Feat: 대댓글 생성 API 구현

Author: joyewon0705

src/main/java/com/back/domain/board/comment/controller/CommentController.java

@@ -3,6 +3,7 @@
 import com.back.domain.board.comment.dto.CommentListResponse;
 import com.back.domain.board.comment.dto.CommentRequest;
 import com.back.domain.board.comment.dto.CommentResponse;
+import com.back.domain.board.comment.dto.ReplyResponse;
 import com.back.domain.board.common.dto.PageResponse;
 import com.back.domain.board.comment.service.CommentService;
 import com.back.global.common.dto.RsData;
@@ -86,4 +87,21 @@ public ResponseEntity<RsData<Void>> deleteComment(
                         null
                 ));
     }
+
+    // 대댓글 생성
+    @PostMapping("/{commentId}/replies")
+    public ResponseEntity<RsData<ReplyResponse>> createReply(
+            @PathVariable Long postId,
+            @PathVariable Long commentId,
+            @RequestBody @Valid CommentRequest request,
+            @AuthenticationPrincipal CustomUserDetails user
+    ) {
+        ReplyResponse response = commentService.createReply(postId, commentId, request, user.getUserId());
+        return ResponseEntity
+                .status(HttpStatus.CREATED)
+                .body(RsData.success(
+                        "대댓글이 생성되었습니다.",
+                        response
+                ));
+    }
 }

src/main/java/com/back/domain/board/comment/dto/ReplyResponse.java

@@ -0,0 +1,39 @@
+package com.back.domain.board.comment.dto;
+
+import com.back.domain.board.comment.entity.Comment;
+import com.back.domain.board.common.dto.AuthorResponse;
+
+import java.time.LocalDateTime;
+
+/**
+ * 대댓글 응답 DTO
+ *
+ * @param commentId 댓글 Id
+ * @param postId    게시글 Id
+ * @param parentId  부모 댓글 Id
+ * @param author    작성자 정보
+ * @param content   댓글 내용
+ * @param createdAt 댓글 생성 일시
+ * @param updatedAt 댓글 수정 일시
+ */
+public record ReplyResponse(
+        Long commentId,
+        Long postId,
+        Long parentId,
+        AuthorResponse author,
+        String content,
+        LocalDateTime createdAt,
+        LocalDateTime updatedAt
+) {
+    public static ReplyResponse from(Comment comment) {
+        return new ReplyResponse(
+                comment.getId(),
+                comment.getPost().getId(),
+                comment.getParent().getId(),
+                AuthorResponse.from(comment.getUser()),
+                comment.getContent(),
+                comment.getCreatedAt(),
+                comment.getUpdatedAt()
+        );
+    }
+}

src/main/java/com/back/domain/board/comment/service/CommentService.java

@@ -3,6 +3,7 @@
 import com.back.domain.board.comment.dto.CommentListResponse;
 import com.back.domain.board.comment.dto.CommentRequest;
 import com.back.domain.board.comment.dto.CommentResponse;
+import com.back.domain.board.comment.dto.ReplyResponse;
 import com.back.domain.board.common.dto.PageResponse;
 import com.back.domain.board.comment.entity.Comment;
 import com.back.domain.board.post.entity.Post;
@@ -120,4 +121,44 @@ public void deleteComment(Long postId, Long commentId, Long userId) {
 
         commentRepository.delete(comment);
     }
+
+    /**
+     * 대댓글 생성 서비스
+     * 1. User 조회
+     * 2. Post 조회
+     * 3. 부모 Comment 조회
+     * 4. 부모 및 depth 검증
+     * 5. 자식 Comment 생성
+     * 6. Comment 저장 및 ReplyResponse 반환
+     */
+    public ReplyResponse createReply(Long postId, Long parentCommentId, CommentRequest request, Long userId) {
+        // User 조회
+        User user = userRepository.findById(userId)
+                .orElseThrow(() -> new CustomException(ErrorCode.USER_NOT_FOUND));
+
+        // Post 조회
+        Post post = postRepository.findById(postId)
+                .orElseThrow(() -> new CustomException(ErrorCode.POST_NOT_FOUND));
+
+        // 부모 Comment 조회
+        Comment parent = commentRepository.findById(parentCommentId)
+                .orElseThrow(() -> new CustomException(ErrorCode.COMMENT_NOT_FOUND));
+
+        // 부모의 게시글 일치 검증
+        if (!parent.getPost().getId().equals(postId)) {
+            throw new CustomException(ErrorCode.COMMENT_PARENT_MISMATCH);
+        }
+
+        // depth 검증: 부모가 이미 대댓글이면 예외
+        if (parent.getParent() != null) {
+            throw new CustomException(ErrorCode.COMMENT_DEPTH_EXCEEDED);
+        }
+
+        // 자식 Comment 생성
+        Comment reply = new Comment(post, user, request.content(), parent);
+
+        // 저장 및 응답 반환
+        commentRepository.save(reply);
+        return ReplyResponse.from(reply);
+    }
 }

src/main/java/com/back/global/exception/ErrorCode.java

@@ -88,6 +88,8 @@ public enum ErrorCode {
     CATEGORY_NOT_FOUND(HttpStatus.NOT_FOUND, "POST_003", "존재하지 않는 카테고리입니다."),
     COMMENT_NOT_FOUND(HttpStatus.NOT_FOUND, "COMMENT_001", "존재하지 않는 댓글입니다."),
     COMMENT_NO_PERMISSION(HttpStatus.FORBIDDEN, "COMMENT_002", "댓글 작성자만 수정/삭제할 수 있습니다."),
+    COMMENT_PARENT_MISMATCH(HttpStatus.BAD_REQUEST, "COMMENT_003", "부모 댓글이 해당 게시글에 속하지 않습니다."),
+    COMMENT_DEPTH_EXCEEDED(HttpStatus.BAD_REQUEST, "COMMENT_004", "대댓글은 한 단계까지만 작성할 수 있습니다."),
 
     // ======================== 공통 에러 ========================
     BAD_REQUEST(HttpStatus.BAD_REQUEST, "COMMON_400", "잘못된 요청입니다."),