Document that test slices should not be combined

[G-Lem]

Could you have a look at this issue ?
https://stackoverflow.com/questions/52527394/in-spring-boot-2-1-many-test-slices-are-not-allowed-anymore-due-to-multiple-boo?noredirect=1#

[philwebb]

To summarize the stackoverflow question, the following test:

@RunWith(SpringRunner.class)
@JdbcTest
@JsonTest
public class MyTest {

	@Test
	public void test() {
		System.out.println("Hello, World !");
	}

}

Fails with Spring Boot 2.1 due to:

java.lang.IllegalStateException: Configuration error: found multiple declarations of @BootstrapWith for test class [scratch.MyTest]: [@org.springframework.test.context.BootstrapWith(value=class org.springframework.boot.test.autoconfigure.jdbc.JdbcTestContextBootstrapper), @org.springframework.test.context.BootstrapWith(value=class org.springframework.boot.test.autoconfigure.json.JsonTestContextBootstrapper)]
	at org.springframework.test.context.BootstrapUtils.resolveExplicitTestContextBootstrapper(BootstrapUtils.java:166)
	at org.springframework.test.context.BootstrapUtils.resolveTestContextBootstrapper(BootstrapUtils.java:127)
	at org.springframework.test.context.TestContextManager.<init>(TestContextManager.java:124)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTestContextManager(SpringJUnit4ClassRunner.java:151)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.<init>(SpringJUnit4ClassRunner.java:142)
	at org.springframework.test.context.junit4.SpringRunner.<init>(SpringRunner.java:49)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
	at org.junit.internal.builders.AnnotatedBuilder.buildRunner(AnnotatedBuilder.java:104)
	at org.junit.internal.builders.AnnotatedBuilder.runnerForClass(AnnotatedBuilder.java:86)
	at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
	at org.junit.internal.builders.AllDefaultPossibilitiesBuilder.runnerForClass(AllDefaultPossibilitiesBuilder.java:26)
	at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
	at org.junit.internal.requests.ClassRequest.getRunner(ClassRequest.java:33)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestLoader.createUnfilteredTest(JUnit4TestLoader.java:87)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestLoader.createTest(JUnit4TestLoader.java:73)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestLoader.loadTests(JUnit4TestLoader.java:46)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:522)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)

[philwebb]

The root cause is the change made for #14052 which means we now have two different bootstrap classes.

[philwebb]

I wonder if we can meta-annotate @JdbcTest with @SpringBootTest and use @AliasFor to get the properties.

[dreis2211]

@philwebb if you don't mind I'd like to give this a shot.

[dreis2211]

Your idea works, but not for @WebMvcTest and @WebFluxTest because they already have dedicated bootstrap classes. Any thoughts? E.g. fixing it for the others with your idea and document the shortcomings of the other two?
In fact they already do that somehow (as well the others):

Can be used when a test focuses **only** on...

I wonder if multiple slices are supported at all officially.

[philwebb]

I wonder if multiple slices are supported at all officially.

@dreis2211 Thinking about this some more, I'm of the opinion that it worked by luck last time and it isn't something we should officially support. It's going to add a lot of complexity if we let people compose these annotations. For example, how would be combine the new properties attributes.

I'll switch this one to a documentation issue.

[philwebb]

@G-Lem I just posted an answer to the stackoverflow question https://stackoverflow.com/a/53033018/1526192

[dreis2211]

@philwebb For learning purposes I made a best effort implementation that seems to be working. If you're interested: master...dreis2211:gh-14981

This also already includes a change in the documentation, which I'm happy to isolate in a dedicated PR if necessary.

Let me know what you think,

[wilkinsona]

@dreis2211 A pull request of that documentation change would be great, please. Perhaps reworded slightly to make it sounds more definite by removing "officially" and replacing "you can pick" with "pick"?

[dreis2211]

@wilkinsona Give me a minute or two

[wilkinsona]

Wow, that was fast! Thank you, @dreis2211. Closing in favour of #15310.