AuthenticationService.java

package com.example.catfactsdaily.service;

import com.example.catfactsdaily.dto.UserDTO;
import com.example.catfactsdaily.entity.User;
import com.example.catfactsdaily.repository.UserRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.security.authentication.AuthenticationManager;
import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
import org.springframework.security.crypto.password.PasswordEncoder;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Isolation;
import org.springframework.transaction.annotation.Transactional;

@Service
public class AuthenticationService {

    private final UserRepository userRepository;

    private final PasswordEncoder passwordEncoder;

    private final AuthenticationManager authenticationManager;

    @Autowired
    public AuthenticationService(
            UserRepository userRepository,
            AuthenticationManager authenticationManager,
            PasswordEncoder passwordEncoder
    ) {
        this.authenticationManager = authenticationManager;
        this.userRepository = userRepository;
        this.passwordEncoder = passwordEncoder;
    }

    @Transactional(isolation = Isolation.SERIALIZABLE)
    public User signup(UserDTO input) {
        User user = new User();
        user.setName(input.getName());
        user.setPassword(passwordEncoder.encode(input.getPassword()));
        return userRepository.save(user);
    }

    @Transactional(isolation = Isolation.SERIALIZABLE)
    public User authenticate(UserDTO input) {
        authenticationManager.authenticate(
                new UsernamePasswordAuthenticationToken(
                        input.getName(),
                        input.getPassword()
                )
        );

        return userRepository.findOneByName(input.getName())
                .orElseThrow();
    }
}