For a project, I'm looking for actual examples of design patterns in real projects or applications; for example, I found Observer and Singleton pattern in Elasticsearch. Does anyone know a well-documented project which uses design patterns and could provide examples in the code?
The Java Spring Framework and its related projects is a framework for developers to build applications on the Java platform.
This was one of the first frameworks to popularise the Dependency Injection design pattern and is built on top of a wide range of design patterns including the Singleton, Builder, Proxy, Template Method, Repository and many others.
You can read about some of the patterns used with some sample code in this Baeldung article.