Validate actual rule object instead of Field#type for @Rule

Currently, JUnit validates field’s type, and it wants the declaration to implement TestRule.

Is there a reason for that? What if JUnit4 validated the actual value rather than field type?

The validation makes it harder to implement code that supports both JUnit4 and JUnit5.

Sample issue: https://github.com/testcontainers/testcontainers-java/issues/970 (/cc @bsideup, @baev)

TL;DR: Testcontainers has to implement junit4-specific interfaces in Testcontainers APIs, thus Testcontainers forces everybody to have JUnit4 on the classpath which is sad.

https://github.com/testcontainers/testcontainers-java/blob/aa273b5c0136d8bf8d9eb308040b30006ad98144/core/src/main/java/org/testcontainers/containers/Network.java#L20

public interface Network extends AutoCloseable, TestRule { // <-- it would be so much better to be able to remove TestRule here
...

However, if Network extends TestRule is replaced with NetworkImpl implements Network, TestRule, then JUnit4 fails as follows:

@RunWith(Enclosed.class)
public class NetworkTest {

    public static class WithRules {

        @Rule
        public Network network = newNetwork();

Gradle Test Executor 1 > org.testcontainers.containers.NetworkTest > org.testcontainers.containers.NetworkTest$WithRules.initializationError FAILED
    org.junit.runners.model.InvalidTestClassError: Invalid test class 'org.testcontainers.containers.NetworkTest$WithRules':
      1. The @Rule 'network' must implement MethodRule or TestRule.
        at org.junit.runners.ParentRunner.validate(ParentRunner.java:525)
        at org.junit.runners.ParentRunner.<init>(ParentRunner.java:102)
        at org.junit.runners.BlockJUnit4ClassRunner.<init>(BlockJUnit4ClassRunner.java:84)
        at org.junit.runners.JUnit4.<init>(JUnit4.java:23)
        at org.junit.internal.builders.JUnit4Builder.runnerForClass(JUnit4Builder.java:10)
        at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:70)
        at org.junit.internal.builders.AllDefaultPossibilitiesBuilder.runnerForClass(AllDefaultPossibilitiesBuilder.java:37)
        at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:70)
        at org.junit.runners.model.RunnerBuilder.runners(RunnerBuilder.java:125)
        at org.junit.runners.model.RunnerBuilder.runners(RunnerBuilder.java:111)
        at org.junit.runners.Suite.<init>(Suite.java:102)
        at org.junit.experimental.runners.Enclosed.<init>(Enclosed.java:31)
        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:107)
        at org.junit.internal.builders.AnnotatedBuilder.runnerForClass(AnnotatedBuilder.java:86)
        at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:70)
        at org.junit.internal.builders.AllDefaultPossibilitiesBuilder.runnerForClass(AllDefaultPossibilitiesBuilder.java:37)
        at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:70)
        at org.junit.internal.requests.ClassRequest.createRunner(ClassRequest.java:28)
        at org.junit.internal.requests.MemoizingRequest.getRunner(MemoizingRequest.java:19)
        at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.runTestClass(JUnitTestClassExecutor.java:78)
        at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:58)
        at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:38)
        at org.gradle.api.internal.tasks.testing.junit.AbstractJUnitTestClassProcessor.processTestClass(AbstractJUnitTestClassProcessor.java:62)
        at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.processTestClass(SuiteTestClassProcessor.java:51)

See also https://github.com/junit-team/junit4/pull/1020

[marcphilipp]

@panchenko What I meant was sth. along these lines:

JUnit 4

public class SomeTests {

    @Rule TestRule rule = new TestcontainersRule();

    Network network = new Network();

	GenericContainer container = new GenericContainer(...);

	@Test public void test() { ... }
}

JUnit Jupiter

@ExtendWith(TestcontainersExtension.class)
public class SomeTests {

    Network network = new Network();

	GenericContainer container = new GenericContainer(...);

	@Test public void test() { ... }
}

[marcphilipp]

There are certainly some details to be worked out which is why I tried to “summon” @bsideup and @rnorth above.