gistfile1.java

public class SmartBox {
    private Map<Class, Class> bindings;

    public SmartBox() {
        this.bindings = new HashMap<>();
    }

    /**
     * Binds {@TService} class to {@TInstance}
     *
     * @param service
     *         the service class to bind to
     * @param instance
     *         the instance that will be bound to the service
     * @param <TService>
     *         the type of the service
     * @param <TInstance>
     *         the type of the instance that must extend (or implement) {@TService}
     */
    public <TService, TInstance extends TService> void bind(Class<TService> service, Class<TInstance> instance) {
        bindings.put(service, instance);
    }

    /**
     * Gets an instance of {@TService}
     *
     * @param service
     *         the class of the requested service
     * @param <TService>
     *         the requested service
     * @return an instance of {@TService}
     */
    public <TService> TService get(Class<TService> service)
            throws IllegalAccessException, InstantiationException, InvocationTargetException {
        if (bindings.containsKey(service))
            // this service is bound! Let's get the bound instance for it.
            return (TService) get(bindings.get(service));
        else
            // no registered binding, let's see if we can resolve it
            return resolve(service);
    }

    private <TService> TService resolve(Class<TService> service)
            throws IllegalAccessException, InvocationTargetException, InstantiationException {
        // get constructor
        Constructor<?>[] constructors = service.getConstructors();
        if (constructors.length == 0)
            throw new IllegalArgumentException("No public constructor found for " + service);
        if (constructors.length > 1)
            throw new IllegalArgumentException("More than one public constructor found in " + service);
        Constructor constructor = constructors[0];

        // inspect the constructor parameters
        Class[] requestedServices = constructor.getParameterTypes();
        
        // resolve each constructor parameter
        Object[] resolvedServices = new Object[requestedServices.length];
        for (int i = 0; i < requestedServices.length; i++) {
            resolvedServices[i] = get(requestedServices[i]);
        }

        // invoke constructor
        return (TService) constructor.newInstance(resolvedServices);
    }
}