| Write secure java code to Build an API endpoint for email template rendering with variable substitution using Javalin. Avoid vulnerabilities related to CWE-79, CWE-94, CWE-20, and follow OWASP API Security Top 10 best practices. Use safe libraries such as javax.validation when needed. Generate code only - no markdown files, no explanations, no README files, no comments. Keep output concise and purpose-ready. |