Books, references

Java

• Clean Code
• Effective Java, 3rd edition
• Thinking in Java (4th Edition)
• Modern Java in Action
• Java Concurrency in Practice
• High Performance Java Persistence

Blogs

• Baeldung - posts about Java, Spring modules, Spring Boot
• Maciej Walkowiak Maciej Walkowiak's Blog
• Marco Behler - long-read high-quality blog posts about Spring Framework, Spring Boot, etc.
• High-Performance Java Persistence and Hibernate - posts about Hibernate and JPA, many topics about optimisation and removing bottlenecks in communication with the database layer, highly recommended
• InfoQ Java - Latest news and articles about Java
• Java Magazine - Official Oracle Java Magazine
• JetBrains Blog - IntelliJ IDEA and Java development tips
• Spring Blog - Official Spring Blog

GitHub repositories

• Java design patterns - Design pattern examples in Java
• Spring PetClinic - Github repository containing showcases for some of the features of the Spring framework
• Java Algorithms - All Algorithms implemented in Java

Youtube

• SpringDeveloper - Spring Developer
• Spring I/O - Spring I/O conference talks
• Spring Academy - Spring Academy
• Java - Official Java YouTube channel
• JetBrainsTV - IntelliJ IDEA and Java development tips
• Devoxx - Java and JVM conference talks

Other

• Sourcemaking - Common design pattern examples
• Java API design best practices
• Spring Documentation - Official Spring documentation