Design patterns are fundamental concepts in software engineering that provide reusable solutions to common problems. They help in creating more maintainable and scalable software. In this article, we will explore several design patterns and provide Java examples for each pattern's practical application.

Presentation Tier

Intercepting Filter Pattern

The Intercepting Filter Pattern is used when the intent is to perform pre or post-processing with the request or response of an application, typically in web applications. In Java, you can implement this pattern using servlet filters. Here's an example:

public class LoggingFilter implements Filter {
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
        throws IOException, ServletException {
        // Pre-processing logic
        // ...

        // Pass the request to the next filter or servlet in the chain
        chain.doFilter(request, response);

        // Post-processing logic
        // ...
    }

    // Other methods for initialization and cleanup
}

Front Controller Pattern

The Front Controller Pattern is employed when the intent is to have a centralized request handling mechanism that dispatches requests to the appropriate handlers. In Java EE applications, a servlet can act as a front controller. Here's a simplified example:

public class FrontControllerServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
        // Determine the appropriate handler for the request
        // ...

        // Dispatch to the appropriate handler
        // ...
    }
}

Business Tier

Business Delegate Pattern

The Business Delegate Pattern is used when the intent is to provide a single point of entry for clients to access business services, especially when separating the presentation tier and the business tier. Here's a simple Java example:

public class BusinessDelegate {
    private BusinessService businessService;

    public BusinessDelegate() {
        businessService = new BusinessService();
    }

    public void doTask() {
        // Delegate the task to the business service
        businessService.doTask();
    }
}

Composite Entity Pattern

The Composite Entity Pattern is often used in database applications to manage a graph of persistent objects using a primary composite entity. Here's a basic Java example:

public class CompositeEntity {
    private CoarseGrainedObject cgo = new CoarseGrainedObject();

    public void setData(String data1, String data2) {
        cgo.setData(data1, data2);
    }

    public String[] getData() {
        return cgo.getData();
    }
}

Service Locator Pattern

The Service Locator Pattern is employed when the intent is to locate services using JNDI (Java Naming and Directory Interface) lookup. Here's a simplified example:

public class ServiceLocator {
    public Service getService(String serviceName) {
        // Perform JNDI lookup to obtain the service
        // ...

        // Return the service
        return service;
    }
}

Session Facade Pattern

The Session Facade Pattern can be used in Java EE applications when the intent is to encapsulate business-tier components and expose a coarse-grained service to remote clients. Here's a basic example:

public class SessionFacade {
    private BusinessComponent businessComponent;

    public SessionFacade() {
        businessComponent = new BusinessComponent();
    }

    public void performBusinessOperation() {
        // Encapsulate business logic and provide a coarse-grained service
        businessComponent.performOperation();
    }
}

Transfer Object (Value Object) Pattern

The Transfer Object Pattern is used to pass data with multiple attributes in one shot from the client to the server. In Java, this pattern is typically implemented using POJOs (Plain Old Java Objects) with getter and setter methods, and the objects must be serializable. Here's a simple example:

public class TransferObject implements Serializable {
    private String attribute1;
    private int attribute2;

    // Getter and setter methods for attributes
    // ...
}

Value List Handler Pattern

The Value List Handler Pattern is used for managing the results of search operations, especially when results can be paged and traversed iteratively. Here's a basic example:

public class ValueListHandler {
    public List<ValueObject> getPage(int pageNumber) {
        // Retrieve a specific page of results
        // ...
    }
}

Integration Tier

Data Access Object (DAO) Pattern

The Data Access Object (DAO) Pattern is commonly used in Java EE applications to separate data persistence logic into a separate layer. Here's a simplified example:

public class UserDao {
    public User getUserById(int userId) {
        // Data access logic to retrieve a user from the database
        // ...
    }
}

Service Activator Pattern

The Service Activator Pattern is used to invoke a service asynchronously. An example of this pattern is a JMS (Java Message Service) listener that waits for a messaging request and delegates it to an appropriate service. Here's a high-level example:

public class ServiceActivator {
    public void activateService(ServiceRequest request) {
        // Asynchronously invoke the service based on the request
        // ...
    }
}

In conclusion, design patterns are invaluable tools for designing and architecting software systems. They provide tested and proven solutions to recurring design problems. By applying these design patterns in your Java applications, you can enhance code reusability, maintainability, and scalability, ultimately leading to more robust and efficient software.