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8.2.1 Service Layer pattern
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Two primary problems can occur when the controller component (like the servlet we just described) accesses business objects, such as
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Web User request Servlet new EventManager updateEvent(...)
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Figure 8.3 An application sequence diagram
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A quick overview of MVC
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an Event instance, in the domain model directly. First, the controller is directly dependent on the domain model. Any changes to the domain model will require changes to the controllers that interact with the model. Second, since there is no consistent way to interact with the domain model, it s easy for controller components to misuse the business objects. For example, if you have numerous controllers interacting with Event instances, you must somehow ensure that all of the necessary error handling and validation is performed. To get around these two problems, you can use the Service Layer pattern, also called the Business Delegate pattern, to provide a consistent interface to classes in the domain model. In this example, the Service Layer pattern is implemented by the EventManager class. Internally, the EventManager delegates persistence calls to an EventDao instance, while performing business operations itself. (You ll recall that we discussed the DAO pattern in the previous chapter.) The EventManager class is shown in listing 8.1. Listing 8.1 EventManager class
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package com.manning.hq.ch08; import com.manning.hq.ch08.Event; import com.manning.hq.ch08.EventDao; public class EventManager { public EventManager() { } public void save(Event e) throws ServiceLayerException { new EventDao().save(e); } public void delete(Event e) throws ServiceLayerException { new EventDao().delete(e); } public void delete(Long id) throws ServiceLayerException {
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Web frameworks: WebWork, Struts, and Tapestry
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Event e = new Event(); e.setId(id); this.delete(e); } public Event get(Long id) throws ServiceLayerException { return new EventDao().find(id); } }
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The version of the EventManager class in listing 8.1 just delegates all calls to the EventDao, like the non-Spring version we worked on in section 7.3. As the application grows and becomes more complex, additional business logic can be placed in the EventManager. Let s look at an example of adding a delegated method call to the EventManager. We ll examine two examples of the kinds of methods you might find in the EventManager class. One is pretty simple and just compares two Events. The second example utilizes our DAO class to associate an Attendee with an Event instance. First, we ll look at the comparator. Suppose you need to compare two Event instances. Start out by creating an EventComparator class, which implements the java.util.Comparator interface. The methods from this comparator are shown in listing 8.2. Listing 8.2 EventComparator
package com.manning.hq.ch08; import java.util.Comparator; import com.manning.hq.ch08.Event; public class EventComparator implements Comparator { public int compare(Object a, Object b) { if ( (a == null) || !(a instanceof Event) ) { throw new RuntimeException("First object is null or"+ " not an instance of Event"); } else if ( (b == null) || !(b instanceof Event) ) { throw new RuntimeException("Second object is null or"+
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" not an instance of Event"); } Event e0 = (Event) a; Event e1 = (Event) b; // just do a simple comparison return e0.getId().compareTo(e1.getId()); } public boolean equals(Object o) { if (o == null) { return false; } else { // this isn't a robust equality check, // but it works for our purposes. return (o instanceof EventComparator); } } }
With the EventComparator done, add the delegate method call to EventManager:
public int compare(Event a, Event b) { return new EventComparator().compare(a, b); }
The controller servlet then calls the compare(Object, Object) method in the EventManager class. Some software developers don t like moving all of the business logic into the service layer. Instead, they prefer to stick with a pure object-oriented design and have all of the business logic operations reside in the domain object. While we agree with this philosophy in principle, it s often more convenient to create a service layer containing the business logic when building web applications. Our comparator example is a bit contrived. In most cases, you d probably find a method comparing domain objects in the domain objects themselves. Next, let s look at the EventManager method that assigns an
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