What's the correct way to test code which performs IO?

Question

Is there a nice way to test code which performs IO? In Java in particular, if you go the Mock route, you need to wrap most of the native APIs in your own mockable classes, something like:

public class Filesystem {
  public void write(String filename, String text) throws FileNotFoundException {
    try (PrintWriter out = new PrintWriter(filename)) {
      out.println(text);
    }
  }
}

and then pass in an instance of Filesystem to any class that needs to write text for a file. Is there a better way?

Answer 1

Just like Frank says in the comment, I don't believe there is need to stub out file system.

Stubs exist because service you call is :

• Too complicated, that your test would mostly be testing that than
• Can change often, making your tests unstable
• Calling it might fail, making your test fragile
• Too slow to call, making your whole test suite slow
• The service might persist data that we send it, and make it hard to remove it

File system is none of those. Trying to stub it out would mean you implement exactly same features, only in-memory. It is extremely stable, so you don't have to worry about it changing. Calling file system rarely fails. File system operations are relatively fast. And while file system is persistent, it is easy to wipe it before every test.

So as long as you keep proper separation between tests, it is perfectly fine to use file system. This can be achieved, for example, by appending test name to each file/folder used by the test.