Answer:

Design patterns are general repeatable solutions to commonly occurring problems in software design. In my previous projects, I have used several design patterns including:

• Singleton:
  This pattern ensures that only one instance of a class exists in the entire application. I have used this pattern to ensure that there is only one instance of a database connection or thread pool in my applications.
• Factory Method:
  This pattern provides an interface for creating objects, but allows subclasses to control their creation. I have used this pattern in situations where the exact class of an object is unknown at runtime. For example, I have used this pattern to create different types of database connections based on configuration parameters.
• Observer:
  This pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. I have used this pattern to notify user interfaces of changes in data that they are displaying.
• Decorator:
  This pattern attaches additional responsibilities to an object dynamically. I have used this pattern to add new functionalities to existing objects without changing their structure or behavior.

In developing a web application using Java and Spring Framework, I would implement these design patterns in the following manner:

• Singleton:
  I would create a singleton class for creating database connections, thread pools, and any other shared resources that need to be accessed across multiple components of the application.
• Factory Method:
  I would create an interface for creating database connections and implement it in different classes based on the type of database being used. This would allow the application to switch between different databases without changing code in other parts of the application.
• Observer:
  I would use Spring's application event mechanism to notify user interfaces of changes in data. This would ensure loose coupling between the components of the application.
• Decorator:
  I would create decorators to add new functionalities to existing objects. For example, I might use a decorator to add caching to a service layer without changing the behavior of the service layer itself.

By using these design patterns, I would be able to improve the modularity, extensibility, and maintainability of the web application.

References:

• Singleton pattern
• Factory Method pattern
• Observer pattern
• Decorator pattern