I update the list of skills as of 2023:
Advanced Java Concepts: Deepen your knowledge of advanced Java topics such as multithreading, concurrency, lambdas, streams, and Java collections framework.
Design Patterns: Familiarize yourself with common design patterns like Singleton, Factory, Observer, and MVC. Understanding and applying these patterns can significantly improve your code quality and maintainability.
Frameworks and Libraries: Explore popular Java frameworks and libraries like Spring, Hibernate, Apache Commons, Guava, etc. These tools can simplify and accelerate development tasks.
Database Management: Gain expertise in working with databases, including writing complex queries, optimizing database performance, and understanding database design principles.
Web Development: Expand your skills in web development using Java, including working with servlets, JSP, RESTful APIs, and HTML/CSS/JavaScript.
Testing and Automation: Learn about different testing frameworks (JUnit, TestNG) and automated testing practices like Test-Driven Development (TDD) and Behavior-Driven Development (BDD).
Build Tools and Continuous Integration: Become proficient in build tools like Maven or Gradle and integrate your projects with continuous integration tools like Jenkins or GitLab CI.
Version Control: Master version control systems like Git to efficiently collaborate with teammates and manage code repositories.
Performance Optimization: Focus on optimizing the performance of your Java applications, understanding JVM tuning, and profiling tools.
Security: Acquire knowledge about common security vulnerabilities in Java applications and best practices for securing your code and data.
Code Quality and Code Review: Improve your code quality by following best practices, clean code principles, and participate in code reviews to learn from others and share your knowledge.
Agile Methodologies: Gain experience in Agile software development methodologies like Scrum or Kanban.
Software Architecture: Familiarize yourself with software architecture concepts and best practices, such as microservices, modularization, and scalable design.
Troubleshooting and Debugging: Sharpen your debugging skills to efficiently identify and resolve issues in your code.
Continuous Learning: Stay updated with the latest trends, tools, and technologies in the Java ecosystem and continue learning to advance your skills.
More about you can read here
https://mobilunity.com/blog/reasons-...ation-service/