Unit testing Singletons and/or "static" classes and possible alternatives

[Dennis Thompson]

Greetings fellow Coderanchers,

I have some questions regarding unit testing singletons and/or "static" classes [in java] and what are the potential alternatives. I'm aware this is a loaded question so I'll try my best to funnel to my issue.

I'm currently creating a simple wrapper around some of the java.nio libraries so I can use them in my application. Here's the code for reference: http://hastebin.com/tadeyicuho.java (sorry for the link, the code is class is 103 lines).

One of the goals when I was writing this is that I wanted to get 100% coverage so as I wrote this class I also wrote unit tests until I hit a bump in the road here:

public static void write(final String file, String message, final boolean appendNewLine) {
	if(checkArgs(file, message)) {
		final Path path = Paths.get(file);
		StandardOpenOption mode = StandardOpenOption.APPEND;
		if(Files.notExists(path)) {
			mode = StandardOpenOption.CREATE_NEW; // difficult to get full coverage
		}
		if(appendNewLine) {
			message += System.getProperty("line.separator");
		}
		try {
			Files.write(path, message.getBytes(), mode);
		} catch(IOException e) { // difficult to unit test without mocking
			handleException(e, "Problem writing to " + file);
		}
	}
}

At this point I turned to mocking using Mockito and PowerMock so I could handle final classes as well as mocking static methods. Unfortunately, PowerMock is not intuitive and I kept getting an unresolvable error that seemed to have even stumped StackOverflow. Anyway, so I turned to a singleton: http://hastebin.com/xipisibuze.java

I updated all my unit tests to reflection this change and my code coverage dropped from 85% coverage to 8%. At this point I found these articles:

http://misko.hevery.com/2008/07/30/top-10-things-which-make-your-code-hard-to-test/

http://programmers.stackexchange.com/questions/40373/so-singletons-are-bad-then-what

Multiple threads on this forum saying Singletons are less than ideal

In my reading I found guice so I could essentially simulate singletons (I think, at least), but I'm wondering if this:

Appropriate

Is there a better, more test-friendly solution/design pattern

At the end of the day, I just want to learn what I should do in situations like this in the future. What can be done? How does one go about unit testing file I/O operations? What about testing static classes? Singletons?

[Dave Tolls]

Guice uses dependency injection, which means it's a lot easier to mock out things in your instance.
It's probably between that and Spring, and I think Guice is a lighter toolkit.

I would recommend either of them over a Singleton which, as you have seen, are a pain to test.