feat(user): optional display name (player tag)

users.display_name (V6), 3-24 letters, digits, space, _ . -, unique ignoring case. Set in settings (409 DISPLAY_NAME_ALREADY_IN_USE, also for the DB constraint race) and optionally at registration. Returned by /api/users/me.

src/main/java/auth_service/modules/registration/api/RegistrationExceptionController.java

@@ -3,6 +3,7 @@ package auth_service.modules.registration.api;
 import auth_service.shared.api.dto.response.ErrorResponse;
 import auth_service.modules.registration.exception.EmailAlreadyInUseException;
 import auth_service.modules.registration.exception.InvalidInvitationCodeException;
+import auth_service.modules.registration.exception.DisplayNameAlreadyRegisteredException;
 import auth_service.modules.registration.exception.SteamIdAlreadyRegisteredException;
 import lombok.NonNull;
 import lombok.extern.slf4j.Slf4j;
@@ -35,6 +36,12 @@ public class RegistrationExceptionController {
         return response(body, HttpStatus.BAD_REQUEST);
     }
 
+    @ExceptionHandler(DisplayNameAlreadyRegisteredException.class)
+    public ResponseEntity<ErrorResponse> clientError(DisplayNameAlreadyRegisteredException ex) {
+        var body = new ErrorResponse("DISPLAY_NAME_ALREADY_IN_USE", ex.getMessage());
+        return response(body, HttpStatus.BAD_REQUEST);
+    }
+
     @ExceptionHandler(SteamIdAlreadyRegisteredException.class)
     public ResponseEntity<ErrorResponse> clientError(SteamIdAlreadyRegisteredException ex) {
         var body = new ErrorResponse("STEAM_ID_ALREADY_IN_USE", ex.getMessage());

src/main/java/auth_service/modules/registration/api/dto/mapper/ApiMapper.java

@@ -12,6 +12,7 @@ public abstract class ApiMapper {
                 email(registrationRequest.email()).
                 rawPassword(registrationRequest.password()).
                 inviteCode(registrationRequest.inviteCode()).
+                displayName(registrationRequest.displayName()).
                 steamId(registrationRequest.steamId() == null ? null : Long.valueOf(registrationRequest.steamId())).
                 build();
     }

src/main/java/auth_service/modules/registration/api/dto/request/RegistrationRequest.java

@@ -1,5 +1,6 @@
 package auth_service.modules.registration.api.dto.request;
 
+import auth_service.shared.api.validation.DisplayNameFormat;
 import auth_service.shared.api.validation.SteamIdDeserializer;
 import auth_service.shared.api.validation.SteamIdFormat;
 import jakarta.validation.constraints.Pattern;
@@ -9,5 +10,6 @@ import tools.jackson.databind.annotation.JsonDeserialize;
 @Builder
 public record RegistrationRequest(String email, String password, String inviteCode,
                                   @Pattern(regexp = SteamIdFormat.PATTERN)
-                                  @JsonDeserialize(using = SteamIdDeserializer.class) String steamId) {
+                                  @JsonDeserialize(using = SteamIdDeserializer.class) String steamId,
+                                  @Pattern(regexp = DisplayNameFormat.PATTERN) String displayName) {
 }

src/main/java/auth_service/modules/registration/exception/DisplayNameAlreadyRegisteredException.java

@@ -0,0 +1,7 @@
+package auth_service.modules.registration.exception;
+
+public class DisplayNameAlreadyRegisteredException extends RuntimeException {
+    public DisplayNameAlreadyRegisteredException(String message) {
+        super(message);
+    }
+}

src/main/java/auth_service/modules/registration/model/Registration.java

@@ -11,4 +11,5 @@ public class Registration {
     private String encryptedPassword;
     private String inviteCode;
     private Long steamId;
+    private String displayName;
 }

src/main/java/auth_service/modules/registration/persistence/RegisteredUserRepository.java

@@ -7,5 +7,7 @@ public interface RegisteredUserRepository {
 
     boolean existsBySteamId(long steamId);
 
+    boolean existsByDisplayName(String displayName);
+
     long save(Registration registration);
 }

src/main/java/auth_service/modules/registration/persistence/RegisteredUserRepositoryImpl.java

@@ -34,6 +34,16 @@ public class RegisteredUserRepositoryImpl implements RegisteredUserRepository {
         return count != null && count > 0;
     }
 
+    @Override
+    public boolean existsByDisplayName(String displayName) {
+        var jpql = "SELECT COUNT(u) FROM UserEntity u WHERE LOWER(u.displayName) = LOWER(:displayName)";
+        var count = entityManager.createQuery(jpql, Long.class)
+                .setParameter("displayName", displayName)
+                .getSingleResult();
+
+        return count != null && count > 0;
+    }
+
     @Override
     public long save(Registration registration) {
         var userToRegisterEntity = EntityMapper.mapToPersistence(registration);

src/main/java/auth_service/modules/registration/persistence/entities/RegisteredUserEntity.java

@@ -30,6 +30,9 @@ public class RegisteredUserEntity {
     @Column(name = "steam_id", unique = true)
     private Long steamId;
 
+    @Column(name = "display_name", length = 24)
+    private String displayName;
+
     @Column(name = "registration_date", nullable = false)
     private LocalDateTime registrationDate;
 

src/main/java/auth_service/modules/registration/persistence/mapper/EntityMapper.java

@@ -18,6 +18,7 @@ public abstract class EntityMapper {
                 password(userModel.getEncryptedPassword()).
                 enabled(true).
                 steamId(userModel.getSteamId()).
+                displayName(userModel.getDisplayName()).
                 lastLoginDate(null).
                 build();
     }

src/main/java/auth_service/modules/registration/service/RegistrationService.java

@@ -2,6 +2,7 @@ package auth_service.modules.registration.service;
 
 import auth_service.modules.registration.exception.EmailAlreadyInUseException;
 import auth_service.modules.registration.exception.InvalidInvitationCodeException;
+import auth_service.modules.registration.exception.DisplayNameAlreadyRegisteredException;
 import auth_service.modules.registration.exception.SteamIdAlreadyRegisteredException;
 import auth_service.modules.registration.model.Registration;
 import auth_service.modules.registration.persistence.RegisteredUserRepository;
@@ -30,6 +31,10 @@ public class RegistrationService {
             throw new SteamIdAlreadyRegisteredException("The steam id is already in use.");
         }
 
+        if (registration.getDisplayName() != null && registeredUserRepository.existsByDisplayName(registration.getDisplayName())) {
+            throw new DisplayNameAlreadyRegisteredException("The display name is already in use.");
+        }
+
         var rawPassword = registration.getRawPassword();
         registration.setEncryptedPassword(passwordEncoder.encode(rawPassword));
 

src/main/java/auth_service/modules/user/api/UserController.java

@@ -42,7 +42,7 @@ public class UserController {
     public ResponseEntity<UserSettingsResponse> updateUserSettings(@AuthenticationPrincipal UserPrincipal principal,
                                                                    @Valid @RequestBody UserSettingsRequest request) {
         var token = this.userService.updateUserSettings(principal.getIdAsLong(), principal.email(),
-                ApiMapper.toSteamId(request), ApiMapper.toDomain(request));
+                ApiMapper.toSteamId(request), request.displayName(), ApiMapper.toDomain(request));
         return ResponseEntity.ok(new UserSettingsResponse(token));
     }
 }

src/main/java/auth_service/modules/user/api/UserExceptionController.java

@@ -1,6 +1,7 @@
 package auth_service.modules.user.api;
 
 import auth_service.shared.api.dto.response.ErrorResponse;
+import auth_service.modules.user.exception.DisplayNameAlreadyInUseException;
 import auth_service.modules.user.exception.SteamIdAlreadyInUseException;
 import auth_service.modules.user.exception.UserNotFoundException;
 import lombok.NonNull;
@@ -17,6 +18,7 @@ import org.springframework.web.bind.annotation.RestControllerAdvice;
 @Slf4j
 public class UserExceptionController {
     private static final String STEAM_ID_CONSTRAINT = "uc_steam_id";
+    private static final String DISPLAY_NAME_CONSTRAINT = "uq_users_display_name_lower";
 
     private static @NonNull ResponseEntity<ErrorResponse> response(ErrorResponse body, HttpStatus status) {
         return ResponseEntity.status(status.value())
@@ -42,17 +44,30 @@ public class UserExceptionController {
         return steamIdInUse();
     }
 
-    // A parallel request can save the same Steam ID after the service check. Then the DB constraint fails.
+    // A parallel request can save the same Steam ID or display name after the service check. Then the DB constraint fails.
     @ExceptionHandler(DataIntegrityViolationException.class)
     public ResponseEntity<ErrorResponse> dataIntegrityError(DataIntegrityViolationException ex) {
         var cause = ex.getMostSpecificCause().getMessage();
         if (cause != null && cause.contains(STEAM_ID_CONSTRAINT)) {
             return steamIdInUse();
         }
+        if (cause != null && cause.contains(DISPLAY_NAME_CONSTRAINT)) {
+            return displayNameInUse();
+        }
         log.error("Data integrity violation in user request", ex);
         return response(new ErrorResponse("INTERNAL_ERROR", "Unexpected database error."), HttpStatus.INTERNAL_SERVER_ERROR);
     }
 
+    @ExceptionHandler(DisplayNameAlreadyInUseException.class)
+    public ResponseEntity<ErrorResponse> clientError(DisplayNameAlreadyInUseException ex) {
+        return displayNameInUse();
+    }
+
+    private static ResponseEntity<ErrorResponse> displayNameInUse() {
+        var body = new ErrorResponse("DISPLAY_NAME_ALREADY_IN_USE", "The display name is already in use.");
+        return response(body, HttpStatus.CONFLICT);
+    }
+
     private static ResponseEntity<ErrorResponse> steamIdInUse() {
         var body = new ErrorResponse("STEAM_ID_ALREADY_IN_USE", "The steam id is already in use.");
         return response(body, HttpStatus.CONFLICT);

src/main/java/auth_service/modules/user/api/dto/request/UserSettingsRequest.java

@@ -1,5 +1,6 @@
 package auth_service.modules.user.api.dto.request;
 
+import auth_service.shared.api.validation.DisplayNameFormat;
 import auth_service.shared.api.validation.SteamIdDeserializer;
 import auth_service.shared.api.validation.SteamIdFormat;
 import jakarta.validation.constraints.Pattern;
@@ -7,8 +8,10 @@ import tools.jackson.databind.annotation.JsonDeserialize;
 
 /**
  * @param steamId     null removes the Steam ID
+ * @param displayName null removes the display name
  */
 public record UserSettingsRequest(boolean useDarkTheme, String language,
                                   @Pattern(regexp = SteamIdFormat.PATTERN)
-                                  @JsonDeserialize(using = SteamIdDeserializer.class) String steamId) {
+                                  @JsonDeserialize(using = SteamIdDeserializer.class) String steamId,
+                                  @Pattern(regexp = DisplayNameFormat.PATTERN) String displayName) {
 }

src/main/java/auth_service/modules/user/api/dto/response/UserProfileResponse.java

@@ -5,5 +5,5 @@ import lombok.Builder;
 import java.time.LocalDateTime;
 
 @Builder
-public record UserProfileResponse(long id, String email, String role, LocalDateTime lastLogin, LocalDateTime createdDate, String steamId, SettingsResponse settings) {
+public record UserProfileResponse(long id, String email, String role, LocalDateTime lastLogin, LocalDateTime createdDate, String steamId, String displayName, SettingsResponse settings) {
 }

src/main/java/auth_service/modules/user/api/mapper/ApiMapper.java

@@ -18,6 +18,7 @@ public abstract class ApiMapper {
                 .createdDate(user.getCreatedDate())
                 .lastLogin(user.getLastLogin())
                 .steamId(user.getSteamId() == null ? null : String.valueOf(user.getSteamId()))
+                .displayName(user.getDisplayName())
                 .role("USER")
                 .settings(SettingsResponse.builder()
                         .id(user.getId())

src/main/java/auth_service/modules/user/exception/DisplayNameAlreadyInUseException.java

@@ -0,0 +1,7 @@
+package auth_service.modules.user.exception;
+
+public class DisplayNameAlreadyInUseException extends RuntimeException {
+    public DisplayNameAlreadyInUseException(String message) {
+        super(message);
+    }
+}

src/main/java/auth_service/modules/user/model/User.java

@@ -14,6 +14,7 @@ public class User {
     private LocalDateTime lastLogin;
     private LocalDateTime createdDate;
     private Long steamId;
+    private String displayName;
 
     private UserSettings settings;
 }

src/main/java/auth_service/modules/user/persistence/UserRepository.java

@@ -24,7 +24,9 @@ public interface UserRepository {
     UserSettings findSettingsByUserId(String userId);
 
     @Transactional
-    boolean updateSettings(long userId, Long steamId, UserSettings settings);
+    boolean updateSettings(long userId, Long steamId, String displayName, UserSettings settings);
 
     boolean isSteamIdUsedByOtherUser(long userId, long steamId);
+
+    boolean isDisplayNameUsedByOtherUser(long userId, String displayName);
 }

src/main/java/auth_service/modules/user/persistence/UserRepositoryImpl.java

@@ -62,7 +62,7 @@ public class UserRepositoryImpl implements UserRepository {
 
     @Transactional
     @Override
-    public boolean updateSettings(long userId, Long steamId, UserSettings settings) {
+    public boolean updateSettings(long userId, Long steamId, String displayName, UserSettings settings) {
         var userEntity = entityManager.find(UserEntity.class, userId);
         if (userEntity == null) {
             throw new UserNotFoundException("Could not find user with id " + userId);
@@ -80,6 +80,7 @@ public class UserRepositoryImpl implements UserRepository {
 
         boolean steamIdChanged = !Objects.equals(userEntity.getSteamId(), steamId);
         userEntity.setSteamId(steamId);
+        userEntity.setDisplayName(displayName);
 
         return steamIdChanged;
     }
@@ -93,4 +94,14 @@ public class UserRepositoryImpl implements UserRepository {
                 .getSingleResult();
         return count > 0;
     }
+
+    @Override
+    public boolean isDisplayNameUsedByOtherUser(long userId, String displayName) {
+        var count = entityManager.createQuery(
+                        "SELECT COUNT(u) FROM UserEntity u WHERE LOWER(u.displayName) = LOWER(:displayName) AND u.id <> :userId", Long.class)
+                .setParameter("displayName", displayName)
+                .setParameter("userId", userId)
+                .getSingleResult();
+        return count > 0;
+    }
 }

src/main/java/auth_service/modules/user/persistence/entities/UserEntity.java

@@ -33,6 +33,9 @@ public class UserEntity {
     @Column(name = "steam_id", unique = true)
     private Long steamId;
 
+    @Column(name = "display_name", length = 24)
+    private String displayName;
+
     @Column(name = "registration_date", nullable = false)
     private LocalDateTime registrationDate;
 

src/main/java/auth_service/modules/user/persistence/mapper/EntityMapper.java

@@ -39,6 +39,7 @@ public abstract class EntityMapper {
                 .lastLogin(entity.getLastLoginDate())
                 .createdDate(entity.getRegistrationDate())
                 .steamId(entity.getSteamId())
+                .displayName(entity.getDisplayName())
                 .settings(settingsModel)
                 .build();
     }

src/main/java/auth_service/modules/user/service/UserService.java

@@ -1,5 +1,6 @@
 package auth_service.modules.user.service;
 
+import auth_service.modules.user.exception.DisplayNameAlreadyInUseException;
 import auth_service.modules.user.exception.SteamIdAlreadyInUseException;
 import auth_service.modules.user.exception.UserNotFoundException;
 import auth_service.modules.user.model.User;
@@ -34,11 +35,14 @@ public class UserService {
 
     @Transactional
     @CacheEvict(value = "usersByEmail", allEntries = true)
-    public String updateUserSettings(long userId, String email, Long steamId, UserSettings userSettings) {
+    public String updateUserSettings(long userId, String email, Long steamId, String displayName, UserSettings userSettings) {
         if (steamId != null && userRepository.isSteamIdUsedByOtherUser(userId, steamId)) {
             throw new SteamIdAlreadyInUseException("The steam id is already in use.");
         }
-        var updated = userRepository.updateSettings(userId, steamId, userSettings);
+        if (displayName != null && userRepository.isDisplayNameUsedByOtherUser(userId, displayName)) {
+            throw new DisplayNameAlreadyInUseException("The display name is already in use.");
+        }
+        var updated = userRepository.updateSettings(userId, steamId, displayName, userSettings);
         if (updated) {
             log.info("Steam id has changed for user {}", userId);
         }

src/main/java/auth_service/shared/api/validation/DisplayNameFormat.java

@@ -0,0 +1,11 @@
+package auth_service.shared.api.validation;
+
+/**
+ * 3 to 24 letters, digits, spaces, "_", "." or "-". No space at the start or end.
+ */
+public final class DisplayNameFormat {
+    public static final String PATTERN = "[\\p{L}\\p{N}_.-][\\p{L}\\p{N} _.-]{1,22}[\\p{L}\\p{N}_.-]";
+
+    private DisplayNameFormat() {
+    }
+}

src/main/resources/db/migration/V6__Add_Display_Name.sql

@@ -0,0 +1,3 @@
+-- Player tag shown instead of the email prefix. Optional, unique ignoring case.
+ALTER TABLE users ADD COLUMN display_name VARCHAR(24);
+CREATE UNIQUE INDEX uq_users_display_name_lower ON users (LOWER(display_name));

src/test/java/auth_service/modules/registration/api/RegistrationControllerTest.java

@@ -168,4 +168,33 @@ class RegistrationControllerTest extends AbstractWebMvcTest {
                 .contentType(MediaType.APPLICATION_JSON)
                 .content(registrationRequestJson.write(request).getJson())));
     }
+
+    @Test
+    void register_InvalidDisplayName() throws Exception {
+        mockMvc.perform(post("/api/registration/new")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"email":"test@example.com","password":"SuperSecret123!","inviteCode":"code","displayName":"x"}"""))
+                .andExpect(status().isBadRequest());
+
+        verifyNoInteractions(registrationService);
+    }
+
+    @Test
+    void register_PassesDisplayName() throws Exception {
+        // Arrange
+        when(registrationService.register(any(Registration.class))).thenReturn(42L);
+
+        // Act
+        mockMvc.perform(post("/api/registration/new")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"email":"test@example.com","password":"SuperSecret123!","inviteCode":"code","displayName":"Lobo"}"""))
+                .andExpect(status().isOk());
+
+        // Assert
+        var captor = ArgumentCaptor.forClass(Registration.class);
+        verify(registrationService).register(captor.capture());
+        assertEquals("Lobo", captor.getValue().getDisplayName());
+    }
 }

src/test/java/auth_service/modules/registration/service/RegistrationServiceTest.java

@@ -2,6 +2,7 @@ package auth_service.modules.registration.service;
 
 import auth_service.modules.registration.exception.EmailAlreadyInUseException;
 import auth_service.modules.registration.exception.InvalidInvitationCodeException;
+import auth_service.modules.registration.exception.DisplayNameAlreadyRegisteredException;
 import auth_service.modules.registration.exception.SteamIdAlreadyRegisteredException;
 import auth_service.modules.registration.model.Registration;
 import auth_service.modules.registration.persistence.RegisteredUserRepository;
@@ -120,4 +121,18 @@ class RegistrationServiceTest extends AbstractUnitTest {
         assertEquals(42L, result);
         verify(registeredUserRepository, never()).existsBySteamId(anyLong());
     }
+
+    @Test
+    void register_DisplayNameTaken() {
+        // Arrange
+        validRegistration.setDisplayName("Lobo");
+        when(codeValidationService.isCodeValid(validRegistration.getInviteCode())).thenReturn(true);
+        when(registeredUserRepository.existsByEmail(validRegistration.getEmail())).thenReturn(false);
+        when(registeredUserRepository.existsBySteamId(validRegistration.getSteamId())).thenReturn(false);
+        when(registeredUserRepository.existsByDisplayName("Lobo")).thenReturn(true);
+
+        // Act & Assert
+        assertThrows(DisplayNameAlreadyRegisteredException.class, () -> registrationService.register(validRegistration));
+        verify(registeredUserRepository, never()).save(any());
+    }
 }

src/test/java/auth_service/modules/user/api/UserControllerTest.java

@@ -3,6 +3,7 @@ package auth_service.modules.user.api;
 import auth_service.configuration.security.JwtAuthenticationFilter;
 import auth_service.configuration.security.JwtService;
 import auth_service.shared.security.UserPrincipal;
+import auth_service.modules.user.exception.DisplayNameAlreadyInUseException;
 import auth_service.modules.user.exception.SteamIdAlreadyInUseException;
 import auth_service.modules.user.exception.UserNotFoundException;
 import auth_service.modules.user.model.User;
@@ -11,6 +12,8 @@ import auth_service.modules.user.service.UserService;
 import auth_service.shared.test.AbstractWebMvcTest;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.autoconfigure.json.AutoConfigureJsonTesters;
 import org.springframework.boot.webmvc.test.autoconfigure.AutoConfigureMockMvc;
@@ -136,7 +139,7 @@ class UserControllerTest extends AbstractWebMvcTest {
         SecurityContextHolder.getContext().setAuthentication(
                 new UsernamePasswordAuthenticationToken(principal, null, Collections.emptyList())
         );
-        when(userService.updateUserSettings(eq(1L), eq(email), eq(76561198000000001L), any())).thenReturn("new-token");
+        when(userService.updateUserSettings(eq(1L), eq(email), eq(76561198000000001L), any(), any())).thenReturn("new-token");
 
         // Act & Assert
         mockMvc.perform(put("/api/users/settings")
@@ -163,7 +166,7 @@ class UserControllerTest extends AbstractWebMvcTest {
                                 {"useDarkTheme":true,"language":"en","steamId":null}"""))
                 .andExpect(status().isOk());
 
-        verify(userService).updateUserSettings(eq(1L), eq(email), isNull(), any());
+        verify(userService).updateUserSettings(eq(1L), eq(email), isNull(), any(), any());
     }
 
     @Test
@@ -192,7 +195,7 @@ class UserControllerTest extends AbstractWebMvcTest {
         SecurityContextHolder.getContext().setAuthentication(
                 new UsernamePasswordAuthenticationToken(principal, null, Collections.emptyList())
         );
-        when(userService.updateUserSettings(eq(1L), eq(email), eq(76561198000000001L), any()))
+        when(userService.updateUserSettings(eq(1L), eq(email), eq(76561198000000001L), any(), any()))
                 .thenThrow(new DataIntegrityViolationException("duplicate key value violates unique constraint \"uc_steam_id\""));
 
         // Act & Assert
@@ -230,7 +233,7 @@ class UserControllerTest extends AbstractWebMvcTest {
         SecurityContextHolder.getContext().setAuthentication(
                 new UsernamePasswordAuthenticationToken(principal, null, Collections.emptyList())
         );
-        when(userService.updateUserSettings(eq(1L), eq(email), eq(76561198000000001L), any()))
+        when(userService.updateUserSettings(eq(1L), eq(email), eq(76561198000000001L), any(), any()))
                 .thenThrow(new SteamIdAlreadyInUseException("taken"));
 
         // Act & Assert
@@ -250,7 +253,7 @@ class UserControllerTest extends AbstractWebMvcTest {
         SecurityContextHolder.getContext().setAuthentication(
                 new UsernamePasswordAuthenticationToken(principal, null, Collections.emptyList())
         );
-        when(userService.updateUserSettings(eq(1L), eq(email), isNull(), any()))
+        when(userService.updateUserSettings(eq(1L), eq(email), isNull(), any(), any()))
                 .thenThrow(new DataIntegrityViolationException("value too long for type character varying(10)"));
 
         // Act & Assert
@@ -261,4 +264,98 @@ class UserControllerTest extends AbstractWebMvcTest {
                 .andExpect(status().isInternalServerError())
                 .andExpect(jsonPath("$.code").value("INTERNAL_ERROR"));
     }
+
+    @Test
+    void getCurrentUserV1_ReturnsDisplayName() throws Exception {
+        // Arrange
+        var email = "test@example.com";
+        var settings = UserSettings.builder().language("en").theme("dark").build();
+        var user = User.builder().email(email).displayName("Lobo").settings(settings).build();
+        var principal = new UserPrincipal("1", email, null);
+        SecurityContextHolder.getContext().setAuthentication(
+                new UsernamePasswordAuthenticationToken(principal, null, Collections.emptyList())
+        );
+        when(userService.getUserByEmail(email)).thenReturn(user);
+
+        // Act & Assert
+        mockMvc.perform(get("/api/users/me").accept(MediaType.APPLICATION_JSON))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.displayName").value("Lobo"));
+    }
+
+    @Test
+    void updateUserSettings_PassesDisplayName() throws Exception {
+        // Arrange
+        var email = "test@example.com";
+        var principal = new UserPrincipal("1", email, null);
+        SecurityContextHolder.getContext().setAuthentication(
+                new UsernamePasswordAuthenticationToken(principal, null, Collections.emptyList())
+        );
+        when(userService.updateUserSettings(eq(1L), eq(email), isNull(), eq("Lobo Corp_1"), any())).thenReturn("t");
+
+        // Act & Assert
+        mockMvc.perform(put("/api/users/settings")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"useDarkTheme":true,"language":"en","steamId":null,"displayName":"Lobo Corp_1"}"""))
+                .andExpect(status().isOk());
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {"ab", " Lobo", "Lobo ", "<script>", "abcdefghijklmnopqrstuvwxy"})
+    void updateUserSettings_InvalidDisplayName(String displayName) throws Exception {
+        // Arrange
+        var principal = new UserPrincipal("1", "test@example.com", null);
+        SecurityContextHolder.getContext().setAuthentication(
+                new UsernamePasswordAuthenticationToken(principal, null, Collections.emptyList())
+        );
+
+        // Act & Assert
+        mockMvc.perform(put("/api/users/settings")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("{\"useDarkTheme\":true,\"language\":\"en\",\"displayName\":\"" + displayName + "\"}"))
+                .andExpect(status().isBadRequest());
+
+        verifyNoInteractions(userService);
+    }
+
+    @Test
+    void updateUserSettings_DisplayNameTaken() throws Exception {
+        // Arrange
+        var email = "test@example.com";
+        var principal = new UserPrincipal("1", email, null);
+        SecurityContextHolder.getContext().setAuthentication(
+                new UsernamePasswordAuthenticationToken(principal, null, Collections.emptyList())
+        );
+        when(userService.updateUserSettings(eq(1L), eq(email), any(), eq("Lobo"), any()))
+                .thenThrow(new DisplayNameAlreadyInUseException("taken"));
+
+        // Act & Assert
+        mockMvc.perform(put("/api/users/settings")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"useDarkTheme":true,"language":"en","displayName":"Lobo"}"""))
+                .andExpect(status().isConflict())
+                .andExpect(jsonPath("$.code").value("DISPLAY_NAME_ALREADY_IN_USE"));
+    }
+
+    @Test
+    void updateUserSettings_DisplayNameRaceIsConflict() throws Exception {
+        // Arrange
+        var email = "test@example.com";
+        var principal = new UserPrincipal("1", email, null);
+        SecurityContextHolder.getContext().setAuthentication(
+                new UsernamePasswordAuthenticationToken(principal, null, Collections.emptyList())
+        );
+        when(userService.updateUserSettings(eq(1L), eq(email), any(), eq("Lobo"), any()))
+                .thenThrow(new DataIntegrityViolationException("duplicate key value violates unique constraint \"uq_users_display_name_lower\""));
+
+        // Act & Assert
+        mockMvc.perform(put("/api/users/settings")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"useDarkTheme":true,"language":"en","displayName":"Lobo"}"""))
+                .andExpect(status().isConflict())
+                .andExpect(jsonPath("$.code").value("DISPLAY_NAME_ALREADY_IN_USE"));
+    }
 }

src/test/java/auth_service/modules/user/persistence/UserRepositoryTest.java

@@ -64,7 +64,7 @@ class UserRepositoryTest extends AbstractPersistenceTest {
                 .build();
 
         // Act
-        var updated = userRepository.updateSettings(userEntity.getId(), 654321L, settings);
+        var updated = userRepository.updateSettings(userEntity.getId(), 654321L, null, settings);
 
         // Assert
         assertTrue(updated);
@@ -109,7 +109,7 @@ class UserRepositoryTest extends AbstractPersistenceTest {
         var settings = UserSettings.builder().language("en").theme("light").build();
 
         // Act
-        userRepository.updateSettings(userEntity.getId(), null, settings);
+        userRepository.updateSettings(userEntity.getId(), null, null, settings);
 
         // Assert
         assertNull(entityManager.find(UserEntity.class, userEntity.getId()).getSteamId());
@@ -143,7 +143,7 @@ class UserRepositoryTest extends AbstractPersistenceTest {
     void updateSettings_UnknownUser() {
         var settings = UserSettings.builder().language("en").theme("light").build();
 
-        assertThrows(UserNotFoundException.class, () -> userRepository.updateSettings(-1L, null, settings));
+        assertThrows(UserNotFoundException.class, () -> userRepository.updateSettings(-1L, null, null, settings));
     }
 
     @Test
@@ -151,4 +151,33 @@ class UserRepositoryTest extends AbstractPersistenceTest {
         // V1 seeded test@example.com with a known password; it must not exist after all migrations.
         assertTrue(userRepository.findByEmail("test@example.com").isEmpty());
     }
+
+    @Test
+    void displayName_StoredAndCheckedCaseInsensitive() {
+        // Arrange
+        var owner = entityManager.persist(UserEntity.builder()
+                .email("owner2@example.com")
+                .password("password")
+                .enabled(true)
+                .registrationDate(LocalDateTime.now())
+                .build());
+        var other = entityManager.persist(UserEntity.builder()
+                .email("other2@example.com")
+                .password("password")
+                .enabled(true)
+                .registrationDate(LocalDateTime.now())
+                .build());
+        entityManager.flush();
+        var settings = UserSettings.builder().language("en").theme("light").build();
+
+        // Act
+        userRepository.updateSettings(owner.getId(), null, "Lobo", settings);
+        entityManager.flush();
+
+        // Assert
+        assertEquals("Lobo", userRepository.findByEmail("owner2@example.com").orElseThrow().getDisplayName());
+        assertFalse(userRepository.isDisplayNameUsedByOtherUser(owner.getId(), "lobo"));
+        assertTrue(userRepository.isDisplayNameUsedByOtherUser(other.getId(), "LOBO"));
+        assertFalse(userRepository.isDisplayNameUsedByOtherUser(other.getId(), "Wolf"));
+    }
 }

src/test/java/auth_service/modules/user/service/UserServiceTest.java

@@ -1,5 +1,6 @@
 package auth_service.modules.user.service;
 
+import auth_service.modules.user.exception.DisplayNameAlreadyInUseException;
 import auth_service.modules.user.exception.SteamIdAlreadyInUseException;
 import auth_service.modules.user.exception.UserNotFoundException;
 import auth_service.modules.user.model.User;
@@ -68,11 +69,11 @@ class UserServiceTest extends AbstractUnitTest {
         when(tokenGenerator.generateToken("1", email, steamId)).thenReturn("new-token");
 
         // Act
-        var token = userService.updateUserSettings(userId, email, steamId, userSettings);
+        var token = userService.updateUserSettings(userId, email, steamId, null, userSettings);
 
         // Assert
         assertEquals("new-token", token);
-        verify(userRepository).updateSettings(userId, steamId, userSettings);
+        verify(userRepository).updateSettings(userId, steamId, null, userSettings);
     }
 
     @Test
@@ -84,11 +85,11 @@ class UserServiceTest extends AbstractUnitTest {
         when(tokenGenerator.generateToken("1", email, null)).thenReturn("new-token");
 
         // Act
-        var token = userService.updateUserSettings(userId, email, null, userSettings);
+        var token = userService.updateUserSettings(userId, email, null, null, userSettings);
 
         // Assert
         assertEquals("new-token", token);
-        verify(userRepository).updateSettings(userId, null, userSettings);
+        verify(userRepository).updateSettings(userId, null, null, userSettings);
         verify(userRepository, never()).isSteamIdUsedByOtherUser(anyLong(), anyLong());
     }
 
@@ -102,7 +103,32 @@ class UserServiceTest extends AbstractUnitTest {
 
         // Act & Assert
         assertThrows(SteamIdAlreadyInUseException.class,
-                () -> userService.updateUserSettings(userId, "test@example.com", steamId, userSettings));
-        verify(userRepository, never()).updateSettings(anyLong(), any(), any());
+                () -> userService.updateUserSettings(userId, "test@example.com", steamId, null, userSettings));
+        verify(userRepository, never()).updateSettings(anyLong(), any(), any(), any());
+    }
+
+    @Test
+    void updateUserSettings_DisplayNameOfOtherUser() {
+        // Arrange
+        var userSettings = UserSettings.builder().language("en").build();
+        when(userRepository.isDisplayNameUsedByOtherUser(1L, "Lobo")).thenReturn(true);
+
+        // Act & Assert
+        assertThrows(DisplayNameAlreadyInUseException.class,
+                () -> userService.updateUserSettings(1L, "test@example.com", null, "Lobo", userSettings));
+        verify(userRepository, never()).updateSettings(anyLong(), any(), any(), any());
+    }
+
+    @Test
+    void updateUserSettings_StoresDisplayName() {
+        // Arrange
+        var userSettings = UserSettings.builder().language("en").build();
+        when(userRepository.isDisplayNameUsedByOtherUser(1L, "Lobo")).thenReturn(false);
+
+        // Act
+        userService.updateUserSettings(1L, "test@example.com", null, "Lobo", userSettings);
+
+        // Assert
+        verify(userRepository).updateSettings(1L, null, "Lobo", userSettings);
     }
 }