[timtebeek]

TemporaryFolderToTempDir turns a TemporaryFolder field without @Rule or @ClassRule into a File initialized with Files.createTempDirectory("junit").toFile(). That method throws the checked IOException, which a field initializer cannot throw, so the class no longer compiles.

Before:

TemporaryFolder serverGitFolder = new TemporaryFolder();

After (does not compile: unreported exception IOException):

File serverGitFolder = Files.createTempDirectory("junit").toFile();

This surfaced while validating the Spring Boot 4.1 migration over open source Spring Boot 3.x repositories, in xm-online/xm-ms-config: LocalJGitRepositoryConfiguration, a Spring @Configuration in the test sources, holds three plain TemporaryFolder fields and calls create() on them itself.

Skipping such fields, or the file, would move the problem: the file also declares public static helpers that take a TemporaryFolder, and JGitRepositoryIntTest passes its @Rule TemporaryFolder fields to them. That test is converted to @TempDir File either way, so the helpers' parameters have to become File too.

This change keeps the conversion, and for a field initializer in Java it calls a private static helper that the recipe adds to the class:

File serverGitFolder = createTempDirectory();

private static File createTempDirectory() {
    try {
        return Files.createTempDirectory("junit").toFile();
    } catch (IOException e) {
        throw new UncheckedIOException(e);
    }
}

When the class already has a no-argument method of that name, the helper is called createTempDirectory2 (and so on). Local variables, arguments and Groovy are unchanged.

Is this the right change?

• Do we already have it? No. Handle non-@Rule TemporaryFolder fields #720 added the conversion of fields without a rule, but not for the checked exception.
• Could it be declarative? No, it depends on where the constructor call sits.
• Is it worth it? Yes, a plain TemporaryFolder field is a common way to get a temporary directory outside a rule, and the result does not compile.
• Does it fix the problem, or move it? It fixes it; the field and every method that receives it stay File, as before.
• Does it do no harm? @Rule/@ClassRule fields still become @TempDir without an initializer. The directory is created when the field is initialized rather than at create(), which the recipe already removed.

Tests: TemporaryFolderToTempDirTest: new fieldNotAsRule (instance and static fields, create() and getRoot() in methods and on a parameter; failed before) and ruleFieldNextToPlainFieldAndExistingHelperName (a @Rule field next to a plain one, with an existing createTempDirectory()). TemporaryFolderToTempDirTest, JUnit5MigrationTest, UseWiremockExtensionTest, HandleExternalResourceRulesTest, RemoveJupiterMigrationSupportTest, TimeoutRuleToClassAnnotationTest and MockitoJUnitToMockitoExtensionTest pass.