Fix student session persistence and question sync on reconnect

exam-system-backend/src/main/java/com/exam/system/repository/StudentRepository.java

@@ -56,6 +56,17 @@ public interface StudentRepository extends JpaRepository<Student, Long> {
      */
     boolean existsBySessionId(String sessionId);
 
+    /**
+     * 根據測驗 ID、Email 和姓名查詢學員
+     * 用於學員重新加入時找回原有 Session
+     *
+     * @param examId 測驗 ID
+     * @param email Email
+     * @param name 姓名
+     * @return 學員實體（Optional）
+     */
+    Optional<Student> findByExamIdAndEmailAndName(Long examId, String email, String name);
+
     /**
      * 根據測驗 ID 統計各分數的學員數量
      * 用於生成累積分數分布圖

exam-system-backend/src/main/java/com/exam/system/service/StudentService.java

@@ -20,6 +20,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.UUID;
 import java.util.stream.Collectors;
 
@@ -87,35 +88,61 @@ public StudentDTO joinExam(StudentDTO studentDTO) {
             }
         }
 
-        // 生成唯一的 sessionId
-        String sessionId = UUID.randomUUID().toString();
-
-        // 建立學員實體
-        Student student = Student.builder()
-                .exam(exam)
-                .sessionId(sessionId)
-                .name(studentDTO.getName())
-                .email(studentDTO.getEmail())
-                .occupation(studentDTO.getOccupation())
-                .surveyData(studentDTO.getSurveyData())
-                .avatarIcon(studentDTO.getAvatarIcon())
-                .totalScore(0)
-                .build();
-
-        student = studentRepository.save(student);
-
-        log.info("Student joined successfully: {} (sessionId: {})", student.getName(), sessionId);
-
-        // 透過 WebSocket 通知講師有新學員加入
-        Map<String, Object> studentData = new HashMap<>();
-        studentData.put("id", student.getId());
-        studentData.put("name", student.getName());
-        studentData.put("avatarIcon", student.getAvatarIcon());
-        studentData.put("totalScore", student.getTotalScore());
-        studentData.put("correctAnswersCount", 0); // 新加入的學員答對題數為 0
-        studentData.put("totalStudents", studentRepository.countByExamId(exam.getId()));
-
-        webSocketService.broadcastStudentJoined(exam.getId(), WebSocketMessage.studentJoined(studentData));
+        // 檢查學員是否已存在（根據 Email 和姓名）
+        // 如果存在，則返回現有學員 Session，實現重新登入功能
+        Optional<Student> existingStudent = studentRepository.findByExamIdAndEmailAndName(
+                exam.getId(), studentDTO.getEmail(), studentDTO.getName());
+
+        Student student;
+        if (existingStudent.isPresent()) {
+            student = existingStudent.get();
+            log.info("Existing student rejoined: {} (sessionId: {})", student.getName(), student.getSessionId());
+
+            // 更新學員資訊（如頭像或調查資料可能變更）
+            boolean updated = false;
+            if (!student.getAvatarIcon().equals(studentDTO.getAvatarIcon())) {
+                student.setAvatarIcon(studentDTO.getAvatarIcon());
+                updated = true;
+            }
+            if (studentDTO.getSurveyData() != null && !studentDTO.getSurveyData().equals(student.getSurveyData())) {
+                student.setSurveyData(studentDTO.getSurveyData());
+                updated = true;
+            }
+
+            if (updated) {
+                student = studentRepository.save(student);
+            }
+        } else {
+            // 生成唯一的 sessionId
+            String sessionId = UUID.randomUUID().toString();
+
+            // 建立學員實體
+            student = Student.builder()
+                    .exam(exam)
+                    .sessionId(sessionId)
+                    .name(studentDTO.getName())
+                    .email(studentDTO.getEmail())
+                    .occupation(studentDTO.getOccupation())
+                    .surveyData(studentDTO.getSurveyData())
+                    .avatarIcon(studentDTO.getAvatarIcon())
+                    .totalScore(0)
+                    .build();
+
+            student = studentRepository.save(student);
+
+            log.info("Student joined successfully: {} (sessionId: {})", student.getName(), sessionId);
+
+            // 透過 WebSocket 通知講師有新學員加入
+            Map<String, Object> studentData = new HashMap<>();
+            studentData.put("id", student.getId());
+            studentData.put("name", student.getName());
+            studentData.put("avatarIcon", student.getAvatarIcon());
+            studentData.put("totalScore", student.getTotalScore());
+            studentData.put("correctAnswersCount", 0); // 新加入的學員答對題數為 0
+            studentData.put("totalStudents", studentRepository.countByExamId(exam.getId()));
+
+            webSocketService.broadcastStudentJoined(exam.getId(), WebSocketMessage.studentJoined(studentData));
+        }
 
         // 如果當前有正在進行的題目，推送給新加入的學員
         if (exam.getCurrentQuestionStartedAt() != null && exam.getCurrentQuestionIndex() < exam.getQuestions().size()) {

exam-system-backend/src/test/java/com/exam/system/service/StudentServiceTest.java

@@ -8,6 +8,7 @@
 import com.exam.system.exception.BusinessException;
 import com.exam.system.exception.ResourceNotFoundException;
 import com.exam.system.repository.ExamRepository;
+import com.exam.system.repository.ExamSurveyFieldConfigRepository;
 import com.exam.system.repository.StudentRepository;
 import com.exam.system.websocket.WebSocketService;
 import org.junit.jupiter.api.BeforeEach;
@@ -18,6 +19,7 @@
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
 
+import java.util.Collections;
 import java.util.Optional;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -42,6 +44,9 @@ class StudentServiceTest {
     @Mock
     private WebSocketService webSocketService;
 
+    @Mock
+    private ExamSurveyFieldConfigRepository examSurveyFieldConfigRepository;
+
     @InjectMocks
     private StudentService studentService;
 
@@ -62,6 +67,10 @@ void setUp() {
     void testJoinExam_Success() {
         // Given
         when(examRepository.findByAccessCode("TEST01")).thenReturn(Optional.of(testExam));
+        when(examSurveyFieldConfigRepository.findByExamIdOrderByDisplayOrderAsc(anyLong())).thenReturn(Collections.emptyList());
+
+        when(studentRepository.findByExamIdAndEmailAndName(anyLong(), anyString(), anyString())).thenReturn(Optional.empty());
+
         when(studentRepository.save(any(Student.class))).thenAnswer(invocation -> {
             Student s = invocation.getArgument(0);
             s.setId(1L);
@@ -86,6 +95,42 @@ void testJoinExam_Success() {
         verify(webSocketService).broadcastStudentJoined(eq(1L), any());
     }
 
+    @Test
+    @DisplayName("測試學員重新加入測驗 (Reconnect)")
+    void testJoinExam_Reconnect() {
+        // Given
+        when(examRepository.findByAccessCode("TEST01")).thenReturn(Optional.of(testExam));
+        when(examSurveyFieldConfigRepository.findByExamIdOrderByDisplayOrderAsc(anyLong())).thenReturn(Collections.emptyList());
+
+        // 模擬已存在的學員
+        Student existingStudent = TestDataBuilder.createStudent(testExam);
+        existingStudent.setId(1L);
+        existingStudent.setSessionId("existing-session-id");
+        existingStudent.setEmail(testStudentDTO.getEmail());
+        existingStudent.setName(testStudentDTO.getName());
+        existingStudent.setAvatarIcon("old-avatar");
+
+        testStudentDTO.setAvatarIcon("new-avatar"); // 模擬頭像變更
+
+        when(studentRepository.findByExamIdAndEmailAndName(eq(1L), eq(testStudentDTO.getEmail()), eq(testStudentDTO.getName())))
+                .thenReturn(Optional.of(existingStudent));
+
+        when(studentRepository.save(any(Student.class))).thenAnswer(invocation -> invocation.getArgument(0));
+
+        // When
+        StudentDTO result = studentService.joinExam(testStudentDTO);
+
+        // Then
+        assertThat(result).isNotNull();
+        assertThat(result.getId()).isEqualTo(1L);
+        assertThat(result.getSessionId()).isEqualTo("existing-session-id");
+        assertThat(result.getAvatarIcon()).isEqualTo("new-avatar"); // 確認頭像已更新
+
+        verify(studentRepository, times(1)).save(any(Student.class)); // 應該被調用一次來更新頭像
+        // 不應該廣播 join 訊息（因為是 reconnect）- 實際上現在的邏輯只在 else 區塊廣播，所以這裡應該 verify never
+        verify(webSocketService, never()).broadcastStudentJoined(anyLong(), any());
+    }
+
     @Test
     @DisplayName("測試學員加入測驗 - 無效的 accessCode")
     void testJoinExam_InvalidAccessCode() {

exam-system-frontend/src/pages/StudentExam.tsx

@@ -185,6 +185,30 @@ export const StudentExam: React.FC = () => {
       handleQuestionStarted
     );
 
+    // 當 WebSocket 連線恢復時，重新獲取最新的學員狀態（包含當前題目）
+    // 這解決了斷線重連後可能錯過 WebSocket 推送的問題
+    console.log('[StudentExam] WebSocket 連線恢復，重新同步狀態');
+    studentApi.getStudent(sessionId)
+      .then(student => {
+        if (student.currentQuestion) {
+          console.log('[StudentExam] 重新同步獲取到當前題目:', student.currentQuestion);
+          setCurrentQuestion({
+            questionId: student.currentQuestion.questionId,
+            questionIndex: student.currentQuestion.questionIndex,
+            questionText: student.currentQuestion.questionText,
+            options: student.currentQuestion.options,
+            expiresAt: student.currentQuestion.expiresAt,
+          });
+          setExamStatus('STARTED');
+        } else {
+             // 如果後端沒回傳 currentQuestion，代表現在可能沒題目或已過期
+             // 但我們只在明確收到 null 時才清除，避免閃爍
+        }
+      })
+      .catch(err => {
+        console.error('[StudentExam] 重新同步狀態失敗:', err);
+      });
+
     // 清理函式
     return () => {
       console.log('[StudentExam] 取消訂閱個人題目主題');