diff --git a/src/main/kotlin/io/csbroker/apiserver/auth/AuthToken.kt b/src/main/kotlin/io/csbroker/apiserver/auth/AuthToken.kt
index 205db063..d48ba7ac 100644
--- a/src/main/kotlin/io/csbroker/apiserver/auth/AuthToken.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/auth/AuthToken.kt
@@ -18,21 +18,17 @@ class AuthToken(
     private val key: Key,
 ) {
     constructor(email: String, expiry: Date, key: Key, role: String? = null) : this("", key) {
-        if (role != null) {
-            token = createAuthToken(email, expiry, role)
+        token = if (role != null) {
+            createAuthToken(email, expiry, role)
         } else {
-            token = createAuthToken(email, expiry)
+            createAuthToken(email, expiry)
         }
     }
 
     val tokenClaims: Claims?
         get() {
             try {
-                return Jwts.parserBuilder()
-                    .setSigningKey(key)
-                    .build()
-                    .parseClaimsJws(token)
-                    .body
+                return parseJwt()
             } catch (e: SecurityException) {
                 log.error("Invalid JWT signature.")
             } catch (e: MalformedJwtException) {
@@ -52,11 +48,7 @@ class AuthToken(
     val expiredTokenClaims: Claims?
         get() {
             try {
-                Jwts.parserBuilder()
-                    .setSigningKey(key)
-                    .build()
-                    .parseClaimsJws(token)
-                    .body
+                parseJwt()
             } catch (e: ExpiredJwtException) {
                 log.info("Expired JWT token.")
                 return e.claims
@@ -77,6 +69,12 @@ class AuthToken(
     val isValid: Boolean
         get() = tokenClaims != null
 
+    private fun parseJwt(): Claims? = Jwts.parserBuilder()
+        .setSigningKey(key)
+        .build()
+        .parseClaimsJws(token)
+        .body
+
     private fun createAuthToken(email: String, expiry: Date): String {
         return Jwts.builder()
             .setSubject(email)
diff --git a/src/main/kotlin/io/csbroker/apiserver/auth/AuthTokenProvider.kt b/src/main/kotlin/io/csbroker/apiserver/auth/AuthTokenProvider.kt
index a609f583..14bde6c9 100644
--- a/src/main/kotlin/io/csbroker/apiserver/auth/AuthTokenProvider.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/auth/AuthTokenProvider.kt
@@ -30,8 +30,8 @@ class AuthTokenProvider(
                 .map(::SimpleGrantedAuthority)
                 .toList()
             return UsernamePasswordAuthenticationToken(User(claims.subject, "", authorities), authToken, authorities)
-        } else {
-            throw UnAuthorizedException(ErrorCode.TOKEN_INVALID, "올바르지 않은 Token입니다.")
         }
+
+        throw UnAuthorizedException(ErrorCode.TOKEN_INVALID, "올바르지 않은 Token입니다.")
     }
 }
diff --git a/src/main/kotlin/io/csbroker/apiserver/auth/GithubOAuth2UserInfo.kt b/src/main/kotlin/io/csbroker/apiserver/auth/GithubOAuth2UserInfo.kt
index 26bbb51a..79b8c3ec 100644
--- a/src/main/kotlin/io/csbroker/apiserver/auth/GithubOAuth2UserInfo.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/auth/GithubOAuth2UserInfo.kt
@@ -2,21 +2,9 @@ package io.csbroker.apiserver.auth
 
 class GithubOAuth2UserInfo(
     attributes: MutableMap<String, Any>,
-) : OAuth2UserInfo(attributes) {
-
-    override fun getId(): String {
-        return (attributes["id"] as Int).toString()
-    }
-
-    override fun getName(): String {
-        return attributes["login"] as String
-    }
-
-    override fun getEmail(): String {
-        return attributes["email"] as String
-    }
-
-    override fun getImageUrl(): String {
-        return attributes["avatar_url"] as String
-    }
-}
+) : OAuth2UserInfo(
+    id = (attributes["id"] as Int).toString(),
+    name = attributes["login"] as String,
+    email = attributes["email"] as String,
+    imageUrl = attributes["avatar_url"] as String,
+)
diff --git a/src/main/kotlin/io/csbroker/apiserver/auth/GoogleOAuth2UserInfo.kt b/src/main/kotlin/io/csbroker/apiserver/auth/GoogleOAuth2UserInfo.kt
index 4f3d46b1..4f02f5f0 100644
--- a/src/main/kotlin/io/csbroker/apiserver/auth/GoogleOAuth2UserInfo.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/auth/GoogleOAuth2UserInfo.kt
@@ -2,21 +2,9 @@ package io.csbroker.apiserver.auth
 
 class GoogleOAuth2UserInfo(
     attributes: MutableMap<String, Any>,
-) : OAuth2UserInfo(attributes) {
-
-    override fun getId(): String {
-        return attributes["sub"] as String
-    }
-
-    override fun getName(): String {
-        return attributes["name"] as String
-    }
-
-    override fun getEmail(): String {
-        return attributes["email"] as String
-    }
-
-    override fun getImageUrl(): String {
-        return attributes["picture"] as String
-    }
-}
+) : OAuth2UserInfo(
+    id = attributes["sub"] as String,
+    name = attributes["name"] as String,
+    email = attributes["email"] as String,
+    imageUrl = attributes["picture"] as String,
+)
diff --git a/src/main/kotlin/io/csbroker/apiserver/auth/OAuth2UserInfo.kt b/src/main/kotlin/io/csbroker/apiserver/auth/OAuth2UserInfo.kt
index 711c8434..0fb52f70 100644
--- a/src/main/kotlin/io/csbroker/apiserver/auth/OAuth2UserInfo.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/auth/OAuth2UserInfo.kt
@@ -1,13 +1,8 @@
 package io.csbroker.apiserver.auth
 
 abstract class OAuth2UserInfo(
-    val attributes: MutableMap<String, Any>,
-) {
-    abstract fun getId(): String
-
-    abstract fun getName(): String
-
-    abstract fun getEmail(): String
-
-    abstract fun getImageUrl(): String
-}
+    val id: String,
+    val name: String,
+    val email: String,
+    val imageUrl: String,
+)
diff --git a/src/main/kotlin/io/csbroker/apiserver/auth/OAuth2UserInfoFactory.kt b/src/main/kotlin/io/csbroker/apiserver/auth/OAuth2UserInfoFactory.kt
index 828dc0fe..5fae279c 100644
--- a/src/main/kotlin/io/csbroker/apiserver/auth/OAuth2UserInfoFactory.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/auth/OAuth2UserInfoFactory.kt
@@ -8,9 +8,7 @@ class OAuth2UserInfoFactory {
             return when (providerType) {
                 ProviderType.GOOGLE -> GoogleOAuth2UserInfo(attributes)
                 ProviderType.GITHUB -> GithubOAuth2UserInfo(attributes)
-                else -> throw OAuthProviderMissMatchException(
-                    "프로바이더 타입이 일치하지 않습니다. ${providerType.name}",
-                )
+                else -> throw OAuthProviderMissMatchException("프로바이더 타입이 일치하지 않습니다. ${providerType.name}")
             }
         }
     }
diff --git a/src/main/kotlin/io/csbroker/apiserver/common/filter/TokenAuthenticationFilter.kt b/src/main/kotlin/io/csbroker/apiserver/common/filter/TokenAuthenticationFilter.kt
index 93f43135..a5a3a285 100644
--- a/src/main/kotlin/io/csbroker/apiserver/common/filter/TokenAuthenticationFilter.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/common/filter/TokenAuthenticationFilter.kt
@@ -17,7 +17,7 @@ class TokenAuthenticationFilter(
         response: HttpServletResponse,
         filterChain: FilterChain,
     ) {
-        val tokenStr = getAccessToken(request)
+        val tokenStr = request.getAccessToken()
 
         tokenStr ?: run {
             filterChain.doFilter(request, response)
diff --git a/src/main/kotlin/io/csbroker/apiserver/common/handler/OAuth2AuthenticationSuccessHandler.kt b/src/main/kotlin/io/csbroker/apiserver/common/handler/OAuth2AuthenticationSuccessHandler.kt
index 2899871f..31b1aee6 100644
--- a/src/main/kotlin/io/csbroker/apiserver/common/handler/OAuth2AuthenticationSuccessHandler.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/common/handler/OAuth2AuthenticationSuccessHandler.kt
@@ -88,9 +88,9 @@ class OAuth2AuthenticationSuccessHandler(
         val user = authentication.principal as OidcUser
         val userInfo = OAuth2UserInfoFactory.getOauth2UserInfo(providerType, user.attributes)
 
-        return userRepository.findByEmailOrProviderId(userInfo.getEmail(), userInfo.getId())
+        return userRepository.findByEmailOrProviderId(userInfo.email, userInfo.id)
             ?: throw EntityNotFoundException(
-                "유저를 찾을 수 없습니다. email = [${userInfo.getEmail()}], providerId = [${userInfo.getId()}] )",
+                "유저를 찾을 수 없습니다. email = [${userInfo.email}], providerId = [${userInfo.id}] )",
             )
     }
 
diff --git a/src/main/kotlin/io/csbroker/apiserver/common/util/CookieUtil.kt b/src/main/kotlin/io/csbroker/apiserver/common/util/CookieUtil.kt
index cbe6688b..12bf824d 100644
--- a/src/main/kotlin/io/csbroker/apiserver/common/util/CookieUtil.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/common/util/CookieUtil.kt
@@ -1,5 +1,6 @@
 package io.csbroker.apiserver.common.util
 
+import org.springframework.security.oauth2.core.endpoint.OAuth2AuthorizationRequest
 import org.springframework.util.SerializationUtils
 import java.util.Base64
 import javax.servlet.http.Cookie
@@ -21,19 +22,17 @@ fun addCookie(response: HttpServletResponse, name: String, value: String, maxAge
 }
 
 fun deleteCookie(request: HttpServletRequest, response: HttpServletResponse, name: String) {
-    request.cookies?.let {
-        it.find { cookie -> cookie.name == name }?.let { cookie ->
-            cookie.value = ""
-            cookie.path = "/"
-            cookie.maxAge = 0
-            response.addCookie(cookie)
-        }
+    request.cookies?.find { cookie -> cookie.name == name }?.let { cookie ->
+        cookie.value = ""
+        cookie.path = "/"
+        cookie.maxAge = 0
+        response.addCookie(cookie)
     }
 }
 
-fun serialize(obj: Any): String {
+fun OAuth2AuthorizationRequest.serialize(): String {
     return Base64.getUrlEncoder()
-        .encodeToString(SerializationUtils.serialize(obj))
+        .encodeToString(SerializationUtils.serialize(this))
 }
 
 fun <T> deserialize(cookie: Cookie, cls: Class<T>): T {
diff --git a/src/main/kotlin/io/csbroker/apiserver/common/util/HeaderUtil.kt b/src/main/kotlin/io/csbroker/apiserver/common/util/HeaderUtil.kt
index e284dcdc..1a5852f2 100644
--- a/src/main/kotlin/io/csbroker/apiserver/common/util/HeaderUtil.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/common/util/HeaderUtil.kt
@@ -5,7 +5,7 @@ import javax.servlet.http.HttpServletRequest
 const val HEADER_AUTHORIZATION = "Authorization"
 const val TOKEN_PREFIX = "Bearer "
 
-fun getAccessToken(request: HttpServletRequest): String? {
-    val headerValue = request.getHeader(HEADER_AUTHORIZATION)
+fun HttpServletRequest.getAccessToken(): String? {
+    val headerValue = this.getHeader(HEADER_AUTHORIZATION)
     return if (headerValue?.startsWith(TOKEN_PREFIX) == true) headerValue.substring(TOKEN_PREFIX.length) else null
 }
diff --git a/src/main/kotlin/io/csbroker/apiserver/common/util/UuidUtil.kt b/src/main/kotlin/io/csbroker/apiserver/common/util/UuidUtil.kt
index c8bf7f68..ddf294e9 100644
--- a/src/main/kotlin/io/csbroker/apiserver/common/util/UuidUtil.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/common/util/UuidUtil.kt
@@ -3,11 +3,9 @@ package io.csbroker.apiserver.common.util
 import java.nio.ByteBuffer
 import java.util.UUID
 
-fun uuidAsByte(uuid: UUID?): ByteArray? {
-    uuid ?: return null
-
+fun UUID.asByte(): ByteArray? {
     val byteBufferWrapper = ByteBuffer.wrap(ByteArray(16))
-    byteBufferWrapper.putLong(uuid.mostSignificantBits)
-    byteBufferWrapper.putLong(uuid.leastSignificantBits)
+    byteBufferWrapper.putLong(this.mostSignificantBits)
+    byteBufferWrapper.putLong(this.leastSignificantBits)
     return byteBufferWrapper.array()
 }
diff --git a/src/main/kotlin/io/csbroker/apiserver/controller/v1/post/response/PostResponseDto.kt b/src/main/kotlin/io/csbroker/apiserver/controller/v1/post/response/PostResponseDto.kt
index ae82fd6b..4cd7eb39 100644
--- a/src/main/kotlin/io/csbroker/apiserver/controller/v1/post/response/PostResponseDto.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/controller/v1/post/response/PostResponseDto.kt
@@ -13,7 +13,7 @@ data class PostResponseDto(
     val comments: List<CommentResponseDto>,
 ) {
     constructor(post: Post, likeCount: Long, isLiked: Boolean, comments: List<CommentResponseDto>) : this(
-        id = post.id!!,
+        id = post.id,
         content = post.content,
         username = post.user.username,
         likeCount = likeCount,
@@ -29,7 +29,7 @@ data class CommentResponseDto(
     val createdAt: LocalDateTime,
 ) {
     constructor(comment: Comment) : this(
-        id = comment.id!!,
+        id = comment.id,
         content = comment.content,
         username = comment.user.username,
         createdAt = comment.createdAt!!,
diff --git a/src/main/kotlin/io/csbroker/apiserver/controller/v1/problem/ProblemController.kt b/src/main/kotlin/io/csbroker/apiserver/controller/v1/problem/ProblemController.kt
index ae2b2c8b..da1f2cc4 100644
--- a/src/main/kotlin/io/csbroker/apiserver/controller/v1/problem/ProblemController.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/controller/v1/problem/ProblemController.kt
@@ -35,13 +35,10 @@ class ProblemController(
         @RequestParam("page") page: Int,
         @RequestParam("size") size: Int,
     ): ApiResponse<ProblemPageResponseDto> {
-        var solvedBy: String? = null
-
-        if (isSolved != null) {
-            solvedBy = getEmailFromSecurityContextHolder()
+        val solvedBy = isSolved?.let {
+            getEmailFromSecurityContextHolder()
                 ?: throw UnAuthorizedException(ErrorCode.FORBIDDEN, "사용자 권한이 없습니다.")
         }
-
         val searchDto = ProblemSearchDto(tags, solvedBy, isSolved, query, type, isGradable, page, size)
         val foundProblems = commonProblemService.findProblems(searchDto)
 
diff --git a/src/main/kotlin/io/csbroker/apiserver/controller/v2/problem/response/ShortProblemDetailResponseV2Dto.kt b/src/main/kotlin/io/csbroker/apiserver/controller/v2/problem/response/ShortProblemDetailResponseV2Dto.kt
index b793cec6..d873d540 100644
--- a/src/main/kotlin/io/csbroker/apiserver/controller/v2/problem/response/ShortProblemDetailResponseV2Dto.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/controller/v2/problem/response/ShortProblemDetailResponseV2Dto.kt
@@ -1,6 +1,6 @@
 package io.csbroker.apiserver.controller.v2.problem.response
 
-class ShortProblemDetailResponseV2Dto(
+data class ShortProblemDetailResponseV2Dto(
     val id: Long,
     val title: String,
     val tags: List<String>,
diff --git a/src/main/kotlin/io/csbroker/apiserver/dto/problem/grade/KeywordGradingRequestDto.kt b/src/main/kotlin/io/csbroker/apiserver/dto/problem/grade/KeywordGradingRequestDto.kt
index e5e9c321..b46dd36f 100644
--- a/src/main/kotlin/io/csbroker/apiserver/dto/problem/grade/KeywordGradingRequestDto.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/dto/problem/grade/KeywordGradingRequestDto.kt
@@ -20,13 +20,13 @@ data class KeywordGradingRequestDto(
     companion object {
         fun createKeywordGradingRequestDto(problem: LongProblem, answer: String): KeywordGradingRequestDto {
             return KeywordGradingRequestDto(
-                problem.id!!,
+                problem.id,
                 answer,
                 problem.gradingStandards.filter {
                     it.type == GradingStandardType.KEYWORD
                 }.map {
                     GradingKeyword(
-                        it.id!!,
+                        it.id,
                         it.content,
                     )
                 },
diff --git a/src/main/kotlin/io/csbroker/apiserver/dto/problem/grade/LongProblemGradingRequestToAiServerDto.kt b/src/main/kotlin/io/csbroker/apiserver/dto/problem/grade/LongProblemGradingRequestToAiServerDto.kt
index 4e3e1399..dffb026f 100644
--- a/src/main/kotlin/io/csbroker/apiserver/dto/problem/grade/LongProblemGradingRequestToAiServerDto.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/dto/problem/grade/LongProblemGradingRequestToAiServerDto.kt
@@ -27,13 +27,13 @@ data class LongProblemGradingRequestToAiServerDto(
     companion object {
         fun createGradingRequestDto(problem: LongProblem, answer: String): LongProblemGradingRequestToAiServerDto {
             return LongProblemGradingRequestToAiServerDto(
-                problem.id!!,
+                problem.id,
                 answer,
                 problem.gradingStandards.filter {
                     it.type == GradingStandardType.KEYWORD
                 }.map {
                     GradingKeyword(
-                        it.id!!,
+                        it.id,
                         it.content,
                     )
                 },
@@ -41,7 +41,7 @@ data class LongProblemGradingRequestToAiServerDto(
                     it.type == GradingStandardType.CONTENT
                 }.map {
                     GradingContent(
-                        it.id!!,
+                        it.id,
                         it.content,
                     )
                 },
diff --git a/src/main/kotlin/io/csbroker/apiserver/dto/problem/longproblem/LongProblemGradingHistoryDto.kt b/src/main/kotlin/io/csbroker/apiserver/dto/problem/longproblem/LongProblemGradingHistoryDto.kt
index 116f85bc..23209554 100644
--- a/src/main/kotlin/io/csbroker/apiserver/dto/problem/longproblem/LongProblemGradingHistoryDto.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/dto/problem/longproblem/LongProblemGradingHistoryDto.kt
@@ -41,7 +41,7 @@ data class LongProblemGradingHistoryDto(
 
             return LongProblemGradingHistoryDto(
                 gradingHistoryId,
-                problem.id!!,
+                problem.id,
                 problem.title,
                 tags,
                 problem.description,
diff --git a/src/main/kotlin/io/csbroker/apiserver/dto/problem/multiplechoiceproblem/MultipleChoiceProblemGradingHistoryDto.kt b/src/main/kotlin/io/csbroker/apiserver/dto/problem/multiplechoiceproblem/MultipleChoiceProblemGradingHistoryDto.kt
index 5dd630d2..8f2866be 100644
--- a/src/main/kotlin/io/csbroker/apiserver/dto/problem/multiplechoiceproblem/MultipleChoiceProblemGradingHistoryDto.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/dto/problem/multiplechoiceproblem/MultipleChoiceProblemGradingHistoryDto.kt
@@ -30,14 +30,14 @@ data class MultipleChoiceProblemGradingHistoryDto(
 
             val choices = problem.choicesList.map {
                 ChoiceResponseDto(
-                    it.id!!,
+                    it.id,
                     it.content,
                 )
             }
 
             return MultipleChoiceProblemGradingHistoryDto(
                 gradingHistoryId,
-                problem.id!!,
+                problem.id,
                 problem.title,
                 commonDetail.tags,
                 problem.description,
diff --git a/src/main/kotlin/io/csbroker/apiserver/dto/problem/shortproblem/ShortProblemGradingHistoryDto.kt b/src/main/kotlin/io/csbroker/apiserver/dto/problem/shortproblem/ShortProblemGradingHistoryDto.kt
index b26a0b87..5d42a47b 100644
--- a/src/main/kotlin/io/csbroker/apiserver/dto/problem/shortproblem/ShortProblemGradingHistoryDto.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/dto/problem/shortproblem/ShortProblemGradingHistoryDto.kt
@@ -31,7 +31,7 @@ data class ShortProblemGradingHistoryDto(
 
             return ShortProblemGradingHistoryDto(
                 gradingHistoryId,
-                problem.id!!,
+                problem.id,
                 problem.title,
                 commonDetail.tags,
                 problem.description,
diff --git a/src/main/kotlin/io/csbroker/apiserver/dto/user/GradingStandardResponseDto.kt b/src/main/kotlin/io/csbroker/apiserver/dto/user/GradingStandardResponseDto.kt
index f1ff4e38..7b44a264 100644
--- a/src/main/kotlin/io/csbroker/apiserver/dto/user/GradingStandardResponseDto.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/dto/user/GradingStandardResponseDto.kt
@@ -12,7 +12,7 @@ data class GradingStandardResponseDto(
     companion object {
         fun fromGradingStandard(gradingStandard: GradingStandard): GradingStandardResponseDto {
             return GradingStandardResponseDto(
-                id = gradingStandard.id!!,
+                id = gradingStandard.id,
                 content = gradingStandard.content,
                 score = gradingStandard.score,
                 type = gradingStandard.type,
diff --git a/src/main/kotlin/io/csbroker/apiserver/dto/useranswer/UserAnswerResponseDto.kt b/src/main/kotlin/io/csbroker/apiserver/dto/useranswer/UserAnswerResponseDto.kt
index 1998ebf4..f591093b 100644
--- a/src/main/kotlin/io/csbroker/apiserver/dto/useranswer/UserAnswerResponseDto.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/dto/useranswer/UserAnswerResponseDto.kt
@@ -27,12 +27,12 @@ data class UserAnswerResponseDto(
                 .map { GradingStandardResponseDto.fromGradingStandard(it) }
 
             val selectedGradingStandards = userAnswer.userAnswerGradingStandards.map {
-                it.gradingStandard.id!!
+                it.gradingStandard.id
             }
 
             return UserAnswerResponseDto(
-                userAnswer.id!!,
-                userAnswer.problem.id!!,
+                userAnswer.id,
+                userAnswer.problem.id,
                 userAnswer.problem.title,
                 userAnswer.problem.description,
                 userAnswer.answer,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/Challenge.kt b/src/main/kotlin/io/csbroker/apiserver/model/Challenge.kt
index c165f56f..c233e13e 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/Challenge.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/Challenge.kt
@@ -16,7 +16,7 @@ class Challenge(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "challenge_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "problem_id")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/ChatResult.kt b/src/main/kotlin/io/csbroker/apiserver/model/ChatResult.kt
index 53fa888c..6fcee1b7 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/ChatResult.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/ChatResult.kt
@@ -13,7 +13,7 @@ class ChatResult(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "chat_result_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     // 연관관계로 엮을 수 있으나, 단순 데이터 저장용이기 때문에.. 굳이 엮지 않겠음.
     @Column(name = "email")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/Choice.kt b/src/main/kotlin/io/csbroker/apiserver/model/Choice.kt
index 08d620b6..e78f930f 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/Choice.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/Choice.kt
@@ -17,7 +17,7 @@ class Choice(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "choice_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @Column(name = "content")
     var content: String,
@@ -31,7 +31,7 @@ class Choice(
 ) {
     fun toChoiceResponseDto(): ChoiceResponseDto {
         return ChoiceResponseDto(
-            id!!,
+            id,
             content,
         )
     }
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/Comment.kt b/src/main/kotlin/io/csbroker/apiserver/model/Comment.kt
index a8b3bce9..e86330b8 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/Comment.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/Comment.kt
@@ -16,7 +16,7 @@ class Comment(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "comment_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "post_id")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/GradingHistory.kt b/src/main/kotlin/io/csbroker/apiserver/model/GradingHistory.kt
index 7daf1630..413ffe03 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/GradingHistory.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/GradingHistory.kt
@@ -17,7 +17,7 @@ class GradingHistory(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "grading_history_id")
-    val gradingHistoryId: Long? = null,
+    val gradingHistoryId: Long = 0,
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "problem_id")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/GradingResultAssessment.kt b/src/main/kotlin/io/csbroker/apiserver/model/GradingResultAssessment.kt
index dd5ba0e8..7a985624 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/GradingResultAssessment.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/GradingResultAssessment.kt
@@ -18,7 +18,7 @@ class GradingResultAssessment(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "grading_result_assessment_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @OneToOne(mappedBy = "gradingResultAssessment", fetch = FetchType.LAZY)
     val gradingHistory: GradingHistory,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/GradingStandard.kt b/src/main/kotlin/io/csbroker/apiserver/model/GradingStandard.kt
index 39418fc2..5ddc5962 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/GradingStandard.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/GradingStandard.kt
@@ -21,7 +21,7 @@ class GradingStandard(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "grading_standard_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @Column(name = "content")
     var content: String,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/LongProblem.kt b/src/main/kotlin/io/csbroker/apiserver/model/LongProblem.kt
index e28f54ed..3180d6fb 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/LongProblem.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/LongProblem.kt
@@ -49,7 +49,7 @@ class LongProblem(
 
     fun toLongProblemResponseDto(): LongProblemResponseDto {
         return LongProblemResponseDto(
-            id!!,
+            id,
             title,
             description,
             standardAnswers.map { it.content },
@@ -70,7 +70,7 @@ class LongProblem(
         }
 
         return LongProblemDataDto(
-            id!!,
+            id,
             title,
             creator.username,
             if (keywordScores.isEmpty()) null else keywordScores.average(),
@@ -94,7 +94,7 @@ class LongProblem(
         val isSolved = gradingHistory.any { it.user.email == email }
 
         return LongProblemDetailResponseDto(
-            id!!,
+            id,
             title,
             tags,
             description,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/Major.kt b/src/main/kotlin/io/csbroker/apiserver/model/Major.kt
index f114cfc3..b4c380e7 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/Major.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/Major.kt
@@ -13,7 +13,7 @@ class Major(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "major_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @Column(name = "major_name", columnDefinition = "VARCHAR(100)")
     val name: String,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/MultipleChoiceProblem.kt b/src/main/kotlin/io/csbroker/apiserver/model/MultipleChoiceProblem.kt
index 400db0aa..197acbb9 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/MultipleChoiceProblem.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/MultipleChoiceProblem.kt
@@ -49,7 +49,7 @@ class MultipleChoiceProblem(
 
     fun toMultipleChoiceProblemResponseDto(): MultipleChoiceProblemResponseDto {
         return MultipleChoiceProblemResponseDto(
-            id!!,
+            id,
             title,
             description,
             problemTags.map {
@@ -75,7 +75,7 @@ class MultipleChoiceProblem(
         }
 
         return MultipleChoiceProblemSearchResponseDto.MultipleChoiceProblemDataDto(
-            id!!,
+            id,
             title,
             creator.username,
             if (answerCnt == 0) null else correctAnswerCnt / answerCnt.toDouble(),
@@ -88,7 +88,7 @@ class MultipleChoiceProblem(
         val commonDetail = ProblemCommonDetailResponse.getCommonDetail(this)
 
         return MultipleChoiceProblemDetailResponseDto(
-            id!!,
+            id,
             title,
             commonDetail.tags,
             description,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/Notification.kt b/src/main/kotlin/io/csbroker/apiserver/model/Notification.kt
index f84b2060..29bf296b 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/Notification.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/Notification.kt
@@ -22,7 +22,7 @@ class Notification(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "notification_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @Column(name = "content", columnDefinition = "VARCHAR(100)")
     val content: String,
@@ -48,7 +48,7 @@ class Notification(
 
     fun toNotificationResponseDto(): NotificationResponseDto {
         return NotificationResponseDto(
-            id!!,
+            id,
             content,
             link,
             isRead,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/Post.kt b/src/main/kotlin/io/csbroker/apiserver/model/Post.kt
index fe046ab5..d2aabe92 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/Post.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/Post.kt
@@ -16,7 +16,7 @@ class Post(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "post_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "problem_id")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/PostLike.kt b/src/main/kotlin/io/csbroker/apiserver/model/PostLike.kt
index 273e77c1..370d48cd 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/PostLike.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/PostLike.kt
@@ -16,7 +16,7 @@ class PostLike(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "post_like_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "post_id")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/Problem.kt b/src/main/kotlin/io/csbroker/apiserver/model/Problem.kt
index 6c40202d..1a88fa69 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/Problem.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/Problem.kt
@@ -25,7 +25,7 @@ abstract class Problem(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "problem_id")
-    var id: Long? = null,
+    var id: Long = 0,
 
     @Column(name = "problem_title", columnDefinition = "VARCHAR(50)")
     var title: String,
@@ -72,7 +72,7 @@ abstract class Problem(
         }
 
         return ProblemResponseDto(
-            id!!,
+            id,
             title,
             tags,
             gradingHistoryStats?.avgScore,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/ProblemBookmark.kt b/src/main/kotlin/io/csbroker/apiserver/model/ProblemBookmark.kt
index 9aba31a0..1d442a75 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/ProblemBookmark.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/ProblemBookmark.kt
@@ -16,7 +16,7 @@ class ProblemBookmark(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "problem_bookmark_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "problem_id")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/ProblemLike.kt b/src/main/kotlin/io/csbroker/apiserver/model/ProblemLike.kt
index 80799b03..642b8b8b 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/ProblemLike.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/ProblemLike.kt
@@ -16,7 +16,7 @@ class ProblemLike(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "problem_like_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "problem_id")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/ProblemSet.kt b/src/main/kotlin/io/csbroker/apiserver/model/ProblemSet.kt
index ba5fcb05..a5d7d70e 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/ProblemSet.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/ProblemSet.kt
@@ -17,7 +17,7 @@ class ProblemSet(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "problem_set_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @Column(name = "problem_set_name")
     var name: String,
@@ -30,7 +30,7 @@ class ProblemSet(
 ) {
     fun toProblemSetResponseDto(): ProblemSetResponseDto {
         return ProblemSetResponseDto(
-            id!!,
+            id,
             problemSetMapping.size,
             name,
             description,
@@ -39,7 +39,7 @@ class ProblemSet(
 
     fun toProblemSetDetailResponseDto(): ProblemSetDetailResponseDto {
         return ProblemSetDetailResponseDto(
-            id!!,
+            id,
             problemSetMapping.map {
                 it.problem.toProblemResponseDto(GradingHistoryStats.toGradingHistoryStats(it.problem.gradingHistory))
             },
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/ProblemSetMapping.kt b/src/main/kotlin/io/csbroker/apiserver/model/ProblemSetMapping.kt
index 4a7c3b44..f1bc9653 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/ProblemSetMapping.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/ProblemSetMapping.kt
@@ -16,7 +16,7 @@ class ProblemSetMapping(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "problem_set_mapping_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "problem_id")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/ProblemTag.kt b/src/main/kotlin/io/csbroker/apiserver/model/ProblemTag.kt
index d3e1cd3a..ba2c86c0 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/ProblemTag.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/ProblemTag.kt
@@ -16,7 +16,7 @@ class ProblemTag(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "problem_tag_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "problem_id")
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/ShortProblem.kt b/src/main/kotlin/io/csbroker/apiserver/model/ShortProblem.kt
index 09e95df5..a49859d7 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/ShortProblem.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/ShortProblem.kt
@@ -37,7 +37,7 @@ class ShortProblem(
 
     fun toShortProblemResponseDto(): ShortProblemResponseDto {
         return ShortProblemResponseDto(
-            id!!,
+            id,
             title,
             description,
             problemTags.map { it.tag.name },
@@ -55,7 +55,7 @@ class ShortProblem(
         }
 
         return ShortProblemSearchResponseDto.ShortProblemDataDto(
-            id!!,
+            id,
             title,
             creator.username,
             if (answerCnt == 0) null else correctAnswerCnt / answerCnt.toDouble(),
@@ -68,7 +68,7 @@ class ShortProblem(
         val commonDetail = ProblemCommonDetailResponse.getCommonDetail(this)
 
         return ShortProblemDetailResponseDto(
-            id!!,
+            id,
             title,
             commonDetail.tags,
             description,
@@ -90,7 +90,7 @@ class ShortProblem(
         val commonDetail = ProblemCommonDetailResponse.getCommonDetail(this)
 
         return ShortProblemDetailResponseV2Dto(
-            id!!,
+            id,
             title,
             commonDetail.tags,
             description,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/StandardAnswer.kt b/src/main/kotlin/io/csbroker/apiserver/model/StandardAnswer.kt
index 136232e6..d7d6d0c4 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/StandardAnswer.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/StandardAnswer.kt
@@ -16,7 +16,7 @@ class StandardAnswer(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "standard_answer_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @Column(name = "content", columnDefinition = "LONGTEXT")
     var content: String,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/Tag.kt b/src/main/kotlin/io/csbroker/apiserver/model/Tag.kt
index a5ca99e1..ae32a839 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/Tag.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/Tag.kt
@@ -14,7 +14,7 @@ class Tag(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "tag_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @Column(name = "tag_name", columnDefinition = "VARCHAR(30)")
     val name: String,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/Tech.kt b/src/main/kotlin/io/csbroker/apiserver/model/Tech.kt
index 1257498d..66158881 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/Tech.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/Tech.kt
@@ -13,7 +13,7 @@ class Tech(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "tech_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @Column(name = "tech_name", columnDefinition = "VARCHAR(100)")
     val name: String,
diff --git a/src/main/kotlin/io/csbroker/apiserver/model/UserAnswer.kt b/src/main/kotlin/io/csbroker/apiserver/model/UserAnswer.kt
index d82792f0..78ff6df3 100644
--- a/src/main/kotlin/io/csbroker/apiserver/model/UserAnswer.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/model/UserAnswer.kt
@@ -20,7 +20,7 @@ class UserAnswer(
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "user_answer_id")
-    val id: Long? = null,
+    val id: Long = 0,
 
     @Column(name = "answer", columnDefinition = "VARCHAR(300)")
     val answer: String,
@@ -68,7 +68,7 @@ class UserAnswer(
 
     fun toUserAnswerDataDto(): UserAnswerDataDto {
         return UserAnswerDataDto(
-            id!!,
+            id,
             problem.title,
             assignedUser?.username,
             validatingUser?.username,
diff --git a/src/main/kotlin/io/csbroker/apiserver/repository/common/OAuth2AuthorizationRequestBasedOnCookieRepository.kt b/src/main/kotlin/io/csbroker/apiserver/repository/common/OAuth2AuthorizationRequestBasedOnCookieRepository.kt
index 04dc1139..60dab9d9 100644
--- a/src/main/kotlin/io/csbroker/apiserver/repository/common/OAuth2AuthorizationRequestBasedOnCookieRepository.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/repository/common/OAuth2AuthorizationRequestBasedOnCookieRepository.kt
@@ -37,7 +37,7 @@ class OAuth2AuthorizationRequestBasedOnCookieRepository : AuthorizationRequestRe
         addCookie(
             response,
             OAUTH2_AUTHORIZATION_REQUEST_COOKIE_NAME,
-            serialize(authorizationRequest),
+            authorizationRequest.serialize(),
             COOKIE_EXPIRE_SECONDS,
         )
 
diff --git a/src/main/kotlin/io/csbroker/apiserver/repository/problem/ProblemRepositoryCustomImpl.kt b/src/main/kotlin/io/csbroker/apiserver/repository/problem/ProblemRepositoryCustomImpl.kt
index 315a2dad..0e10cfd2 100644
--- a/src/main/kotlin/io/csbroker/apiserver/repository/problem/ProblemRepositoryCustomImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/repository/problem/ProblemRepositoryCustomImpl.kt
@@ -73,7 +73,7 @@ class ProblemRepositoryCustomImpl(
     override fun getProblemIdToStatMap(problems: List<Problem>): Map<Long, GradingHistoryStats> =
         getGradingHistoriesRelatedProblems(problems)
             .groupBy {
-                it.problem.id!!
+                it.problem.id
             }.map {
                 it.key to GradingHistoryStats.toGradingHistoryStats(it.value)
             }.toMap()
diff --git a/src/main/kotlin/io/csbroker/apiserver/repository/problem/UserAnswerRepositoryCustomImpl.kt b/src/main/kotlin/io/csbroker/apiserver/repository/problem/UserAnswerRepositoryCustomImpl.kt
index 2c2dc87e..3403d441 100644
--- a/src/main/kotlin/io/csbroker/apiserver/repository/problem/UserAnswerRepositoryCustomImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/repository/problem/UserAnswerRepositoryCustomImpl.kt
@@ -2,7 +2,7 @@ package io.csbroker.apiserver.repository.problem
 
 import com.querydsl.core.types.dsl.BooleanExpression
 import com.querydsl.jpa.impl.JPAQueryFactory
-import io.csbroker.apiserver.common.util.uuidAsByte
+import io.csbroker.apiserver.common.util.asByte
 import io.csbroker.apiserver.dto.useranswer.UserAnswerUpsertDto
 import io.csbroker.apiserver.model.QLongProblem.longProblem
 import io.csbroker.apiserver.model.QUser.user
@@ -37,9 +37,9 @@ class UserAnswerRepositoryCustomImpl(
                     ps.setString(1, userAnswer.answer)
                     ps.setBoolean(2, false)
                     ps.setBoolean(3, false)
-                    ps.setBytes(4, uuidAsByte(userAnswer.assignedUserId))
+                    ps.setBytes(4, userAnswer.assignedUserId?.asByte())
                     ps.setLong(5, userAnswer.problemId)
-                    ps.setBytes(6, uuidAsByte(userAnswer.validatingUserId))
+                    ps.setBytes(6, userAnswer.assignedUserId?.asByte())
                     ps.setTimestamp(7, Timestamp.valueOf(LocalDateTime.now()))
                     ps.setTimestamp(8, Timestamp.valueOf(LocalDateTime.now()))
                 }
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/auth/AuthServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/auth/AuthServiceImpl.kt
index 420e43ac..8902af45 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/auth/AuthServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/auth/AuthServiceImpl.kt
@@ -94,7 +94,7 @@ class AuthServiceImpl(
     }
 
     override fun refreshUserToken(request: HttpServletRequest): TokenDto {
-        val accessToken = getAccessToken(request)
+        val accessToken = request.getAccessToken()
             ?: throw UnAuthorizedException(ErrorCode.ACCESS_TOKEN_NOT_EXIST, "Access Token이 존재하지 않습니다.")
 
         val convertAccessToken = authTokenProvider.convertAuthToken(accessToken)
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/auth/CustomOAuth2UserService.kt b/src/main/kotlin/io/csbroker/apiserver/service/auth/CustomOAuth2UserService.kt
index c08a4576..b1312431 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/auth/CustomOAuth2UserService.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/auth/CustomOAuth2UserService.kt
@@ -64,19 +64,19 @@ class CustomOAuth2UserService(
         attributes: MutableMap<String, Any>,
     ): User {
         val userInfo = OAuth2UserInfoFactory.getOauth2UserInfo(providerType, attributes)
-        val savedUser = userRepository.findByEmail(userInfo.getEmail())
-            ?: userRepository.findUserByProviderId(userInfo.getId())
+        val savedUser = userRepository.findByEmail(userInfo.email)
+            ?: userRepository.findUserByProviderId(userInfo.id)
 
         return savedUser ?: createUser(userInfo, providerType)
     }
 
     private fun createUser(userInfo: OAuth2UserInfo, providerType: ProviderType): User {
         val user = User(
-            email = userInfo.getEmail(),
-            username = userInfo.getName(),
+            email = userInfo.email,
+            username = userInfo.name,
             providerType = providerType,
-            profileImageUrl = userInfo.getImageUrl(),
-            providerId = userInfo.getId(),
+            profileImageUrl = userInfo.imageUrl,
+            providerId = userInfo.id,
         )
 
         return userRepository.saveAndFlush(user)
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/common/NotificationServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/common/NotificationServiceImpl.kt
index 31c2fcda..b42c605a 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/common/NotificationServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/common/NotificationServiceImpl.kt
@@ -23,7 +23,7 @@ class NotificationServiceImpl(
 
         val notification = Notification(notificationRequestDto, findUser)
 
-        return notificationRepository.save(notification).id!!
+        return notificationRepository.save(notification).id
     }
 
     @Transactional
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/post/CommentServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/post/CommentServiceImpl.kt
index d9d7751a..e9f4dcb2 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/post/CommentServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/post/CommentServiceImpl.kt
@@ -25,7 +25,7 @@ class CommentServiceImpl(
             "${postId}번 답변은 존재하지 않는 답변입니다",
         )
         val comment = commentRepository.save(Comment(content = content, post = post, user = user))
-        return comment.id!!
+        return comment.id
     }
 
     @Transactional
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/post/PostServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/post/PostServiceImpl.kt
index d8439328..a23111f7 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/post/PostServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/post/PostServiceImpl.kt
@@ -53,7 +53,7 @@ class PostServiceImpl(
             "${problemId}번 문제는 존재하지 않는 문제입니다",
         )
         val post = postRepository.save(Post(content = content, problem = problem, user = user))
-        return post.id!!
+        return post.id
     }
 
     @Transactional
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/problem/CommonProblemServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/problem/CommonProblemServiceImpl.kt
index 2474f67f..09e4c746 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/problem/CommonProblemServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/problem/CommonProblemServiceImpl.kt
@@ -87,7 +87,7 @@ class CommonProblemServiceImpl(
 
         gradingHistory.gradingResultAssessment = gradingResultAssessment
 
-        return gradingResultAssessment.id!!
+        return gradingResultAssessment.id
     }
 
     @Transactional
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/problem/LongProblemServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/problem/LongProblemServiceImpl.kt
index 2a48e27c..4b78d771 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/problem/LongProblemServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/problem/LongProblemServiceImpl.kt
@@ -65,7 +65,7 @@ class LongProblemServiceImpl(
 
         // get keywords
         val correctKeywordListDto = gradeResultDto.correctKeywordIds.map {
-            val keyword = findProblem.gradingStandards.find { gs -> gs.id!! == it }
+            val keyword = findProblem.gradingStandards.find { gs -> gs.id == it }
                 ?: throw EntityNotFoundException("${it}번 채점 기준을 찾을 수 없습니다.")
             if (keyword.type != GradingStandardType.KEYWORD) {
                 throw InternalServiceException(
@@ -75,7 +75,7 @@ class LongProblemServiceImpl(
             }
             userGradedScore += keyword.score
             KeywordDto(
-                keyword.id!!,
+                keyword.id,
                 keyword.content,
                 true,
                 gradeResultDto.predictKeywordPositions[it]
@@ -86,7 +86,7 @@ class LongProblemServiceImpl(
         val notCorrectKeywordListDto = findProblem.gradingStandards.filter {
             it.type == GradingStandardType.KEYWORD && it.id !in gradeResultDto.correctKeywordIds
         }.map {
-            KeywordDto(it.id!!, it.content)
+            KeywordDto(it.id, it.content)
         }.toList()
 
         // get score from content standards
@@ -100,13 +100,13 @@ class LongProblemServiceImpl(
                 )
             }
             userGradedScore += it.score
-            ContentDto(it.id!!, it.content, true)
+            ContentDto(it.id, it.content, true)
         }
 
         val notCorrectContentListDto = findProblem.gradingStandards.filter {
             it.type == GradingStandardType.CONTENT && it.id !in gradeResultDto.correctContentIds
         }.map {
-            ContentDto(it.id!!, it.content)
+            ContentDto(it.id, it.content)
         }.toList()
 
         if (correctContentListDto.size != gradeResultDto.correctContentIds.size) {
@@ -130,7 +130,7 @@ class LongProblemServiceImpl(
 
         // create dto
         return LongProblemGradingHistoryDto.createDto(
-            gradingHistoryId = gradingHistory.gradingHistoryId!!,
+            gradingHistoryId = gradingHistory.gradingHistoryId,
             problem = findProblem,
             userAnswer = answer,
             score = userGradedScore,
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/problem/MultipleProblemServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/problem/MultipleProblemServiceImpl.kt
index cda304a2..3692ee23 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/problem/MultipleProblemServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/problem/MultipleProblemServiceImpl.kt
@@ -40,7 +40,7 @@ class MultipleProblemServiceImpl(
         val correctAnswer = findProblem.choicesList.filter {
             it.isAnswer
         }.map {
-            it.id!!
+            it.id
         }
 
         val isAnswer = correctAnswer.size == answerIds.size && correctAnswer.containsAll(answerIds)
@@ -58,7 +58,7 @@ class MultipleProblemServiceImpl(
 
         // create dto
         return MultipleChoiceProblemGradingHistoryDto.createDto(
-            gradingHistoryId = gradingHistory.gradingHistoryId!!,
+            gradingHistoryId = gradingHistory.gradingHistoryId,
             problem = findProblem,
             userAnswerIds = answerIds,
             score = score,
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/problem/ProblemSetServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/problem/ProblemSetServiceImpl.kt
index a2093ce3..764f96b9 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/problem/ProblemSetServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/problem/ProblemSetServiceImpl.kt
@@ -31,16 +31,16 @@ class ProblemSetServiceImpl(
     override fun createProblemSet(problemSetUpsertRequestDto: ProblemSetUpsertRequestDto): Long {
         val problemSet = problemSetRepository.save(problemSetUpsertRequestDto.toProblemSet())
         createProblemSetMapping(problemSetUpsertRequestDto.problemIds, problemSet)
-        return problemSet.id!!
+        return problemSet.id
     }
 
     @Transactional
     override fun updateProblemSet(id: Long, problemSetUpsertRequestDto: ProblemSetUpsertRequestDto): Long {
         val problemSet = findById(id)
         problemSet.updateContents(problemSetUpsertRequestDto.name, problemSetUpsertRequestDto.description)
-        problemSetMappingRepository.deleteAllByProblemSetId(problemSet.id!!)
+        problemSetMappingRepository.deleteAllByProblemSetId(problemSet.id)
         createProblemSetMapping(problemSetUpsertRequestDto.problemIds, problemSet)
-        return problemSet.id!!
+        return problemSet.id
     }
 
     private fun createProblemSetMapping(
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/problem/ShortProblemServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/problem/ShortProblemServiceImpl.kt
index 01214b7c..6d8cd66c 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/problem/ShortProblemServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/problem/ShortProblemServiceImpl.kt
@@ -51,7 +51,7 @@ class ShortProblemServiceImpl(
 
         // create dto
         return ShortProblemGradingHistoryDto.createDto(
-            gradingHistoryId = gradingHistory.gradingHistoryId!!,
+            gradingHistoryId = gradingHistory.gradingHistoryId,
             problem = findProblem,
             userAnswer = answer,
             score = score,
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/problem/UserAnswerServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/problem/UserAnswerServiceImpl.kt
index c584d5ff..e21cccd3 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/problem/UserAnswerServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/problem/UserAnswerServiceImpl.kt
@@ -56,7 +56,7 @@ class UserAnswerServiceImpl(
             validatingUser = validatingUser,
         )
 
-        return userAnswerRepository.save(createUserAnswer).id!!
+        return userAnswerRepository.save(createUserAnswer).id
     }
 
     override fun findUserAnswerById(id: Long): UserAnswerResponseDto {
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminLongProblemServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminLongProblemServiceImpl.kt
index b9247711..205b7e8b 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminLongProblemServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminLongProblemServiceImpl.kt
@@ -63,7 +63,7 @@ class AdminLongProblemServiceImpl(
             },
         )
 
-        return savedProblem.id!!
+        return savedProblem.id
     }
 
     @Transactional
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminMultipleProblemServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminMultipleProblemServiceImpl.kt
index 2c081697..f5864464 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminMultipleProblemServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminMultipleProblemServiceImpl.kt
@@ -58,7 +58,7 @@ class AdminMultipleProblemServiceImpl(
 
         multipleChoiceProblem.addChoices(choiceDataList)
 
-        return problemRepository.save(multipleChoiceProblem).id!!
+        return problemRepository.save(multipleChoiceProblem).id
     }
 
     @Transactional
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminShortProblemServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminShortProblemServiceImpl.kt
index 67078fbf..1a7811eb 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminShortProblemServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/problem/admin/AdminShortProblemServiceImpl.kt
@@ -46,7 +46,7 @@ class AdminShortProblemServiceImpl(
 
         tagUpserter.setTags(shortProblem, createRequestDto.tags)
 
-        return problemRepository.save(shortProblem).id!!
+        return problemRepository.save(shortProblem).id
     }
 
     @Transactional
diff --git a/src/main/kotlin/io/csbroker/apiserver/service/user/UserServiceImpl.kt b/src/main/kotlin/io/csbroker/apiserver/service/user/UserServiceImpl.kt
index 4b1ec13f..5ae0f178 100644
--- a/src/main/kotlin/io/csbroker/apiserver/service/user/UserServiceImpl.kt
+++ b/src/main/kotlin/io/csbroker/apiserver/service/user/UserServiceImpl.kt
@@ -77,7 +77,7 @@ class UserServiceImpl(
         val gradingHistories = gradingHistoryRepository.findGradingHistoriesByUserId(findUser.id!!)
 
         val resultMap = gradingHistories.groupBy {
-            it.problem.id!!
+            it.problem.id
         }.mapValues { v ->
             v.value.maxByOrNull { it.score } ?: v.value.first()
         }
@@ -148,7 +148,7 @@ class UserServiceImpl(
     override fun calculateRank() {
         val userScoreMap = userRepository.findAll().associate {
             val scoreMap = it.gradingHistories.groupBy { gradingHistory ->
-                gradingHistory.problem.id!!
+                gradingHistory.problem.id
             }.mapValues { map ->
                 map.value.maxOfOrNull { gradingHistory -> gradingHistory.score } ?: 0.0
             }
diff --git a/src/test/kotlin/io/csbroker/apiserver/controller/IntegrationTest.kt b/src/test/kotlin/io/csbroker/apiserver/controller/IntegrationTest.kt
index b61f0d2d..08cfad10 100644
--- a/src/test/kotlin/io/csbroker/apiserver/controller/IntegrationTest.kt
+++ b/src/test/kotlin/io/csbroker/apiserver/controller/IntegrationTest.kt
@@ -65,6 +65,7 @@ class IntegrationTest {
         entity
     }
 
+    @Suppress("UNCHECKED_CAST")
     fun <T> findOne(
         @Language("JPAQL") qlString: String,
         params: Map<String, Any> = emptyMap(),
@@ -74,6 +75,7 @@ class IntegrationTest {
         query.singleResult as T
     }
 
+    @Suppress("UNCHECKED_CAST")
     fun <T> findAll(
         @Language("JPAQL") qlString: String,
         params: Map<String, Any> = emptyMap(),
diff --git a/src/test/kotlin/io/csbroker/apiserver/controller/v1/admin/problem/AdminProblemControllerIntegrationTest.kt b/src/test/kotlin/io/csbroker/apiserver/controller/v1/admin/problem/AdminProblemControllerIntegrationTest.kt
index f3e859b3..ac84403b 100644
--- a/src/test/kotlin/io/csbroker/apiserver/controller/v1/admin/problem/AdminProblemControllerIntegrationTest.kt
+++ b/src/test/kotlin/io/csbroker/apiserver/controller/v1/admin/problem/AdminProblemControllerIntegrationTest.kt
@@ -41,7 +41,7 @@ class AdminProblemControllerIntegrationTest : IntegrationTest() {
         // then
         response.andExpect(status().isOk)
             .andExpect {
-                val problems = findAll<Problem>("SELECT p FROM Problem p where p.id = :id", mapOf("id" to problem.id!!))
+                val problems = findAll<Problem>("SELECT p FROM Problem p where p.id = :id", mapOf("id" to problem.id))
                 problems.size shouldBe 0
             }
     }
@@ -78,7 +78,7 @@ class AdminProblemControllerIntegrationTest : IntegrationTest() {
             url = "/api/admin/problems",
             isAdmin = true,
             body = ProblemDeleteRequestDto(
-                ids = listOf(problem1.id!!, problem2.id!!),
+                ids = listOf(problem1.id, problem2.id),
             ),
         )
 
@@ -87,7 +87,7 @@ class AdminProblemControllerIntegrationTest : IntegrationTest() {
             .andExpect {
                 val problems = findAll<Problem>(
                     "SELECT p FROM Problem p where p.id in :ids",
-                    mapOf("ids" to listOf(problem1.id!!, problem2.id!!)),
+                    mapOf("ids" to listOf(problem1.id, problem2.id)),
                 )
                 problems.size shouldBe 0
             }
diff --git a/src/test/kotlin/io/csbroker/apiserver/controller/v1/admin/problem/AdminProblemSetControllerIntegrationTest.kt b/src/test/kotlin/io/csbroker/apiserver/controller/v1/admin/problem/AdminProblemSetControllerIntegrationTest.kt
index c1eec980..d05280ed 100644
--- a/src/test/kotlin/io/csbroker/apiserver/controller/v1/admin/problem/AdminProblemSetControllerIntegrationTest.kt
+++ b/src/test/kotlin/io/csbroker/apiserver/controller/v1/admin/problem/AdminProblemSetControllerIntegrationTest.kt
@@ -49,7 +49,7 @@ class AdminProblemSetControllerIntegrationTest : IntegrationTest() {
         val response = request(
             method = HttpMethod.POST,
             url = "/api/admin/problem-sets",
-            body = ProblemSetUpsertRequestDto(listOf(longProblem.id!!, shortProblem.id!!), "name", "description"),
+            body = ProblemSetUpsertRequestDto(listOf(longProblem.id, shortProblem.id), "name", "description"),
             isAdmin = true,
         )
 
@@ -125,7 +125,7 @@ class AdminProblemSetControllerIntegrationTest : IntegrationTest() {
         val response = request(
             method = HttpMethod.PUT,
             url = "/api/admin/problem-sets/${problemSet.id}",
-            body = ProblemSetUpsertRequestDto(listOf(multipleChoiceProblem.id!!), "name", "description"),
+            body = ProblemSetUpsertRequestDto(listOf(multipleChoiceProblem.id), "name", "description"),
             isAdmin = true,
         )
 
diff --git a/src/test/kotlin/io/csbroker/apiserver/service/problem/MultipleProblemServiceTest.kt b/src/test/kotlin/io/csbroker/apiserver/service/problem/MultipleProblemServiceTest.kt
index 3cd74802..d56ac4ae 100644
--- a/src/test/kotlin/io/csbroker/apiserver/service/problem/MultipleProblemServiceTest.kt
+++ b/src/test/kotlin/io/csbroker/apiserver/service/problem/MultipleProblemServiceTest.kt
@@ -101,8 +101,8 @@ class MultipleProblemServiceTest {
         // given
         val email = user.email
         val problem = createProblem()
-        val problemId = problem.id!!
-        val answerId = problem.choicesList.find { it.isAnswer }?.id!!
+        val problemId = problem.id
+        val answerId = problem.choicesList.first { it.isAnswer }.id
         val answerIds = listOf(answerId)
         val gradingRequest = MultipleProblemGradingRequestDto(email, problemId, answerIds)
         val gradingHistory = GradingHistory(
@@ -126,8 +126,8 @@ class MultipleProblemServiceTest {
         // given
         val email = user.email
         val problem = createProblem()
-        val problemId = problem.id!!
-        val answerId = problem.choicesList.find { !it.isAnswer }?.id!!
+        val problemId = problem.id
+        val answerId = problem.choicesList.first { !it.isAnswer }.id
         val answerIds = listOf(answerId)
         val gradingRequest = MultipleProblemGradingRequestDto(email, problemId, answerIds)
         val gradingHistory = GradingHistory(
diff --git a/src/test/kotlin/io/csbroker/apiserver/service/problem/ShortProblemServiceTest.kt b/src/test/kotlin/io/csbroker/apiserver/service/problem/ShortProblemServiceTest.kt
index c1bbfde9..88cb9d68 100644
--- a/src/test/kotlin/io/csbroker/apiserver/service/problem/ShortProblemServiceTest.kt
+++ b/src/test/kotlin/io/csbroker/apiserver/service/problem/ShortProblemServiceTest.kt
@@ -96,7 +96,7 @@ class ShortProblemServiceTest {
         val problem = createProblem()
         val gradingRequest = ShortProblemGradingRequestDto(
             user.email,
-            problem.id!!,
+            problem.id,
             problem.answer,
         )
         val gradingHistory = GradingHistory(
@@ -107,7 +107,7 @@ class ShortProblemServiceTest {
             score = problem.score,
         )
         every { userRepository.findByEmail(user.email) } returns user
-        every { shortProblemRepository.findByIdOrNull(problem.id!!) } returns problem
+        every { shortProblemRepository.findByIdOrNull(problem.id) } returns problem
         every { gradingHistoryRepository.save(any()) } returns gradingHistory
 
         // when
@@ -116,7 +116,7 @@ class ShortProblemServiceTest {
         // then
         assertEquals(problem.score, result.score)
         verify { userRepository.findByEmail(user.email) }
-        verify { shortProblemRepository.findByIdOrNull(problem.id!!) }
+        verify { shortProblemRepository.findByIdOrNull(problem.id) }
         verify { gradingHistoryRepository.save(any()) }
     }
 
@@ -127,7 +127,7 @@ class ShortProblemServiceTest {
         val wrongAnswer = "wrongAnswer"
         val gradingRequest = ShortProblemGradingRequestDto(
             user.email,
-            problem.id!!,
+            problem.id,
             wrongAnswer,
         )
         val gradingHistory = GradingHistory(
@@ -138,7 +138,7 @@ class ShortProblemServiceTest {
             score = 10.0,
         )
         every { userRepository.findByEmail(user.email) } returns user
-        every { shortProblemRepository.findByIdOrNull(problem.id!!) } returns problem
+        every { shortProblemRepository.findByIdOrNull(problem.id) } returns problem
         every { gradingHistoryRepository.save(any()) } returns gradingHistory
         // when
         val result = service.gradingProblem(gradingRequest)
@@ -146,7 +146,7 @@ class ShortProblemServiceTest {
         // then
         assertEquals(0.0, result.score)
         verify { userRepository.findByEmail(user.email) }
-        verify { shortProblemRepository.findByIdOrNull(problem.id!!) }
+        verify { shortProblemRepository.findByIdOrNull(problem.id) }
         verify { gradingHistoryRepository.save(any()) }
     }
 
diff --git a/src/test/kotlin/io/csbroker/apiserver/service/problem/UserAnswerServiceTest.kt b/src/test/kotlin/io/csbroker/apiserver/service/problem/UserAnswerServiceTest.kt
index 969deeb6..04e0d8b7 100644
--- a/src/test/kotlin/io/csbroker/apiserver/service/problem/UserAnswerServiceTest.kt
+++ b/src/test/kotlin/io/csbroker/apiserver/service/problem/UserAnswerServiceTest.kt
@@ -86,7 +86,7 @@ class UserAnswerServiceTest {
     @Test
     fun `createUserAnswer - 존재하지 않는 유저가 담당자로 등록되면 예외가 발생합니다 `() {
         // given
-        val problemId = problem.id!!
+        val problemId = problem.id
         val notExistUserId = UUID.randomUUID()
         val requestDto = UserAnswerUpsertDto(
             assignedUserId = notExistUserId,
@@ -106,7 +106,7 @@ class UserAnswerServiceTest {
     @Test
     fun `createUserAnswer - 존재하지 않는 유저가 검수자로 등록되면 예외가 발생합니다 `() {
         // given
-        val problemId = problem.id!!
+        val problemId = problem.id
         val notExistUserId = UUID.randomUUID()
         val requestDto = UserAnswerUpsertDto(
             assignedUserId = assignee.id!!,
@@ -128,7 +128,7 @@ class UserAnswerServiceTest {
     @Test
     fun `createUserAnswer - success`() {
         // given
-        val problemId = problem.id!!
+        val problemId = problem.id
         val answer = "answer"
         val requestDto = UserAnswerUpsertDto(
             assignedUserId = assignee.id!!,
@@ -187,7 +187,7 @@ class UserAnswerServiceTest {
     fun `labelUserAnswer - 담당자가 존재하지 않는 답안은 라벨링을 할 수 없습니다`() {
         // given
         val anotherUserEmail = "another@email.com"
-        val userAnswerId = userAnswer.id!!
+        val userAnswerId = userAnswer.id
         userAnswer.assignedUser = null
         val selectedGradingStandardIds = listOf(1L, 2L, 3L)
         every { userAnswerRepository.findByIdOrNull(userAnswerId) } returns userAnswer
@@ -203,7 +203,7 @@ class UserAnswerServiceTest {
     fun `labelUserAnswer - 담당자가 아닌 유저가 라벨링을 진행할 시 예외가 발생합니다`() {
         // given
         val anotherUserEmail = "another@email.com"
-        val userAnswerId = userAnswer.id!!
+        val userAnswerId = userAnswer.id
         val selectedGradingStandardIds = listOf(1L, 2L, 3L)
         every { userAnswerRepository.findByIdOrNull(userAnswerId) } returns userAnswer
 
@@ -217,7 +217,7 @@ class UserAnswerServiceTest {
     @Test
     fun `labelUserAnswer - 존재하지 않는 채점기준이 존재하면 예외가 발생합니다`() {
         // given
-        val userAnswerId = userAnswer.id!!
+        val userAnswerId = userAnswer.id
         val selectedGradingStandardIds = listOf(1L, 2L, 3L)
 
         every { userAnswerRepository.findByIdOrNull(userAnswerId) } returns userAnswer
@@ -234,7 +234,7 @@ class UserAnswerServiceTest {
     @Test
     fun `labelUserAnswer - 라벨링이 완료되면 완료 표식을 남깁니다`() {
         // given
-        val userAnswerId = userAnswer.id!!
+        val userAnswerId = userAnswer.id
         val selectedGradingStandardIds = listOf(1L, 2L, 3L)
 
         every { userAnswerRepository.findByIdOrNull(userAnswerId) } returns userAnswer