How should I unit test code that uses the google guava libraries, especially stuff in the io package?

Question

A lot of the functionality in guava is provided by static methods. I haven't figured out how to merge the use of guava libraries and good Dependency Injection practice.

For example, if I were to use

Files.readLines(File, Charset)

then I find I have a hard time writing a unit test which doesn't touch the filesystem, which I only like to do for integration testing.

I guess it's possible that I could write an adapter for all of the ones I'm interested in? But that could possibly end up being a lot of work...

I find it odd that the guava libraries come from the same set of people that provide guice and write blog posts like this

Answer 1

Ugh, the dreaded static methods. I have heard that JMockit is capable of mocking out statics, but I've never tried it myself. The solution I typically use is an Adapter.

public class FilesAdapter {

    private final File file;

    public FilesAdapter( File file ) {
        this.file = file;
    }

    public List<String> readLines( Charset charset ) {
        return Files.readLines( file, charset );
    }
}

You can optionally have FilesAdapter implements an interface, although since this is a single purpose object, I typically wouldn't.

GUICE is capable of injecting concrete objects, and mocking frameworks such as JMock2 & Mockito are able to mock concretes as well. This is all a matter of academics and different people will have different opinions.

If you were using GUICE, you would wrap this guy in a factory for injection goodness.

public class FilesAdapter {

    private final File file;

    @Inject
    protected FilesAdapter( @Assisted File file ) {
        this.file = file;
    }

    public List<String> readLines( Charset charset ) {
        return Files.readLines( file, charset );
    }

    public interface Factory {
        FilesAdapter create( File file );
    }
}