Symptoms

A test-case class is directly instantiating a PicoContainer and registering components within it.

Causes

Its too easy to use a PicoContainer instance and setup the component to test with it

It goes really bad when you're setting up consequential transitive dependencies For example:

public void testCocktailWithVodkaIsAlcoholic() {
    DefaultPicoContainer container = new DefaultPicoContainer();
    container.addComponent(Cocktail.class, BananaPopsicleCocktail.class);
    // needed by Cocktail
    container.addComponent(BananaLiqueur.class);
    container.addComponent(OrangeJuice.class);
    container.addComponent(PineappleJuice.class);
    container.addComponent(VanillaVodka.class);
    // needed by Cocktail's ingredients
    container.addComponent(LiqueurMaker.class);
    container.addComponent(StolichnayaDistillery.class);
    container.addComponent(FruitJuiceDistributor.class);

    Cocktail cocktail = container.getComponent(Cocktail.class);

    assertTrue(cocktail.isAlcoholic());
}

What To Do

For unit tests like this, the class being tested should be instantiated directly by the test. Mock Objects should be used to "mock" the dependent objects, i.e. supplying a fake implementation that can have expectations set and verified on it, rather than a real implementation. So, the test becomes:

public void testCocktailWithVodkaIsAlcoholic() {

    BananaLiqueur bl = mock(BananaLiqueur.class);
    OrangeJuice oj = mock(OrangeJuice.class);
    PineappleJuice pj = mock(PineappleJuice.class);
    VanillaVodka vv = mock(VanillaVodka.class);

    // set expectations these four according to your mocking framework

    Cocktail cocktail = new BananaPopsicleCocktail(bl, oj, pj, vv);
    assertTrue(cocktail.isAlcoholic());

    // verify expectations on the four mocks here.
}

The implementation details of creating a mock object and setting and verifying expectations have been left out of the example, as the details depend on which mock object library/technique is used. Search for EasyMock, JMock or Mockito in Google.

Exceptions

The container has to be instantiated somewhere of course.

Bootstrappers

A common place to instantiate containers is in some bootstrap class in the application.

Functional Tests

There may be a requirement to write a high-level "functional" test, in which you wish to test the interactions between a set of real components (not mocks). In this case, you may wish to create a container with the appropriate components registered for your test.