pom.xml

@@ -6,11 +6,11 @@
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-parent</artifactId>
         <version>4.0.6</version>
-        <relativePath/> <!-- lookup parent from repository -->
+        <relativePath/>
     </parent>
     <groupId>de.zendric.app</groupId>
     <artifactId>XpenselyServer</artifactId>
-    <version>1.0.0</version>
+    <version>1.1.0</version>
     <name>XpenselyServer</name>
     <description>XpenselyServer used to handle the Xpensely App</description>
     <url/>
@@ -52,10 +52,6 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-oauth2-resource-server</artifactId>
         </dependency>
-        <dependency>
-            <groupId>org.springframework.boot</groupId>
-            <artifactId>spring-boot-starter-oauth2-client</artifactId>
-        </dependency>
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-web</artifactId>

src/main/java/de/zendric/app/xpensely_server/controller/AppUserController.java

@@ -9,6 +9,7 @@ import org.springframework.web.server.ResponseStatusException;
 
 import de.zendric.app.xpensely_server.model.AppUser;
 import de.zendric.app.xpensely_server.model.AppUserCreateRequest;
+import de.zendric.app.xpensely_server.model.UsernameUpdateRequest;
 import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
 import de.zendric.app.xpensely_server.services.UserService;
 
@@ -31,11 +32,6 @@ public class AppUserController {
         return ResponseEntity.ok(userService.getUser(id));
     }
 
-    @GetMapping("/byName")
-    public AppUser getUserByName(@RequestParam String username) {
-        return userService.getUserByName(username);
-    }
-
     @GetMapping("/byGoogleId")
     public ResponseEntity<AppUser> getUserByGoogleId(@RequestParam String id, Authentication authentication) {
         AppUser self = authenticatedUserResolver.resolveCurrentUser(authentication);
@@ -51,6 +47,14 @@ public class AppUserController {
         return new ResponseEntity<>(nUser, HttpStatus.CREATED);
     }
 
+    @PutMapping("/username")
+    public ResponseEntity<AppUser> updateUsername(@RequestBody @Valid UsernameUpdateRequest request,
+            Authentication authentication) {
+        AppUser self = authenticatedUserResolver.resolveCurrentUser(authentication);
+        AppUser updated = userService.updateUsername(self.getId(), request.getUsername());
+        return ResponseEntity.ok(updated);
+    }
+
     @DeleteMapping
     public ResponseEntity<String> deleteUser(@RequestParam Long id, Authentication authentication) {
         AppUser self = authenticatedUserResolver.resolveCurrentUser(authentication);

src/main/java/de/zendric/app/xpensely_server/controller/ExpenseListController.java

@@ -17,7 +17,6 @@ import de.zendric.app.xpensely_server.model.*;
 import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
 import de.zendric.app.xpensely_server.services.CategoryService;
 import de.zendric.app.xpensely_server.services.ExpenseListService;
-import de.zendric.app.xpensely_server.services.UserService;
 
 @RestController
 @RequestMapping("/api/expenselist")
@@ -26,14 +25,12 @@ public class ExpenseListController {
     private static final Logger log = LoggerFactory.getLogger(ExpenseListController.class);
 
     private final ExpenseListService expenseListService;
-    private final UserService userService;
     private final CategoryService categoryService;
     private final AuthenticatedUserResolver authenticatedUserResolver;
 
-    public ExpenseListController(ExpenseListService expenseListService, UserService userService,
+    public ExpenseListController(ExpenseListService expenseListService,
             CategoryService categoryService, AuthenticatedUserResolver authenticatedUserResolver) {
         this.expenseListService = expenseListService;
-        this.userService = userService;
         this.categoryService = categoryService;
         this.authenticatedUserResolver = authenticatedUserResolver;
     }
@@ -93,7 +90,7 @@ public class ExpenseListController {
         if (listOpt.isEmpty())
             return new ResponseEntity<>(HttpStatus.NOT_FOUND);
         assertMember(user, listOpt.get());
-        AppUser expenseOwner = userService.getUserByName(expenseInput.getOwner());
+        AppUser expenseOwner = resolveListMember(listOpt.get(), expenseInput.getOwner());
         Expense expense = expenseInput.convertToExpense(expenseOwner.getId());
         Expense addedExpense = expenseListService.addExpenseToList(expenseListId, expense);
         return new ResponseEntity<>(addedExpense, HttpStatus.CREATED);
@@ -109,7 +106,7 @@ public class ExpenseListController {
         if (expenseListOpt.isEmpty())
             return new ResponseEntity<>(HttpStatus.NOT_FOUND);
         assertMember(user, expenseListOpt.get());
-        AppUser expenseOwner = userService.getUserByName(expenseChangeRequest.getOwnerName());
+        AppUser expenseOwner = resolveListMember(expenseListOpt.get(), expenseChangeRequest.getOwnerName());
         Expense expense = expenseChangeRequest.convertToExpense(expenseOwner.getId(), expenseListOpt.get());
         Expense updatedExpense = expenseListService.updateExpense(expenseListId, expense);
         return new ResponseEntity<>(updatedExpense, HttpStatus.OK);
@@ -156,6 +153,11 @@ public class ExpenseListController {
         if (list.getOwner().getId().equals(authenticatedUser.getId())) {
             return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("You cannot join your own list");
         }
+        if (list.getOwner().getUsername().equals(authenticatedUser.getUsername())) {
+            return ResponseEntity.status(HttpStatus.CONFLICT).body(
+                    "You and the list owner both use the username \"" + authenticatedUser.getUsername()
+                            + "\". Please change your username before joining this list.");
+        }
         list.setSharedWith(authenticatedUser);
         expenseListService.save(list);
         return ResponseEntity.ok("User added to the list");
@@ -173,4 +175,20 @@ public class ExpenseListController {
         if (!isOwner && !isShared)
             throw new ResponseStatusException(HttpStatus.FORBIDDEN);
     }
+
+    /**
+     * Resolves an expense owner by username among the list's members. Usernames are
+     * not globally unique, so the owner must be one of this list's (at most two)
+     * members — never a global lookup.
+     */
+    private AppUser resolveListMember(ExpenseList list, String username) {
+        if (list.getOwner() != null && list.getOwner().getUsername().equals(username)) {
+            return list.getOwner();
+        }
+        if (list.getSharedWith() != null && list.getSharedWith().getUsername().equals(username)) {
+            return list.getSharedWith();
+        }
+        throw new ResponseStatusException(HttpStatus.BAD_REQUEST,
+                "Expense owner must be a member of this list");
+    }
 }

src/main/java/de/zendric/app/xpensely_server/controller/GlobalExceptionHandler.java

@@ -1,5 +1,6 @@
 package de.zendric.app.xpensely_server.controller;
 
+import de.zendric.app.xpensely_server.model.Exception.GoogleAccountAlreadyRegisteredException;
 import de.zendric.app.xpensely_server.model.Exception.ResourceNotFoundException;
 import de.zendric.app.xpensely_server.model.Exception.UsernameAlreadyExistsException;
 import org.slf4j.Logger;
@@ -47,6 +48,13 @@ public class GlobalExceptionHandler {
                 .body(Map.of("error", ex.getMessage()));
     }
 
+    @ExceptionHandler(GoogleAccountAlreadyRegisteredException.class)
+    public ResponseEntity<Map<String, String>> handleGoogleAccountConflict(
+            GoogleAccountAlreadyRegisteredException ex) {
+        return ResponseEntity.status(HttpStatus.CONFLICT)
+                .body(Map.of("error", ex.getMessage()));
+    }
+
     @ExceptionHandler(ResponseStatusException.class)
     public ResponseEntity<Map<String, String>> handleResponseStatus(ResponseStatusException ex) {
         return ResponseEntity.status(ex.getStatusCode())

src/main/java/de/zendric/app/xpensely_server/model/AppUser.java

@@ -26,9 +26,10 @@ public class AppUser {
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     private Long id;
 
-    @Column(name = "username", nullable = false, unique = true)
+    @Column(name = "username", nullable = false)
     private String username;
 
+    @Column(name = "google_id", unique = true)
     @JsonIgnore
     private String googleId;
 

src/main/java/de/zendric/app/xpensely_server/model/Exception/GoogleAccountAlreadyRegisteredException.java

@@ -0,0 +1,11 @@
+package de.zendric.app.xpensely_server.model.Exception;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(HttpStatus.CONFLICT)
+public class GoogleAccountAlreadyRegisteredException extends RuntimeException {
+    public GoogleAccountAlreadyRegisteredException(String message) {
+        super(message);
+    }
+}

src/main/java/de/zendric/app/xpensely_server/model/UsernameUpdateRequest.java

@@ -0,0 +1,15 @@
+package de.zendric.app.xpensely_server.model;
+
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.Pattern;
+import jakarta.validation.constraints.Size;
+import lombok.Data;
+
+@Data
+public class UsernameUpdateRequest {
+
+    @NotBlank(message = "Username is required")
+    @Size(min = 3, max = 30, message = "Username must be between 3 and 30 characters")
+    @Pattern(regexp = "^[a-zA-Z0-9_.\\-]+$", message = "Username may only contain letters, digits, underscores, dots, and hyphens")
+    private String username;
+}

src/main/java/de/zendric/app/xpensely_server/repo/UserRepository.java

@@ -14,4 +14,6 @@ public interface UserRepository extends JpaRepository<AppUser, Long> {
     Optional<AppUser> findByGoogleId(String id);
 
     Boolean existsByUsername(String username);
+
+    Boolean existsByGoogleId(String googleId);
 }

src/main/java/de/zendric/app/xpensely_server/services/UserService.java

@@ -5,8 +5,8 @@ import java.util.List;
 import org.springframework.stereotype.Service;
 
 import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.Exception.GoogleAccountAlreadyRegisteredException;
 import de.zendric.app.xpensely_server.model.Exception.ResourceNotFoundException;
-import de.zendric.app.xpensely_server.model.Exception.UsernameAlreadyExistsException;
 import de.zendric.app.xpensely_server.repo.UserRepository;
 
 @Service
@@ -22,9 +22,18 @@ public class UserService {
     }
 
     public AppUser createUser(AppUser user) {
-        if (Boolean.TRUE.equals(userRepository.existsByUsername(user.getUsername()))) {
+        if (Boolean.TRUE.equals(userRepository.existsByGoogleId(user.getGoogleId()))) {
-            throw new UsernameAlreadyExistsException("Username already exists");
+            throw new GoogleAccountAlreadyRegisteredException(
+                    "An account already exists for this Google account");
         }
+        // Usernames are not unique — identity is the Google account (googleId).
+        return userRepository.save(user);
+    }
+
+    public AppUser updateUsername(Long userId, String newUsername) {
+        AppUser user = userRepository.findById(userId)
+                .orElseThrow(() -> new ResourceNotFoundException("User not found with id: " + userId));
+        user.setUsername(newUsername);
         return userRepository.save(user);
     }
 

src/test/java/de/zendric/app/xpensely_Server/controller/AppUserControllerTest.java

@@ -6,8 +6,6 @@ import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
 import de.zendric.app.xpensely_server.services.UserService;
 import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
-import org.springframework.boot.security.oauth2.client.autoconfigure.OAuth2ClientAutoConfiguration;
-import org.springframework.boot.security.oauth2.client.autoconfigure.servlet.OAuth2ClientWebSecurityAutoConfiguration;
 import org.springframework.boot.security.oauth2.server.resource.autoconfigure.servlet.OAuth2ResourceServerAutoConfiguration;
 import org.springframework.boot.webmvc.test.autoconfigure.AutoConfigureMockMvc;
 import org.springframework.boot.webmvc.test.autoconfigure.WebMvcTest;
@@ -22,8 +20,6 @@ import static org.springframework.test.web.servlet.request.MockMvcRequestBuilder
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
 
 @WebMvcTest(value = AppUserController.class, excludeAutoConfiguration = {
-        OAuth2ClientAutoConfiguration.class,
-        OAuth2ClientWebSecurityAutoConfiguration.class,
         OAuth2ResourceServerAutoConfiguration.class
 })
 @AutoConfigureMockMvc(addFilters = false)
@@ -99,4 +95,38 @@ class AppUserControllerTest {
         mockMvc.perform(delete("/api/users").param("id", "99"))
                 .andExpect(status().isForbidden());
     }
+
+    // --- Rename username ---
+
+    @Test
+    void updateUsername_valid_returns200WithUpdatedUser() throws Exception {
+        AppUser self = new AppUser(); self.setId(1L); self.setUsername("old");
+        AppUser updated = new AppUser(); updated.setId(1L); updated.setUsername("newname");
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(self);
+        when(userService.updateUsername(1L, "newname")).thenReturn(updated);
+
+        mockMvc.perform(put("/api/users/username")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"username\":\"newname\"}"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.username").value("newname"));
+    }
+
+    @Test
+    void updateUsername_invalidPattern_returns400() throws Exception {
+        mockMvc.perform(put("/api/users/username")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"username\":\"bad name!\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.username").exists());
+    }
+
+    @Test
+    void updateUsername_tooShort_returns400() throws Exception {
+        mockMvc.perform(put("/api/users/username")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"username\":\"ab\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.username").exists());
+    }
 }

src/test/java/de/zendric/app/xpensely_Server/controller/ExpenseListControllerTest.java

@@ -2,15 +2,13 @@ package de.zendric.app.xpensely_Server.controller;
 
 import de.zendric.app.xpensely_server.controller.ExpenseListController;
 import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.Expense;
 import de.zendric.app.xpensely_server.model.ExpenseList;
 import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
 import de.zendric.app.xpensely_server.services.CategoryService;
 import de.zendric.app.xpensely_server.services.ExpenseListService;
-import de.zendric.app.xpensely_server.services.UserService;
 import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
-import org.springframework.boot.security.oauth2.client.autoconfigure.OAuth2ClientAutoConfiguration;
-import org.springframework.boot.security.oauth2.client.autoconfigure.servlet.OAuth2ClientWebSecurityAutoConfiguration;
 import org.springframework.boot.security.oauth2.server.resource.autoconfigure.servlet.OAuth2ResourceServerAutoConfiguration;
 import org.springframework.boot.webmvc.test.autoconfigure.AutoConfigureMockMvc;
 import org.springframework.boot.webmvc.test.autoconfigure.WebMvcTest;
@@ -19,17 +17,17 @@ import org.springframework.test.context.ActiveProfiles;
 import org.springframework.test.context.bean.override.mockito.MockitoBean;
 import org.springframework.test.web.servlet.MockMvc;
 
+import java.time.LocalDateTime;
 import java.util.List;
 import java.util.Optional;
 
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.when;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
 
 @WebMvcTest(value = ExpenseListController.class, excludeAutoConfiguration = {
-        OAuth2ClientAutoConfiguration.class,
-        OAuth2ClientWebSecurityAutoConfiguration.class,
         OAuth2ResourceServerAutoConfiguration.class
 })
 @AutoConfigureMockMvc(addFilters = false)
@@ -38,7 +36,6 @@ class ExpenseListControllerTest {
 
     @Autowired MockMvc mockMvc;
     @MockitoBean ExpenseListService expenseListService;
-    @MockitoBean UserService userService;
     @MockitoBean CategoryService categoryService;
     @MockitoBean AuthenticatedUserResolver authenticatedUserResolver;
 
@@ -178,4 +175,55 @@ class ExpenseListControllerTest {
                 .andExpect(status().isBadRequest())
                 .andExpect(jsonPath("$.name").exists());
     }
+
+    // --- Owner resolution within list members ---
+
+    @Test
+    void addExpense_ownerNotAListMember_returns400() throws Exception {
+        AppUser member = new AppUser(); member.setId(1L); member.setUsername("alice");
+        ExpenseList list = new ExpenseList(); list.setId(1L); list.setOwner(member);
+
+        when(expenseListService.findById(1L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(member);
+
+        mockMvc.perform(post("/api/expenselist/1/add")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"title\":\"Lunch\",\"owner\":\"ghost\",\"amount\":10.0,\"date\":\"2026-05-04\",\"category\":\"Food\"}"))
+                .andExpect(status().isBadRequest());
+    }
+
+    @Test
+    void addExpense_ownerIsAListMember_returns201() throws Exception {
+        AppUser member = new AppUser(); member.setId(1L); member.setUsername("alice");
+        ExpenseList list = new ExpenseList(); list.setId(1L); list.setOwner(member);
+
+        when(expenseListService.findById(1L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(member);
+        when(expenseListService.addExpenseToList(eq(1L), any())).thenReturn(new Expense());
+
+        mockMvc.perform(post("/api/expenselist/1/add")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"title\":\"Lunch\",\"owner\":\"alice\",\"amount\":10.0,\"date\":\"2026-05-04\",\"category\":\"Food\"}"))
+                .andExpect(status().isCreated());
+    }
+
+    // --- Duplicate-username invite guard ---
+
+    @Test
+    void acceptInvite_sameUsernameAsOwner_returns409() throws Exception {
+        AppUser owner = new AppUser(); owner.setId(1L); owner.setUsername("cedric");
+        AppUser joiner = new AppUser(); joiner.setId(2L); joiner.setUsername("cedric");
+        ExpenseList list = new ExpenseList();
+        list.setId(1L);
+        list.setOwner(owner);
+        list.setInviteCodeExpiration(LocalDateTime.now().plusHours(1));
+
+        when(expenseListService.findByInviteCode("ABC123")).thenReturn(list);
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(joiner);
+
+        mockMvc.perform(post("/api/expenselist/accept-invite")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"inviteCode\":\"ABC123\"}"))
+                .andExpect(status().isConflict());
+    }
 }