Providing Mock API's at Runtime

This article is about how I provide a mock api's to my applications at runtime so I can test them without external services being involved.

Many of the application I work on make use of api's to servers, communications engines and so on. I find it useful to be able to test my application with mock API's so I have more control of the tests on it and can test it standalone. Also the mechanism provided allows me to run my application when a service has yet to be delivered but is defined. I could make use of dependency injection if I was to drag JEE api's into my build but I prefer the simple approach. It might not be perfect but it works.

Lets say I have an external interface that my application makes use of:


public interface Service {
    void addListener(final ServiceListener listener);
    void removeListener(final ServiceListener listener);
}


Now in my application I would typically construct a concrete class either directly  or via a factory method which implements the interface contract. Whilst testing I would like to actually provide another version of the implementing class which can be used to test my application with the interface:

public class MockService implements Service {

    private static List<ServiceListener> listeners = new ArrayList<ServiceListener>();

    public void addListener(ServiceListener listener) {
        listeners.add(listener);
    }

    public void removeListener(ServiceListener listener) {
        listeners.remove(listener);
    }

    public void newRequestReceived(final ExternalRequest request) {
        RequestEvent event = new RequestEvent(request);
        for(ServiceListener listener : listeners) {
            listener.notifyRequestEvent(event);
        }
    }

}

I provide the mock class making use of a command line property:


java -Dservice.api.classname=com.webbyit.MockService -jar myapp.jar



Now in the code of my app I make use of the property (maybe within a factory method) and construct the class provided rather than the default class:

private static Service getService() {
  
    synchronized (ServiceLock) {
        if (Service == null) { 
            String serviceClassName = System.getProperty("service.api.classname");

                if (serviceClassName == null) {
                     Service = new RealService();
                } else {
                    try {
                        Service = (Service) Class.forName(serviceClassName).newInstance();
                    } catch (InstantiationException e) {
                        throwCallServiceInitialisationException(serviceClassName, e);
                    } catch (IllegalAccessException e) {
                        throwCallServiceInitialisationException(serviceClassName, e);
                    } catch (ClassNotFoundException e) {
                        throwCallServiceInitialisationException(serviceClassName, e);
                    }
                }
            }
        }
        return Service;
    } 
}

So what I have done is test for an alternative class being specified by a property (via the command line). If set the code attempts to construct the alternative class and use that instead of the default class usually used by the application.