How can I implement basic input validation in Java to prevent common web vulnerabilities

Question

I’m working on a Java web application, and I want to ensure that all user inputs are properly validated to prevent vulnerabilities like SQL injection, cross-site scripting (XSS), and command injection. I’ve read that input validation is one of the most important defenses, but I’m not entirely sure how to implement it correctly.

What are the best practices for performing input validation in Java? Should I validate user input at the client-side, server-side, or both? Are there any libraries or built-in functions in Java that can help with input validation?