[Fix] #588 - 친구 추천 시 Platform 조회 전 사전 샘플링으로 414 방지 + RandomPicker를 ThreadLocalRandom으로 전환

src/main/java/org/sopt/app/application/friend/FriendRecommender.java

@@ -27,6 +27,8 @@ public class FriendRecommender {
     private final PlaygroundUserIdsProvider playgroundUserIdsProvider;
     private final PlatformService platformService;
 
+    private static final int MAX_OVERSAMPLE = 50;
+
     public RecommendedFriendsRequest recommendFriendsByTypeList(List<FriendRecommendType> typeList, int size,
         Long userId) {
         typeList = this.adjustTypeList(typeList);
@@ -37,7 +39,7 @@ public RecommendedFriendsRequest recommendFriendsByTypeList(List<FriendRecommend
         List<RecommendedFriendsByType> recommendedFriends = new ArrayList<>();
         for (FriendRecommendType type : typeList) {
             List<UserProfile> recommendableUserProfiles =
-                    this.getRecommendableUserProfiles(type, ownProfile, friendFilter);
+                    this.getRecommendableUserProfiles(type, ownProfile, friendFilter, size);
             if (!recommendableUserProfiles.isEmpty()) {
                 List<UserProfile> pickedUserProfiles = RandomPicker.pickRandom(recommendableUserProfiles, size);
                 List<SimplePokeProfile> simplePokeProfiles =
@@ -75,17 +77,37 @@ private List<SimplePokeProfile> makeSimplePokeProfilesForNotFriend(
     }
 
     private List<UserProfile> getRecommendableUserProfiles(
-            FriendRecommendType type, OwnPlaygroundProfile ownProfile, FriendFilter friendFilter) {
+            FriendRecommendType type, OwnPlaygroundProfile ownProfile, FriendFilter friendFilter, int size) {
+        // size 가드 : 이상값 들어오면 500 대신 빈 리스트
+        if (size <= 0) return List.of();
+
+        // ALL_USER 아닐 경우 playground에서 후보 id 수집
         Set<Long> userIds;
         if (type == FriendRecommendType.ALL_USER) {
             userIds = Set.copyOf(userService.getAllUserIds());
         } else {
             userIds = playgroundUserIdsProvider.findPlaygroundIdsByType(ownProfile, type);
         }
+        if (userIds.isEmpty()) return List.of();
+
+        List<Long> candidates = new ArrayList<>(userIds);
+
+        // oversample: size*5 (최대 50). 플랫폼 요청 URI 길이 보호(414 방지), friendFilter 탈락 대비 여유 확보 목적.
+        int oversampleCount = Math.min(MAX_OVERSAMPLE, size *5);
+        oversampleCount = Math.min(oversampleCount, candidates.size());
+
+        // 플랫폼 조회 전 ID를 oversampleCount 만큼만 추출 (URI 길이 보호, 414 방지)
+        List<Long> toLookup = (candidates.size() <= oversampleCount)
+            ? candidates
+            : RandomPicker.pickRandom(candidates, oversampleCount);
+
+        // 플랫폼에서 유저 정보 조회
+        List <UserProfile> unFilteredUserProfiles = platformService.getPlatformUserInfosResponse(List.copyOf(toLookup))
+            .stream()
+            .map(user -> UserProfile.of((long)user.userId(), user.name()))
+            .toList();
 
-        List <UserProfile> unFilteredUserProfiles = platformService.getPlatformUserInfosResponse(List.copyOf(userIds))
-                .stream()
-                .map(user -> UserProfile.of((long)user.userId(), user.name())).toList();
+        // 이미 친구인 유저 제외(앱 내 filter)
         return friendFilter.excludeAlreadyFriendUserIds(unFilteredUserProfiles);
     }
 

src/main/java/org/sopt/app/common/utils/RandomPicker.java

@@ -7,21 +7,19 @@
 @NoArgsConstructor(access = lombok.AccessLevel.PRIVATE)
 public class RandomPicker {
 
-    private static final Random random = new Random();
-
     public static <T> List<T> pickRandom(List<T> data, int pickLimit) {
-        if (data.size() <= pickLimit) {
-            return new ArrayList<>(data);
-        }
+        int n = data.size();
+        if (pickLimit <= 0 || n == 0) return List.of();     // 가드
+        if (pickLimit >= n) return new ArrayList<>(data);   // 전부 반환
 
-        int[] indices = IntStream.range(0, data.size()).toArray();
-        List<T> randomList = new ArrayList<>();
+        int[] indices = IntStream.range(0, n).toArray();
+        List<T> randomList = new ArrayList<>(pickLimit);
         for (int i = 0; i < pickLimit; ++i) {
-            int randomIndex = random.nextInt(data.size() - i);
+            int randomIndex = java.util.concurrent.ThreadLocalRandom.current().nextInt(n - i);
             randomList.add(data.get(indices[randomIndex]));
-            indices[randomIndex] = indices[data.size() - 1 - i];
+            // Fisher–Yates 선택: 사용한 슬롯을 뒤쪽 미사용 구간의 마지막으로 대체
+            indices[randomIndex] = indices[n - 1 - i];
         }
         return randomList;
     }
-
 }