Initialize Spring Boot backend with JWT auth

src/main/java/com/openisle/service/UserService.java

@@ -0,0 +1,33 @@
+package com.openisle.service;
+
+import com.openisle.model.User;
+import com.openisle.repository.UserRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
+import org.springframework.security.crypto.password.PasswordEncoder;
+import org.springframework.stereotype.Service;
+
+import java.util.Optional;
+
+@Service
+@RequiredArgsConstructor
+public class UserService {
+    private final UserRepository userRepository;
+    private final PasswordEncoder passwordEncoder = new BCryptPasswordEncoder();
+
+    public User register(String username, String email, String password) {
+        if (userRepository.findByUsername(username).isPresent() || userRepository.findByEmail(email).isPresent()) {
+            throw new RuntimeException("User already exists");
+        }
+        User user = new User();
+        user.setUsername(username);
+        user.setEmail(email);
+        user.setPassword(passwordEncoder.encode(password));
+        return userRepository.save(user);
+    }
+
+    public Optional<User> authenticate(String username, String password) {
+        return userRepository.findByUsername(username)
+                .filter(user -> passwordEncoder.matches(password, user.getPassword()));
+    }
+}