TempDir cleanup timing uncertain after test execution

Problem Statement

JUnit 5.7+ introduces the @TempDir annotation for automatic creation of temporary directories in test methods. The Javadoc states that directories are deleted after test execution, but the exact moment of deletion is not guaranteed to be immediate. This uncertainty can lead to memory or file‑handle leaks, especially when tests spawn long‑running processes or rely on external tools that keep the files open.

Desired Goal

Determine whether the cleanup performed by @TempDir reliably frees all resources immediately after a test finishes, and identify the conditions under which residual files or handles may remain.

Constraints & Uncertainty

• JUnit 5.7‑5.10 show varying cleanup behavior; 5.8 introduced improvements but no definitive guarantee.
• Cleanup may be deferred until the JVM exits if the test terminates abruptly or if the test framework delays the deletion.
• Platform‑specific temp directory handling can affect the actual deletion timing.

Specific Questions

1. What internal JUnit mechanisms control the timing of @TempDir cleanup, and how do they differ across 5.7, 5.8, and later releases?
2. Under what test or JVM conditions can @TempDir directories persist beyond test completion, potentially causing a memory or file‑handle leak?
3. Is there a documented or recommended strategy for ensuring immediate cleanup when using @TempDir in long‑running or multi‑threaded tests?