What is the equivalent of ExternalResource and TemporaryFolder in JUnit 5?

Question

According to the JUnit 5 User Guide, JUnit Jupiter provides backwards compatibility for some JUnit 4 Rules in order to assist with migration.

As stated above, JUnit Jupiter does not and will not support JUnit 4 rules natively. The JUnit team realizes, however, that many organizations, especially large ones, are likely to have large JUnit 4 codebases including custom rules. To serve these organizations and enable a gradual migration path the JUnit team has decided to support a selection of JUnit 4 rules verbatim within JUnit Jupiter.

The guide goes on to say that one of the rules is ExternalResource, which is a parent for TemporaryFolder.

However, the guide unfortunately doesn't go on to say what the migration path is, or what the equivalent is for those writing new JUnit 5 tests. So what should we use?

Answer 1

JUnit 5.4 comes with a built-in extension to handle temporary directories in tests.

@org.junit.jupiter.api.io.TempDir annotation can be used in order to annotate class field or a parameter in a lifecycle (e.g. @BeforeEach) or test method of type File or Path.

import org.junit.jupiter.api.io.TempDir;

@Test
void writesContentToFile(@TempDir Path tempDir) throws IOException {
    // arrange
    Path output = tempDir
            .resolve("output.txt");

    // act
    fileWriter.writeTo(output.toString(), "test");

    // assert
    assertAll(
            () -> assertTrue(Files.exists(output)),
            () -> assertLinesMatch(List.of("test"), Files.readAllLines(output))
    );
}

You can read more on this in my blog post, where you will find some more examples on utilizing this built-in extension: https://blog.codeleak.pl/2019/03/temporary-directories-in-junit-5-tests.html.