OS 002: Unit Test Conventions

OS-002 Define Unit Test Conventions

Date: 2020-06-15




OS 002 head

Some libraries need to create files on a file system. For example, you want to generate human-readable paper invoices.


We recommend the following conventions when writing unit tests which need to create files. Use Google Java in memory file system Jimfs. Additional file systems can only be used in Java if all operations are performed through Path objects. Path objects have a reference to the file system; File objects always referenced the default file system. When you are writing to the default file system use the root directory /tmp/ available in all Unix like operating systems.

Avoid writing files to a local file system. This approach is slow, cumbersome and error-prone. Use in-memory file systems.


  • We have a dependency on an open source product currently managed by Google.

  • Use of Jimfs requires use of Path instances. We shall remove all references to File instances. File instances do not support alternative file systems. Therefore, we shall use libraries providing a workaround File instances either by using Path instances or reader and writer to access files.

  • Operations on the physical file system have the risk that rights problems could rise in some continuous integration environment.