How to Choose Design Patterns for Web-Based Applications Using Servlets?

How should responsibilities be distributed among Servlets in a web-based application design?

// Example of Servlet handling multiple actions for an entity
@WebServlet("/entity")
public class EntityServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String action = request.getParameter("action");
        switch (action) {
            case "add":
                addEntity(request, response);
                break;
            case "edit":
                editEntity(request, response);
                break;
            case "delete":
                deleteEntity(request, response);
                break;
            default:
                throw new ServletException("Unknown action: " + action);
        }
    }
}

In web-based applications, proper distribution of responsibilities among Servlets is crucial for maintaining architecture that is both scalable and maintainable. This involves strategic design patterns that can simplify interactions and lead to cleaner code organization.

// Service Layer Example 
public class EntityService {
    public void addEntity(Entity entity) {
        // logic to add entity
    }
    public void editEntity(Entity entity) {
        // logic to edit entity
    }
    public void deleteEntity(int id) {
        // logic to delete entity
    }
}

Causes

• Complexity introduced by having multiple Servlets for similar functionalities.
• Difficulty in managing and maintaining numerous Servlet classes due to proliferation.
• Increased overhead in server resource management from maintaining many Servlet instances.

Solutions

• Adopt a Single Servlet per Entity approach that handles multiple actions (add, edit, delete) for that entity.
• Utilize a Front Controller design pattern with a single Servlet to direct all requests, improving centralized request handling.
• Implement a Service Layer that abstracts business logic, allowing Servlets to focus on request handling and routing rather than business rules.

Mistake: Creating too many Servlets leads to fragmented logic and increased complexity.

Solution: Consolidate related actions in a single Servlet per entity.

Mistake: Not utilizing the Service Layer results in Servlets holding too much business logic.

Solution: Ensure that complex business operations are handled within a separate Service Layer.

Mistake: Passing the entire request object to the Service Layer can expose too much data.

Solution: Only pass necessary parameters to the Service Layer for greater encapsulation.