source: src/main/java/com/example/task/service/UserAuthenticationService.java

package com.example.task.service;

import com.example.task.entity.StudentEntity;
import com.example.task.repository.StudentRepository;
import lombok.AllArgsConstructor;
import org.springframework.security.core.Authentication;
import org.springframework.security.core.context.SecurityContextHolder;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.stereotype.Service;

@Service
@AllArgsConstructor
public class UserAuthenticationService {

    private final StudentRepository studentRepository;

    public StudentEntity getLoggedInUser() {
        Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
        if (authentication != null && authentication.isAuthenticated()) {

            Object principal = authentication.getPrincipal();
            if (principal instanceof UserDetails) {
                String username = ((UserDetails) principal).getUsername();
                StudentEntity user = studentRepository.findByUsername(username).get();
                return user;
            }
        }
        return null;
    }
}