How do I test something which uses the System Filesystem? Apache NetBeans Wiki Index Note: These pages are being reviewed. There is a fake System FileSystem provided by as soon as FileSystem API is on classpath. It understands NetBeans layer definitions and is generally suitable for running unit tests. In case you see difference from expected behavior: Try to run tests in real mode Report NetBeans a bug (product platform/filesystems) with a test case See this page in GitHub.