测试类取决于它的静态final字段

Question

I wonder if it is possible to test somehow class which is dependent on static final field? For example I have the following class:

public final class Foo
{
    private static Foo instance;
    public static final String BAR_FILE = "/usr/bin/bar.bar";

    private Foo()
    {
        loadBar();
    }

    public static synchronized Foo getInstance()
    {
        if (instance == null)
        {
            instance = new Foo();
        }
        return instance;
    }

    private void loadBar()
    {
        final Properties prop = new Properties();
        try
        {
            final FileInputStream fis = new FileInputStream(BAR_FILE);
            prop.load(fis);
            fis.close();
        }
        catch (final IOException ex)
        {
            System.out.println("Exception!");
        }
    }
}

So how can I test getInstance() (I know there is nothing to test, but this is just an example) method on windows if BAR_FILE is hardcoded and equals to some unix-style path. I tried to change this field via reflection, but no luck, and, moreover, in accordance with this discussion it is not even possible. Any help is appreciated!

Answer 1

Separate your business logic from the constructional logic. Your class should not care how it is created, leave that to either a factory or DI framework.

Using a DI framework this is very easy:

@Singleton
public final class Foo {
  public Foo(String fileName) { loadBar(fileName);}  // Use in tests
  public Foo() { loadBar(DEFAULT_FILENAME);} 
  private void loadBar(String file)
    {
         // SNIP
    }
}


public class ThingieThatUsesFoo {
     public ThingieThatUsesFoo(Foo foo) { ... } // The DI framework (Guice/Spring) will handle the details of WHAT gets injected.
}

Using a factory is a little more effort, but not a lot.

// You could make FooFactory a singleton if that is your thing.
public class FooFactory {
     private Foo fooInstance = new Foo();
     public Foo getFoo() {
         return fooInstance;
     }
     public void setFooForTestingOnly(Foo fooInstance) { ... } // Set this to a mock or what have thee when testing clients.
}

In both cases testing the client of Foo is easy as you can inject a mock and testing Foo is easier as you can now set the file to read (if you want to be clever don't pass in a file but a Reader or InputStream).