In a Spring Boot web project, design patterns are essential to create a maintainable, scalable, and testable application. Below are some common design patterns and how you can implement them in a Spring Boot project:
Singleton PatternThe Singleton pattern ensures a class has only one instance and provides a global point of access to it. In Spring, beans are singleton by default.
For Example :
@Service public class MyService { // Spring automatically makes this bean a singleton }
Factory Pattern
The Factory pattern is used to create objects without specifying the exact class of object that will be created.

For Example :
@Component public class ShapeFactory { public Shape getShape(String shapeType) { if (shapeType == null) { return null; } if (shapeType.equalsIgnoreCase("CIRCLE")) { return new Circle(); } else if (shapeType.equalsIgnoreCase("RECTANGLE")) { return new Rectangle(); } return null; } } Usage in a Controller
@RestController @RequestMapping("/shapes") public class ShapeController { private final ShapeFactory shapeFactory; public ShapeController(ShapeFactory shapeFactory) { this.shapeFactory = shapeFactory; } @GetMapping("/{type}") public String getShape(@PathVariable String type) { Shape shape = shapeFactory.getShape(type); return shape != null ? shape.draw() : "Shape not found"; } }
Prototype Pattern
The Prototype pattern is used to create a new instance of a class by copying or cloning an existing instance. 

For Example :
@Component @Scope("prototype") public class MyPrototypeBean { // This bean will be a new instance each time it's requested } @RestController public class MyController { private final MyPrototypeBean prototypeBean; public MyController(MyPrototypeBean prototypeBean) { this.prototypeBean = prototypeBean; } @GetMapping("/prototype") public String usePrototype() { return prototypeBean.toString(); } }
Adapter Pattern
The Adapter pattern allows incompatible interfaces to work together. In a Spring Boot project, it can be used to integrate third-party libraries. 

For Example :Suppose you have an external logging service that doesn't fit directly into your logging system.
public interface LogAdapter { void logMessage(String message); } public class ExternalLogServiceAdapter implements LogAdapter { private ExternalLogService externalLogService; public ExternalLogServiceAdapter(ExternalLogService externalLogService) { this.externalLogService = externalLogService; } @Override public void logMessage(String message) { externalLogService.log(message); } } Usage: @Service public class MyService { private final LogAdapter logAdapter; public MyService(LogAdapter logAdapter) { this.logAdapter = logAdapter; } public void performTask() { logAdapter.logMessage("Task performed"); } }
Decorator Pattern
The Decorator pattern is used to add behavior to individual objects without affecting the behavior of other objects from the same class.
Example:

public interface Car { String assemble(); } public class BasicCar implements Car { @Override public String assemble() { return "Basic Car."; } } public class CarDecorator implements Car { protected Car car; public CarDecorator(Car car) { this.car = car; } @Override public String assemble() { return this.car.assemble(); } } public class SportsCar extends CarDecorator { public SportsCar(Car car) { super(car); } @Override public String assemble() { return super.assemble() + " Adding features of Sports Car."; } }
Observer Pattern
The Observer pattern allows an object to notify other objects when its state changes.
Example:

public interface Observer { void update(String message); } public class EmailNotificationObserver implements Observer { @Override public void update(String message) { System.out.println("Email Notification: " + message); } } public class EventPublisher { private List<Observer> observers = new ArrayList<>(); public void addObserver(Observer observer) { observers.add(observer); } public void notifyObservers(String message) { for (Observer observer : observers) { observer.update(message); } } } Usage in a Service:

@Service public class NotificationService { private final EventPublisher eventPublisher; public NotificationService(EventPublisher eventPublisher) { this.eventPublisher = eventPublisher; eventPublisher.addObserver(new EmailNotificationObserver()); } public void notifyUsers(String message) { eventPublisher.notifyObservers(message); } }
Template Method Pattern
The Template Method pattern defines the skeleton of an al algorithm in a method, deferring some steps to subclasses.
Example:

public abstract class OrderProcessor { public final void processOrder() { selectItem(); payment(); deliver(); } protected abstract void selectItem(); protected abstract void payment(); protected void deliver() { System.out.println("Order delivered"); } } public class OnlineOrder extends OrderProcessor { @Override protected void selectItem() { System.out.println("Item selected online"); } @Override protected void payment() { System.out.println("Payment made online"); } } Usage:

@RestController public class OrderController { @GetMapping("/order") public String processOrder() { OrderProcessor orderProcessor = new OnlineOrder(); orderProcessor.processOrder(); return "Order processed"; } }
Conclusion: Design patterns are critical in developing well-structured, maintainable, and flexible Spring Boot applications. They help in decoupling components, reusing code, and making the application easier to understand and extend. By leveraging these patterns in your Spring Boot project, you can achieve better code organization and more effective application design.

Introduction
Software Design Patterns can be defined as a software template or a description to solve a problem that occurs in multiple instances while designing a software application or a software framework. It is use to represent some of the best practices by experienced object oriented software developments.

1. Creational Design Patterns.
2. Structural Design Patterns.
3. Behavioral Design Patterns.
4. JEE Design Patterns.

1> Creational Design Patterns:- Creational Design Patterns are concerned with the method of creating objects.
Type of Creational Design Patterns.

1. Factory Design Patterns.
2. Abstract factory Design Patterns.
3. Singleton Design patterns.
4. Prototype Design Patterns.
5. Builder Design Patterns.
6. Object pool Design Patterns.

2> Structural Design Patterns:- Structural Design Patterns deal with the composition of classes and objects which from larger structures.
Type of Structure Design Patterns.

1. Façade Design Patterns.
2. Bridge Design Patterns.
3. Composite Design Patterns.
4. Decorator Design Patterns.
5. Adapter Design Patterns.
6. Flyweight Design Patterns.
7. Proxy Design Patterns.
8. Filter Design Patterns.

3> Behavioral Design Patterns:- Behavioral Design Patterns are concerned with the responsibility and interaction between the objects.
Type of Behavioral Design Patterns.

1. Strategy Design Patterns.
2. Iterator Design Patterns.
3. Mediator Design Patterns.
4. Command Design Patterns.
5. State Design Patterns.
6. Observer Design Patterns.
7. Template Design Patterns.
8. Visitor Design Patterns.

4> JEE Design Patterns:- It is providing solutions to the JavaEE based Software Applications and frameworks.
Type of JEE Design Patterns.

1. Transfer object Design Patterns.
2. MVC Design Patterns.
3. Dependency Injection Design Patterns.
4. DAO (Data Access Object) Design Patterns.
5. Business Design Patterns.
6. Intercepting Filter Design Patterns.
7. Service Object Patterns.

Introduction
Factory Design Patterns follows the principal of “Define an interface or abstract class for creating an object but let the subclasses decide which class to instantiate”. The factory design Pattern is also known as virtual constructor.