConfigurationParameter("param1").