Enterprise-level Java applications require design patterns to improve code maintainability, decouple and deal with complex business logic. 1. Factory/Dependency Injection is used to decouple object creation and use, and supports dynamic switching implementation at runtime; 2. Strategy Pattern encapsulates variable algorithms and supports runtime switching strategies; 3. Template Method defines a fixed process skeleton, allowing subclasses to implement variable steps; 4. Observer/Event Listener implements decoupling between components and supports event-driven development. The rational use of these modes can improve system scalability and testability and avoid over-design.

In enterprise-level Java application development, the use of design patterns is almost standard. They not only help us write clearer and maintainable code, but also cope with complex business logic and system scaling needs. The key is not how many modes are used, but the right one is used in the right place.

Why do enterprise applications require design patterns?

Enterprise applications often involve collaboration between large amounts of data processing, transaction control, security, and multi-layer architectures. In these scenarios, writing the "noodle style" code directly will soon get out of control. Design Patterns provide a set of widely verified solution templates, allowing developers to solve problems on the shoulders of their predecessors.

for example:

• Module decoupling in hierarchical architecture
• Separation of data access and business logic
• Changeable configuration management
• Asynchronous task scheduling

These problems can all be simplified by appropriate patterns, improve testability and scalability.

What are the common and practical design patterns?

1. Factory/Dependency Injection (Dependency Injection)

This is the most common and most misunderstood combination. Factory pattern is used to encapsulate object creation logic, while DI is a higher level of abstraction and is often used in frameworks such as Spring.

For example: You have a payment service interface PaymentService , which may have multiple implementation classes (such as Alipay and WeChat). With a factory or DI container, it is possible to decide which implementation to use at runtime instead of hard-coded.

 @Service
class AlipayService implements PaymentService { ... }

@RestController
class PaymentController {
    private final PaymentService paymentService;

    // Spring automatically injects the correct implementation public PaymentController(PaymentService paymentService) {
        this.paymentService = paymentService;
    }
}

suggestion :

• Try to program through interfaces, not specific classes
• Avoid new objects in business logic and handing them over to containers or factories for processing
• Use Spring's @Primary or @Qualifier to distinguish multiple beans of the same type

2. Strategy Pattern

The policy pattern is very suitable when you have a set of algorithms or behaviors and want to switch based on conditions at runtime.

For example, order discount calculations, logistics cost calculations in different regions, etc. can all be encapsulated into strategy categories.

 public interface DiscountStrategy {
    double applyDiscount(double price);
}

public class MemberDiscount implements DiscountStrategy {
    public double applyDiscount(double price) {
        return price * 0.9; // 10% off for members}
}

// Use discountStrategy.applyDiscount(100);

suggestion :

• Used in conjunction with factories or enumerations to facilitate finding corresponding strategies
• You can combine caching mechanism to avoid repeated creation of policy objects
• If there are too many strategies, consider introducing Map to manage it uniformly

3. Template Method

Suitable for scenarios where the process is fixed but some steps are variable. For example, the skeleton of an approval process remains unchanged, but each node may be processed differently.

 abstract class ApprovalProcess {
    void process() {
        prepare();
        if (needReview()) {
            review();
        }
        finalizeApproval();
    }

    abstract void review();
    void prepare() { ... }
    boolean needReview() { return true; }
    void finalizeApproval() { ... }
}

suggestion :

• Don't overuse the final method unless you really don't want to subclass modifications
• Leave hook method appropriately for subclass extension
• If the logic is too complex, consider splitting it into multiple small templates

4. Observer/Event Listener (Observer/Event Listener)

When multiple related components need to be notified after an action occurs, it is clearer to use the event-driven model.

Spring provides ApplicationEventPublisher, which is very suitable for this asynchronous notification.

 @Component
class OrderService {

    @Autowired
    private ApplicationEventPublisher eventPublisher;

    public void placeOrder(Order order) {
        // ...
        eventPublisher.publishEvent(new OrderPlacedEvent(order));
    }
}

@Component
class EmailService {

    @EventListener
    public void sendEmail(OrderPlacedEvent event) {
        // Send mail}
}

suggestion :

• Clear the boundaries of events and don't make everything into events
• Consider whether asynchronous execution is required (@Async)
• Pay attention to exception handling to avoid event failure affecting the main process

Let's summarize

There are actually not many commonly used design patterns in enterprise-level Java applications. The key is to understand their applicable scenarios and limitations. Like Factory DI is the foundation, Strategy and Template Method are used to encapsulate change points, and Observer is used to decouple processes. Each mode has its own "comfort zone". Don't use it for the sake of using the mode, as it will make the code more difficult to maintain.

Basically that's it.

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