How to Implement the Command Pattern in Java Servlets?

Question

What are the steps to implement the Command Pattern in a Java Servlet application?

// Command Interface
public interface Command {
    void execute(HttpServletRequest request, HttpServletResponse response);
}

// Concrete Command
public class ConcreteCommand implements Command {
    @Override
    public void execute(HttpServletRequest request, HttpServletResponse response) {
        // Processing logic here
    }
}

// Command Invoker
public class CommandInvoker {
    private Command command;

    public void setCommand(Command command) {
        this.command = command;
    }

    public void executeCommand(HttpServletRequest request, HttpServletResponse response) {
        command.execute(request, response);
    }
}

Answer

The Command Pattern is a behavioral design pattern that turns a request into a stand-alone object, thereby allowing for parameterization of clients with queues, requests, and operations. In the context of Java Servlets, it helps in separating the request handling logic from business logic, making the code more maintainable and scalable.

// Main servlet class
@WebServlet("/controller")
public class ControllerServlet extends HttpServlet {
    private CommandInvoker invoker;

    @Override
    public void init() {
        invoker = new CommandInvoker();
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String action = request.getParameter("action");
        Command command = getCommand(action);
        invoker.setCommand(command);
        invoker.executeCommand(request, response);
    }

    private Command getCommand(String action) {
        switch (action) {
            case "create":
                return new CreateCommand();
            case "update":
                return new UpdateCommand();
            default:
                return new DefaultCommand();
        }
    }
}

Causes

Complex request handling logic in servlets.
Difficulty in testing servlet behaviors.
Tight coupling between request processing and business logic.

Solutions

Define a Command interface with an execute method.
Create concrete command classes implementing the Command interface for each specific request.
Use a Command Invoker to execute commands based on the incoming requests.

Common Mistakes

Mistake: Not encapsulating request logic in commands.

Solution: Always ensure that command classes handle only the logic related to that specific command.

Mistake: Failing to maintain a consistent interface for commands.

Solution: Ensure all command implementations adhere to the same interface for uniformity.

Helpers

Java Servlet
Command Pattern
design patterns in Java
Servlet architecture
Java design patterns