test rule

TestComponentRule.java

public class TestComponentRule implements TestRule {

    private ApplicationTestModule module;
    private TestComponent testComponent;

    private TestComponentRule(ApplicationTestModule applicationTestModule) {
        this.module = applicationTestModule;
    }

    private void setupDaggerTestComponentInApplication() {
        App application = App.get();
        if (application == null) { // this will be null in Unit Tests
            application = new App();
        }
        // this needs to be set here, because during the builder process
        // the Application has not been instantiated.
        module.setApplication(application);
        testComponent = DaggerTestComponent.builder()
                .applicationTestModule(module)
                .build();
        application.setComponent(testComponent);
    }

    @Override
    public Statement apply(final Statement base, Description description) {
        return new Statement() {
            @Override
            public void evaluate() throws Throwable {
                try {
                    setupDaggerTestComponentInApplication();
                    base.evaluate();
                } finally {
                    testComponent = null;
                }
            }
        };
    }
    ...
}