Can @Nested class be non-static?

[vlsi]

Testcontainers hava a case with @RunWith(Enclosed.class) where they use static field in a nested class to hold a container:

See https://github.com/testcontainers/testcontainers-java/blob/63189c8678f81c8ef4c431e4468ee74640682654/modules/localstack/src/test/java/org/testcontainers/containers/localstack/LocalstackContainerTest.java#L78-L91

public class LocalstackContainerTest {

    public static class WithoutNetwork {

        // without_network {
        @ClassRule
        public static LocalStackContainer localstack = new LocalStackContainer(LocalstackTestImages.LOCALSTACK_IMAGE)
            .withServices(
                Service.S3,
                Service.SQS,
                Service.CLOUDWATCHLOGS,
                Service.KMS,
                LocalStackContainer.EnabledService.named("events")
            );

I don't understand the way to rewrite it to JUnit 5 since @Nested classes must be non-static.

Of course, I can try making the field non-static, however, it would cause the container to initialize multiple times (once for every test).
A workaround could be moving all the fields to a top-level, however, it would make the coder harder to read.

PS. I think I've created the question, however, it seems to disappear somehow.

[marcphilipp]

While @Nested required classes to be non-static, you can use static member classes in the same way as top-level classes. Depending on how you run them, you might have to rename them to have a Tests suffix (for example, for Maven Surefire), though.

[vlsi]

Ok, got it.
At the same time, it would be great to have a single "run" button for all the nested classes. If I go with individual static classes, then it looks like IDE won't provide "run all" button for "running all the nested classes.

It looks like there's an alternative like @TestInstance(Lifecycle.PER_CLASS), however, it might be great to just allow static classes for @Nested case.

[vlsi]

Wow. Finally I learned that Java 16+ allows static members in inner classes, so it would be possible to use static container field with a non-static nested test class if the codebase uses Java 17+. That is something new to me.

However, typically I prefer making nested classes static if they do not need to access outer class fields, so it still feels odd for JUnit to require inner classes for the tests :-/

[marcphilipp]

Allowing static @Nested classes will have implications that might complicate matters. Could you please create an issue?

[vlsi]

I've filed #4622