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copied!<blockquote> <p>But what is not clear to me is where the factory "layer" lies with a DDD architecture? Should the factory be calling directly into the repository to get its data or the service library?</p> </blockquote> <p>The factory should be the one-stop shop to construct domain objects. Any other part of the code that needs to do this should use the factory.</p> <p>Typically, there are at least three sources of data that are used as input into a factory for domain object construction: input from the UI, the results of queries from persistence, and domain-meaningful requests. So to answer your specific question, the repository would use the factory.</p> <p>Here is an example. I am using <a href="http://www.javaworld.com/javaworld/jw-01-2004/jw-0102-toolbox.html?page=4" rel="noreferrer">Holub's Builder pattern</a> here. <strong>Edit</strong>: disregard the use of this pattern. I've started realizing that it <a href="http://moffdub.wordpress.com/2009/02/22/i-suffer-from-ocd-odorless-code-dreamer/" rel="noreferrer">doesn't mix too well with DDD factories</a>. </p> <pre><code>// domain layer class Order { private Integer ID; private Customer owner; private List<Product> ordered; // can't be null, needs complicated rules to initialize private Product featured; // can't be null, needs complicated rules to initialize, not part of Order aggregate private Itinerary schedule; void importFrom(Importer importer) { ... } void exportTo(Exporter exporter) { ... } ... insert business logic methods here ... interface Importer { Integer importID(); Customer importOwner(); Product importOrdered(); } interface Exporter { void exportID(Integer id); void exportOwner(Customer owner); void exportOrdered(Product ordered); } } // domain layer interface OrderEntryScreenExport { ... } // UI class UIScreen { public UIScreen(OrderEntryDTO dto) { ... } } // App Layer class OrderEntryDTO implements OrderEntryScreenExport { ... } </code></pre> <p>Here is what the OrderFactory might look like:</p> <pre><code>interface OrderFactory { Order createWith(Customer owner, Product ordered); Order createFrom(OrderEntryScreenExport to); Order createFrom(List<String> resultSets); } </code></pre> <p>The logic for the featured Product and the generation of the Itinerary go in the OrderFactory.</p> <p>Now here is how the factory might be used in each instance.</p> <p>In OrderRepository:</p> <pre><code>public List<Order> findAllMatching(Criteria someCriteria) { ResultSet rcds = this.db.execFindOrdersQueryWith(someCriteria.toString()); List<List<String>> results = convertToStringList(rcds); List<Order> returnList = new ArrayList<Order>(); for(List<String> row : results) returnList.add(this.orderFactory.createFrom(row)); return returnList; } </code></pre> <p>In your application layer:</p> <pre><code>public void submitOrder(OrderEntryDTO dto) { Order toBeSubmitted = this.orderFactory.createFrom(dto); this.orderRepo.add(toBeSubmitted); // do other stuff, raise events, etc } </code></pre> <p>Within your domain layer, a unit test perhaps:</p> <pre><code>Customer carl = customerRepo.findByName("Carl"); List<Product> weapons = productRepo.findAllByName("Ruger P-95 9mm"); Order weaponsForCarl = orderFactory.createWith(carl, weapons); weaponsForCarl.place(); assertTrue(weaponsForCarl.isPlaced()); assertTrue(weaponsForCarl.hasSpecialShippingNeeds()); </code></pre> <blockquote> <p>Where does the factory fit into the following framework: UI > App > Domain > Service > Data</p> </blockquote> <p>Domain.</p> <blockquote> <p>Also, because the factory is the only place allowed for object creation would'nt you get circular references if you wanted to create your objects in your data and service layers?</p> </blockquote> <p>In my example, all dependencies flow from top to bottom. I used the <a href="http://www.objectmentor.com/resources/articles/dip.pdf" rel="noreferrer">Dependency Inversion Principle</a> (PDF link) to avoid the problem you speak of.</p> <blockquote> <p>If the role of the factory class is for object creation then what benefits does the service layer have?</p> </blockquote> <p>When you have logic that doesn't fit into any single domain object OR you have an algorithm that involves orchestrating multiple domain objects, use a service. The service would encapsulate any logic that doesn't fit in anything else and delegate to domain objects where it does fit. </p> <p>In the example I scribbled here, I imagine that coming up with an Itinerary for the Order would involve multiple domain objects. The OrderFactory could delegate to such a service.</p> <p>BTW, the hierarchy you described should probably be UI > App > Domain Services > Domain > Infrastructure (Data)</p> <blockquote> <p>I've asked a lot of questions and appreciate any response. What i'am lacking is a sample application which demonstrates how all the layers in a domain driven design project come together...Is there anything out there?</p> </blockquote> <p><a href="http://rads.stackoverflow.com/amzn/click/0321268202" rel="noreferrer">Applying Domain Driven Design and Patterns</a> by Jimmy Nilsson is a great compliment to Eric Evans' <a href="http://rads.stackoverflow.com/amzn/click/0321125215" rel="noreferrer">Domain-Driven Design</a>. It has lots of code examples, though I don't know if there is an emphasis on layering. Layering can be tricky and is almost a topic separate from DDD.</p> <p>In the Evans book, there is a very small example of layering you might want to check out. Layering is an enterprise pattern, and Martin Fowler wrote <a href="http://rads.stackoverflow.com/amzn/click/0321127420" rel="noreferrer">Patterns of Enterprise Application Architecture</a>, which you might find useful too.</p>

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