    @Test(expected = IllegalStateException.class)
    public void should_throw_IllegalStateException_if_already_installed() {

        ClassInjector injector = mock(ClassInjector.class, withSettings().stubOnly());
        given(instrumentation.getTargetContext()).willReturn(context);
        ClassLoader loader = new ClassLoader(injector) {
        };
        given(context.getClassLoader()).willReturn(loader);
        DexOpener.install(instrumentation);
    }