feat : 수강신청 도메인 구성

src/main/java/com/study/course_registration/config/ClockConfig.java

@@ -0,0 +1,14 @@
+package com.study.course_registration.config;
+
+import java.time.Clock;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration
+public class ClockConfig {
+    @Bean
+    Clock clock() {
+        return Clock.systemUTC();
+    }
+}

src/main/java/com/study/course_registration/config/OpenApiConfig.java

@@ -0,0 +1,18 @@
+package com.study.course_registration.config;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+import io.swagger.v3.oas.models.OpenAPI;
+import io.swagger.v3.oas.models.info.Info;
+
+@Configuration
+public class OpenApiConfig {
+    @Bean
+    OpenAPI courseRegistrationOpenApi() {
+        return new OpenAPI().info(new Info()
+                .title("Course Registration API")
+                .version("v1")
+                .description("수강신청, 취소, 신청 목록 및 개설 강의 조회 API"));
+    }
+}

src/main/java/com/study/course_registration/controller/CourseRegistrationController.java

@@ -1,38 +1,70 @@
 package com.study.course_registration.controller;
 
-import io.swagger.v3.oas.annotations.Operation;
-import io.swagger.v3.oas.annotations.responses.ApiResponse;
-import io.swagger.v3.oas.annotations.responses.ApiResponses;
-
+import org.springframework.http.HttpStatus;
 import org.springframework.validation.annotation.Validated;
+import org.springframework.web.bind.annotation.DeleteMapping;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PathVariable;
 import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.ResponseStatus;
 import org.springframework.web.bind.annotation.RestController;
 
+import com.study.course_registration.dto.registration.CourseRegistrationRequest;
+import com.study.course_registration.dto.registration.CourseRegistrationResponse;
+import com.study.course_registration.dto.registration.RegistrationListResponse;
 import com.study.course_registration.service.CourseRegistrationService;
 
-import com.study.course_registration.dto.RequestDto;
-import com.study.course_registration.dto.ResponseDto;
+import io.swagger.v3.oas.annotations.Operation;
+import io.swagger.v3.oas.annotations.media.Content;
+import io.swagger.v3.oas.annotations.responses.ApiResponse;
+import io.swagger.v3.oas.annotations.responses.ApiResponses;
+import io.swagger.v3.oas.annotations.tags.Tag;
+import jakarta.validation.Valid;
 
 @RestController
-@RequestMapping ("/api/v1")
+@Validated
+@RequestMapping("/api/v1/users/{userId}/registrations")
+@Tag(name = "Registrations", description = "수강신청/조회/취소 API")
 public class CourseRegistrationController {
-
     private final CourseRegistrationService courseRegistrationService;
-    
+
     public CourseRegistrationController(CourseRegistrationService courseRegistrationService) {
         this.courseRegistrationService = courseRegistrationService;
     }
 
-    @PostMapping("/course-register")
-    @Operation(summary = "Register a course",
-            description = "Registers a new course with the provided details.")
+    @PostMapping
+    @ResponseStatus(HttpStatus.CREATED)
+    @Operation(summary = "수강신청")
     @ApiResponses({
-        @ApiResponse (responseCode = "200", description = "Course registered successfully"),
-        @ApiResponse (responseCode = "400", description = "Invalid request data")
+            @ApiResponse(responseCode = "201", description = "신청 성공"),
+            @ApiResponse(responseCode = "403", description = "학년 또는 역할 제한", content = @Content),
+            @ApiResponse(responseCode = "404", description = "사용자 또는 강의 없음", content = @Content),
+            @ApiResponse(responseCode = "409", description = "기간/중복/시간표/학점/정원 충돌", content = @Content),
+            @ApiResponse(responseCode = "503", description = "락 대기 타임아웃", content = @Content)
     })
-    public ResponseDto registerCourse(@Validated RequestDto requestDto) {
-        return courseRegistrationService.registerCourse(requestDto);
+    public CourseRegistrationResponse register(
+            @PathVariable String userId,
+            @Valid @RequestBody CourseRegistrationRequest request) {
+        return courseRegistrationService.register(userId, request);
     }
 
+    @GetMapping
+    @Operation(summary = "내 수강신청 목록 조회")
+    public RegistrationListResponse getRegistrations(
+            @PathVariable String userId,
+            @RequestParam(required = false) String semesterId) {
+        return courseRegistrationService.getRegistrations(userId, semesterId);
+    }
+
+    @DeleteMapping("/{registrationId}")
+    @ResponseStatus(HttpStatus.NO_CONTENT)
+    @Operation(summary = "수강신청 취소")
+    public void cancel(
+            @PathVariable String userId,
+            @PathVariable String registrationId) {
+        courseRegistrationService.cancel(userId, registrationId);
+    }
 }

src/main/java/com/study/course_registration/controller/LessonController.java

@@ -0,0 +1,45 @@
+package com.study.course_registration.controller;
+
+import org.springframework.validation.annotation.Validated;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.RestController;
+
+import com.study.course_registration.dto.lesson.LessonDetailResponse;
+import com.study.course_registration.dto.lesson.LessonListResponse;
+import com.study.course_registration.service.LessonQueryService;
+
+import io.swagger.v3.oas.annotations.Operation;
+import io.swagger.v3.oas.annotations.tags.Tag;
+import jakarta.validation.constraints.Max;
+import jakarta.validation.constraints.Min;
+
+@RestController
+@Validated
+@RequestMapping("/api/v1/lessons")
+@Tag(name = "Lessons", description = "개설 강의 조회 API")
+public class LessonController {
+    private final LessonQueryService lessonQueryService;
+
+    public LessonController(LessonQueryService lessonQueryService) {
+        this.lessonQueryService = lessonQueryService;
+    }
+
+    @GetMapping
+    @Operation(summary = "개설 강의 목록 조회")
+    public LessonListResponse getLessons(
+            @RequestParam(required = false) String semesterId,
+            @RequestParam(required = false) String subjectId,
+            @RequestParam(defaultValue = "0") @Min(0) int page,
+            @RequestParam(defaultValue = "20") @Min(1) @Max(100) int size) {
+        return lessonQueryService.getLessons(semesterId, subjectId, page, size);
+    }
+
+    @GetMapping("/{lessonId}")
+    @Operation(summary = "개설 강의 상세 조회")
+    public LessonDetailResponse getLesson(@PathVariable String lessonId) {
+        return lessonQueryService.getLesson(lessonId);
+    }
+}

src/main/java/com/study/course_registration/dto/RequestDto.java

@@ -1,7 +0,0 @@
-package com.study.course_registration.dto;
-
-public record RequestDto(
-    
-) {
-    
-}

src/main/java/com/study/course_registration/dto/ResponseDto.java

@@ -1,7 +0,0 @@
-package com.study.course_registration.dto;
-
-public record ResponseDto(
-    
-) {
-    
-}

src/main/java/com/study/course_registration/dto/ScheduleResponse.java

@@ -0,0 +1,15 @@
+package com.study.course_registration.dto;
+
+import java.time.DayOfWeek;
+import java.time.LocalTime;
+
+import com.study.course_registration.entity.LessonSchedule;
+
+public record ScheduleResponse(
+        DayOfWeek dayOfWeek,
+        LocalTime startTime,
+        LocalTime endTime) {
+    public static ScheduleResponse from(LessonSchedule schedule) {
+        return new ScheduleResponse(schedule.getDayOfWeek(), schedule.getStartTime(), schedule.getEndTime());
+    }
+}

src/main/java/com/study/course_registration/dto/lesson/LessonDetailResponse.java

@@ -0,0 +1,25 @@
+package com.study.course_registration.dto.lesson;
+
+import java.time.Instant;
+import java.util.List;
+
+import com.study.course_registration.dto.ScheduleResponse;
+import com.study.course_registration.enums.UserRole;
+
+public record LessonDetailResponse(
+        String lessonId,
+        String lessonName,
+        String subjectCode,
+        String subjectName,
+        String subjectDescription,
+        Integer credit,
+        String professorName,
+        String semesterName,
+        Integer capacity,
+        Long enrolledCount,
+        Long minGrade,
+        UserRole allowedRole,
+        Instant registrationStartAt,
+        Instant registrationEndAt,
+        List<ScheduleResponse> schedules) {
+}

src/main/java/com/study/course_registration/dto/lesson/LessonItemResponse.java

@@ -0,0 +1,20 @@
+package com.study.course_registration.dto.lesson;
+
+import java.util.List;
+
+import com.study.course_registration.dto.ScheduleResponse;
+import com.study.course_registration.enums.UserRole;
+
+public record LessonItemResponse(
+        String lessonId,
+        String lessonName,
+        String subjectCode,
+        String subjectName,
+        Integer credit,
+        String professorName,
+        Integer capacity,
+        Long enrolledCount,
+        Long minGrade,
+        UserRole allowedRole,
+        List<ScheduleResponse> schedules) {
+}

src/main/java/com/study/course_registration/dto/lesson/LessonListResponse.java

@@ -0,0 +1,10 @@
+package com.study.course_registration.dto.lesson;
+
+import java.util.List;
+
+public record LessonListResponse(
+        Integer page,
+        Integer size,
+        Long totalElements,
+        List<LessonItemResponse> lessons) {
+}

src/main/java/com/study/course_registration/dto/registration/CourseRegistrationRequest.java

@@ -0,0 +1,7 @@
+package com.study.course_registration.dto.registration;
+
+import jakarta.validation.constraints.NotBlank;
+
+public record CourseRegistrationRequest(
+        @NotBlank(message = "lessonId는 필수입니다.") String lessonId) {
+}

src/main/java/com/study/course_registration/dto/registration/CourseRegistrationResponse.java

@@ -0,0 +1,14 @@
+package com.study.course_registration.dto.registration;
+
+import java.time.Instant;
+
+public record CourseRegistrationResponse(
+        String registrationId,
+        String userId,
+        String lessonId,
+        String lessonName,
+        String subjectCode,
+        Integer credit,
+        Instant registeredAt,
+        Integer totalCredits) {
+}

src/main/java/com/study/course_registration/dto/registration/RegistrationItemResponse.java

@@ -0,0 +1,18 @@
+package com.study.course_registration.dto.registration;
+
+import java.time.Instant;
+import java.util.List;
+
+import com.study.course_registration.dto.ScheduleResponse;
+
+public record RegistrationItemResponse(
+        String registrationId,
+        String lessonId,
+        String lessonName,
+        String subjectCode,
+        String subjectName,
+        Integer credit,
+        String professorName,
+        List<ScheduleResponse> schedules,
+        Instant registeredAt) {
+}

src/main/java/com/study/course_registration/dto/registration/RegistrationListResponse.java

@@ -0,0 +1,12 @@
+package com.study.course_registration.dto.registration;
+
+import java.util.List;
+
+public record RegistrationListResponse(
+        String userId,
+        String semesterId,
+        String semesterName,
+        Integer totalCredits,
+        Integer maxCredits,
+        List<RegistrationItemResponse> registrations) {
+}

src/main/java/com/study/course_registration/entity/BaseCreateEntity.java

@@ -5,11 +5,23 @@ import java.time.Instant;
 import org.springframework.data.annotation.CreatedDate;
 import org.springframework.data.jpa.domain.support.AuditingEntityListener;
 
-import jakarta.persistence.*;
+import jakarta.persistence.Column;
+import jakarta.persistence.EntityListeners;
+import jakarta.persistence.MappedSuperclass;
+import lombok.Getter;
 
-@MappedSuperclass 
-@EntityListeners (AuditingEntityListener.class)
-public class BaseCreateEntity {
-    @CreatedDate 
+@Getter
+@MappedSuperclass
+@EntityListeners(AuditingEntityListener.class)
+public abstract class BaseCreateEntity extends BaseEntity {
+    @CreatedDate
+    @Column(nullable = false, updatable = false)
     private Instant createdAt;
+
+    protected BaseCreateEntity() {
+    }
+
+    protected BaseCreateEntity(String id) {
+        super(id);
+    }
 }

src/main/java/com/study/course_registration/entity/BaseEntity.java

@@ -1,15 +1,31 @@
 package com.study.course_registration.entity;
 
-import jakarta.persistence.GeneratedValue;
-import jakarta.persistence.GenerationType;
+import java.util.UUID;
+
+import jakarta.persistence.Column;
 import jakarta.persistence.Id;
 import jakarta.persistence.MappedSuperclass;
+import jakarta.persistence.PrePersist;
 import lombok.Getter;
 
 @Getter
-@MappedSuperclass 
-public class BaseEntity {
-    @Id 
-    @GeneratedValue(strategy = GenerationType.UUID)
+@MappedSuperclass
+public abstract class BaseEntity {
+    @Id
+    @Column(length = 36, nullable = false, updatable = false)
     private String id;
+
+    protected BaseEntity() {
+    }
+
+    protected BaseEntity(String id) {
+        this.id = id;
+    }
+
+    @PrePersist
+    protected void assignId() {
+        if (id == null) {
+            id = UUID.randomUUID().toString();
+        }
+    }
 }

src/main/java/com/study/course_registration/entity/BaseTimeZoneEntity.java

@@ -5,15 +5,28 @@ import java.time.Instant;
 import org.springframework.data.annotation.CreatedDate;
 import org.springframework.data.annotation.LastModifiedDate;
 import org.springframework.data.jpa.domain.support.AuditingEntityListener;
+
+import jakarta.persistence.Column;
 import jakarta.persistence.EntityListeners;
 import jakarta.persistence.MappedSuperclass;
+import lombok.Getter;
 
-@MappedSuperclass 
-@EntityListeners (AuditingEntityListener.class)
-public class BaseTimeZoneEntity extends BaseEntity {
-    @CreatedDate 
+@Getter
+@MappedSuperclass
+@EntityListeners(AuditingEntityListener.class)
+public abstract class BaseTimeZoneEntity extends BaseEntity {
+    @CreatedDate
+    @Column(nullable = false, updatable = false)
     private Instant createdAt;
 
-    @LastModifiedDate 
+    @LastModifiedDate
+    @Column(nullable = false)
     private Instant updatedAt;
+
+    protected BaseTimeZoneEntity() {
+    }
+
+    protected BaseTimeZoneEntity(String id) {
+        super(id);
+    }
 }

src/main/java/com/study/course_registration/entity/Lesson.java

@@ -1,53 +1,74 @@
 package com.study.course_registration.entity;
 
-import jakarta.persistence.*;
-import lombok.Getter;
-import lombok.NoArgsConstructor;
-
-import java.time.Instant;
+import java.util.Objects;
 
 import com.study.course_registration.enums.UserRole;
 
-@Entity 
-@NoArgsConstructor 
-@Getter 
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.EnumType;
+import jakarta.persistence.Enumerated;
+import jakarta.persistence.FetchType;
+import jakarta.persistence.ForeignKey;
+import jakarta.persistence.JoinColumn;
+import jakarta.persistence.ManyToOne;
+import jakarta.persistence.Table;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+
+@Entity
+@NoArgsConstructor
+@Getter
 @Table(name = "lessons")
 public class Lesson extends BaseTimeZoneEntity {
+    @Column(nullable = false)
     private String name;
 
-    @ManyToOne(fetch = FetchType.LAZY)
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "subject_id", nullable = false, foreignKey = @ForeignKey(name = "fk_lesson_subject"))
     private Subject subject;
 
-    @ManyToOne(fetch = FetchType.LAZY)
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "professor_id", nullable = false, foreignKey = @ForeignKey(name = "fk_lesson_professor"))
     private Professor professor;
 
-    private Instant startTime;
-
-    private Instant endTime;
-
-    @ManyToOne(fetch = FetchType.LAZY)
-    @Column (name = "semester_id", nullable = false)
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "semester_id", nullable = false, foreignKey = @ForeignKey(name = "fk_lesson_semester"))
     private Semester semester;
 
-    @Column (name = "capacity", nullable = false)
+    @Column(nullable = false)
     private Integer capacity;
 
-    @Column (name = "min_grade", nullable = false)
+    @Column(name = "min_grade")
     private Long minGrade;
 
-    @Column (name = "allowed_role", nullable = false)
+    @Enumerated(EnumType.STRING)
+    @Column(name = "allowed_role")
     private UserRole allowedRole;
 
-    public Lesson(String name, Subject subject, Professor professor, Instant startTime, Instant endTime, Semester semester, Integer capacity, Long minGrade, UserRole allowedRole) {
+    public Lesson(String name, Subject subject, Professor professor, Semester semester,
+                  Integer capacity, Long minGrade, UserRole allowedRole) {
+        this(null, name, subject, professor, semester, capacity, minGrade, allowedRole);
+    }
+
+    public Lesson(String id, String name, Subject subject, Professor professor, Semester semester,
+                  Integer capacity, Long minGrade, UserRole allowedRole) {
+        super(id);
+        if (name == null || name.isBlank()) {
+            throw new IllegalArgumentException("name must not be blank");
+        }
+        if (capacity == null || capacity < 1) {
+            throw new IllegalArgumentException("capacity must be at least 1");
+        }
+        if (minGrade != null && minGrade < 1) {
+            throw new IllegalArgumentException("minGrade must be at least 1 when present");
+        }
         this.name = name;
-        this.subject = subject;
-        this.professor = professor;
-        this.startTime = startTime;
-        this.endTime = endTime;
-        this.semester = semester;
+        this.subject = Objects.requireNonNull(subject, "subject");
+        this.professor = Objects.requireNonNull(professor, "professor");
+        this.semester = Objects.requireNonNull(semester, "semester");
         this.capacity = capacity;
         this.minGrade = minGrade;
         this.allowedRole = allowedRole;
-
     }
 }

src/main/java/com/study/course_registration/entity/LessonSchedule.java

@@ -2,26 +2,51 @@ package com.study.course_registration.entity;
 
 import java.time.DayOfWeek;
 import java.time.LocalTime;
+import java.util.Objects;
 
 import jakarta.persistence.Column;
 import jakarta.persistence.Entity;
+import jakarta.persistence.EnumType;
+import jakarta.persistence.Enumerated;
 import jakarta.persistence.FetchType;
+import jakarta.persistence.ForeignKey;
+import jakarta.persistence.JoinColumn;
 import jakarta.persistence.ManyToOne;
+import jakarta.persistence.Table;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 
-@Entity 
+@Entity
 @Getter
-@NoArgsConstructor 
+@NoArgsConstructor
+@Table(name = "lesson_schedules")
 public class LessonSchedule extends BaseTimeZoneEntity {
-    @ManyToOne(fetch = FetchType.LAZY)
-    @Column()
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "lesson_id", nullable = false, foreignKey = @ForeignKey(name = "fk_schedule_lesson"))
     private Lesson lesson;
 
-    @Column()
+    @Enumerated(EnumType.STRING)
+    @Column(nullable = false, length = 16)
     private DayOfWeek dayOfWeek;
 
+    @Column(nullable = false)
     private LocalTime startTime;
 
+    @Column(nullable = false)
     private LocalTime endTime;
+
+    public LessonSchedule(Lesson lesson, DayOfWeek dayOfWeek, LocalTime startTime, LocalTime endTime) {
+        this(null, lesson, dayOfWeek, startTime, endTime);
+    }
+
+    public LessonSchedule(String id, Lesson lesson, DayOfWeek dayOfWeek, LocalTime startTime, LocalTime endTime) {
+        super(id);
+        this.lesson = Objects.requireNonNull(lesson, "lesson");
+        this.dayOfWeek = Objects.requireNonNull(dayOfWeek, "dayOfWeek");
+        this.startTime = Objects.requireNonNull(startTime, "startTime");
+        this.endTime = Objects.requireNonNull(endTime, "endTime");
+        if (!startTime.isBefore(endTime)) {
+            throw new IllegalArgumentException("startTime must be before endTime");
+        }
+    }
 }

src/main/java/com/study/course_registration/entity/Professor.java

@@ -1,17 +1,28 @@
 package com.study.course_registration.entity;
 
-import jakarta.persistence.*;
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.Table;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 
-@Entity 
-@NoArgsConstructor 
+@Entity
+@NoArgsConstructor
 @Getter
 @Table(name = "professors")
 public class Professor extends BaseTimeZoneEntity {
+    @Column(nullable = false)
     private String name;
 
     public Professor(String name) {
+        this(null, name);
+    }
+
+    public Professor(String id, String name) {
+        super(id);
+        if (name == null || name.isBlank()) {
+            throw new IllegalArgumentException("name must not be blank");
+        }
         this.name = name;
     }
 }

src/main/java/com/study/course_registration/entity/Semester.java

@@ -2,28 +2,67 @@ package com.study.course_registration.entity;
 
 import java.time.Instant;
 import java.time.LocalDate;
+import java.util.Objects;
 
-import jakarta.persistence.*;
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.Table;
+import jakarta.persistence.UniqueConstraint;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 
-@Entity 
-@NoArgsConstructor 
-@Getter 
+@Entity
+@Getter
+@NoArgsConstructor
+@Table(name = "semesters", uniqueConstraints = @UniqueConstraint(name = "uk_semester_name", columnNames = "name"))
 public class Semester extends BaseTimeZoneEntity {
+    @Column(nullable = false)
     private String name;
 
+    @Column(nullable = false)
     private LocalDate startDate;
 
+    @Column(nullable = false)
     private LocalDate endDate;
 
+    @Column(nullable = false)
     private Instant registrationStartAt;
 
+    @Column(nullable = false)
     private Instant registrationEndAt;
 
+    @Column(nullable = false)
     private Integer maxCredits;
 
-    public Semester(String name) {
-        this.name = name;
+    public Semester(String name, LocalDate startDate, LocalDate endDate,
+                    Instant registrationStartAt, Instant registrationEndAt, Integer maxCredits) {
+        this(null, name, startDate, endDate, registrationStartAt, registrationEndAt, maxCredits);
+    }
+
+    public Semester(String id, String name, LocalDate startDate, LocalDate endDate,
+                    Instant registrationStartAt, Instant registrationEndAt, Integer maxCredits) {
+        super(id);
+        this.name = requireText(name, "name");
+        this.startDate = Objects.requireNonNull(startDate, "startDate");
+        this.endDate = Objects.requireNonNull(endDate, "endDate");
+        this.registrationStartAt = Objects.requireNonNull(registrationStartAt, "registrationStartAt");
+        this.registrationEndAt = Objects.requireNonNull(registrationEndAt, "registrationEndAt");
+        if (endDate.isBefore(startDate)) {
+            throw new IllegalArgumentException("endDate must be on or after startDate");
+        }
+        if (!registrationStartAt.isBefore(registrationEndAt)) {
+            throw new IllegalArgumentException("registrationStartAt must be before registrationEndAt");
+        }
+        if (maxCredits == null || maxCredits < 1) {
+            throw new IllegalArgumentException("maxCredits must be at least 1");
+        }
+        this.maxCredits = maxCredits;
+    }
+
+    private static String requireText(String value, String field) {
+        if (value == null || value.isBlank()) {
+            throw new IllegalArgumentException(field + " must not be blank");
+        }
+        return value;
     }
 }

src/main/java/com/study/course_registration/entity/Subject.java

@@ -1,23 +1,48 @@
 package com.study.course_registration.entity;
 
-import jakarta.persistence.*;
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.Table;
+import jakarta.persistence.UniqueConstraint;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 
-@Entity 
-@NoArgsConstructor 
-@Getter 
-@Table(name = "subjects")
+@Entity
+@NoArgsConstructor
+@Getter
+@Table(name = "subjects", uniqueConstraints = @UniqueConstraint(name = "uk_subject_code", columnNames = "code"))
 public class Subject extends BaseTimeZoneEntity {
+    @Column(nullable = false)
     private String name;
 
+    @Column(nullable = false)
     private String code;
 
+    @Column(nullable = false, length = 1000)
     private String description;
 
-    public Subject(String name, String code, String description) {
-        this.name = name;
-        this.code = code;
-        this.description = description;
+    @Column(nullable = false)
+    private Integer credit;
+
+    public Subject(String name, String code, String description, Integer credit) {
+        this(null, name, code, description, credit);
+    }
+
+    public Subject(String id, String name, String code, String description, Integer credit) {
+        super(id);
+        if (credit == null || credit < 1) {
+            throw new IllegalArgumentException("credit must be at least 1");
+        }
+        this.name = requireText(name, "name");
+        this.code = requireText(code, "code");
+        this.description = description == null ? "" : description;
+        this.credit = credit;
+    }
+
+    private static String requireText(String value, String field) {
+        if (value == null || value.isBlank()) {
+            throw new IllegalArgumentException(field + " must not be blank");
+        }
+        return value;
     }
 }

src/main/java/com/study/course_registration/entity/User.java

@@ -1,26 +1,46 @@
 package com.study.course_registration.entity;
 
+import java.util.Objects;
+
 import com.study.course_registration.enums.UserRole;
 
-import jakarta.persistence.*;
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.EnumType;
+import jakarta.persistence.Enumerated;
+import jakarta.persistence.Table;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 
 @Entity
-@Getter 
+@Getter
 @NoArgsConstructor
 @Table(name = "users")
 public class User extends BaseTimeZoneEntity {
+    @Column(nullable = false)
     private String name;
 
+    @Column(nullable = false)
     private Long grade;
 
     @Enumerated(EnumType.STRING)
+    @Column(nullable = false)
     private UserRole role;
 
     public User(String name, Long grade, UserRole role) {
+        this(null, name, grade, role);
+    }
+
+    public User(String id, String name, Long grade, UserRole role) {
+        super(id);
+        if (name == null || name.isBlank()) {
+            throw new IllegalArgumentException("name must not be blank");
+        }
+        if (grade == null || grade < 1) {
+            throw new IllegalArgumentException("grade must be at least 1");
+        }
         this.name = name;
         this.grade = grade;
-        this.role = role;
+        this.role = Objects.requireNonNull(role, "role");
     }
 }

src/main/java/com/study/course_registration/entity/UserLesson.java

@@ -1,36 +1,52 @@
 package com.study.course_registration.entity;
 
-import jakarta.persistence.*;
+import java.time.Instant;
+import java.util.Objects;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.FetchType;
+import jakarta.persistence.ForeignKey;
+import jakarta.persistence.JoinColumn;
+import jakarta.persistence.ManyToOne;
+import jakarta.persistence.Table;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 
-@Entity 
+@Entity
 @NoArgsConstructor
 @Getter
 @Table(name = "user_lessons")
 public class UserLesson extends BaseCreateEntity {
-
-    @EmbeddedId 
-    private UserLessonId id;
-
-    @MapsId("userId")
-    @ManyToOne(fetch = FetchType.LAZY)
-    @JoinColumn (
-        name = "user_id",
-        foreignKey = @ForeignKey(name = "fk_user_lesson_user_id")
-    )
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "user_id", nullable = false, foreignKey = @ForeignKey(name = "fk_user_lesson_user"))
     private User user;
 
-    @MapsId("lessonId")
-    @ManyToOne(fetch = FetchType.LAZY)
-    @JoinColumn (
-        name = "lesson_id",
-        foreignKey = @ForeignKey(name = "fk_user_lesson_lesson_id")
-    )
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "lesson_id", nullable = false, foreignKey = @ForeignKey(name = "fk_user_lesson_lesson"))
     private Lesson lesson;
 
+    @Column(name = "canceled_at")
+    private Instant canceledAt;
+
     public UserLesson(User user, Lesson lesson) {
-        this.user = user;
-        this.lesson = lesson;
+        this(null, user, lesson);
+    }
+
+    public UserLesson(String id, User user, Lesson lesson) {
+        super(id);
+        this.user = Objects.requireNonNull(user, "user");
+        this.lesson = Objects.requireNonNull(lesson, "lesson");
+    }
+
+    public void cancel(Instant canceledAt) {
+        if (this.canceledAt != null) {
+            throw new IllegalStateException("registration is already canceled");
+        }
+        this.canceledAt = Objects.requireNonNull(canceledAt, "canceledAt");
+    }
+
+    public boolean isCanceled() {
+        return canceledAt != null;
     }
 }

src/main/java/com/study/course_registration/entity/UserLessonId.java

@@ -1,21 +0,0 @@
-package com.study.course_registration.entity;
-
-import jakarta.persistence.Embeddable;
-import java.io.Serializable;
-import lombok.EqualsAndHashCode;
-import lombok.Getter;
-import lombok.NoArgsConstructor;
-
-@Embeddable
-@Getter 
-@NoArgsConstructor 
-@EqualsAndHashCode 
-public class UserLessonId implements Serializable {
-    private String userId;
-    private String lessonId;
-    
-    public UserLessonId(String userId, String lessonId) {
-        this.userId = userId;
-        this.lessonId = lessonId;
-    }
-}

src/main/java/com/study/course_registration/enums/RegistrationErrorCode.java

@@ -0,0 +1,32 @@
+package com.study.course_registration.enums;
+
+import org.springframework.http.HttpStatus;
+
+public enum RegistrationErrorCode {
+    VALIDATION_ERROR(HttpStatus.BAD_REQUEST),
+    USER_NOT_FOUND(HttpStatus.NOT_FOUND),
+    LESSON_NOT_FOUND(HttpStatus.NOT_FOUND),
+    SEMESTER_NOT_FOUND(HttpStatus.NOT_FOUND),
+    REGISTRATION_NOT_FOUND(HttpStatus.NOT_FOUND),
+    NOT_ELIGIBLE_GRADE(HttpStatus.FORBIDDEN),
+    NOT_ELIGIBLE_ROLE(HttpStatus.FORBIDDEN),
+    REGISTRATION_FORBIDDEN(HttpStatus.FORBIDDEN),
+    REGISTRATION_PERIOD_CLOSED(HttpStatus.CONFLICT),
+    ALREADY_REGISTERED(HttpStatus.CONFLICT),
+    DUPLICATE_SUBJECT(HttpStatus.CONFLICT),
+    SCHEDULE_CONFLICT(HttpStatus.CONFLICT),
+    CREDIT_LIMIT_EXCEEDED(HttpStatus.CONFLICT),
+    CAPACITY_EXCEEDED(HttpStatus.CONFLICT),
+    ALREADY_CANCELED(HttpStatus.CONFLICT),
+    LOCK_TIMEOUT(HttpStatus.SERVICE_UNAVAILABLE);
+
+    private final HttpStatus status;
+
+    RegistrationErrorCode(HttpStatus status) {
+        this.status = status;
+    }
+
+    public HttpStatus getStatus() {
+        return status;
+    }
+}

src/main/java/com/study/course_registration/exception/ErrorResponse.java

@@ -0,0 +1,6 @@
+package com.study.course_registration.exception;
+
+import java.time.Instant;
+
+public record ErrorResponse(String code, String message, Instant timestamp) {
+}

src/main/java/com/study/course_registration/exception/GlobalExceptionHandler.java

@@ -0,0 +1,54 @@
+package com.study.course_registration.exception;
+
+import java.time.Clock;
+
+import org.springframework.dao.PessimisticLockingFailureException;
+import org.springframework.http.ResponseEntity;
+import org.springframework.http.converter.HttpMessageNotReadableException;
+import org.springframework.web.bind.MethodArgumentNotValidException;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.RestControllerAdvice;
+
+import com.study.course_registration.enums.RegistrationErrorCode;
+
+import jakarta.persistence.LockTimeoutException;
+import jakarta.persistence.PessimisticLockException;
+import jakarta.validation.ConstraintViolationException;
+
+@RestControllerAdvice
+public class GlobalExceptionHandler {
+    private final Clock clock;
+
+    public GlobalExceptionHandler(Clock clock) {
+        this.clock = clock;
+    }
+
+    @ExceptionHandler(RegistrationException.class)
+    ResponseEntity<ErrorResponse> handleRegistrationException(RegistrationException exception) {
+        return response(exception.getErrorCode(), exception.getMessage());
+    }
+
+    @ExceptionHandler(MethodArgumentNotValidException.class)
+    ResponseEntity<ErrorResponse> handleValidation(MethodArgumentNotValidException exception) {
+        String message = exception.getBindingResult().getFieldErrors().stream()
+                .findFirst()
+                .map(error -> error.getField() + ": " + error.getDefaultMessage())
+                .orElse("요청 값이 올바르지 않습니다.");
+        return response(RegistrationErrorCode.VALIDATION_ERROR, message);
+    }
+
+    @ExceptionHandler({ConstraintViolationException.class, HttpMessageNotReadableException.class})
+    ResponseEntity<ErrorResponse> handleBadRequest(Exception exception) {
+        return response(RegistrationErrorCode.VALIDATION_ERROR, "요청 값이 올바르지 않습니다.");
+    }
+
+    @ExceptionHandler({LockTimeoutException.class, PessimisticLockException.class, PessimisticLockingFailureException.class})
+    ResponseEntity<ErrorResponse> handleLockTimeout(Exception exception) {
+        return response(RegistrationErrorCode.LOCK_TIMEOUT, "다른 요청이 처리 중입니다. 잠시 후 다시 시도해 주세요.");
+    }
+
+    private ResponseEntity<ErrorResponse> response(RegistrationErrorCode code, String message) {
+        return ResponseEntity.status(code.getStatus())
+                .body(new ErrorResponse(code.name(), message, clock.instant()));
+    }
+}

src/main/java/com/study/course_registration/exception/RegistrationException.java

@@ -0,0 +1,16 @@
+package com.study.course_registration.exception;
+
+import com.study.course_registration.enums.RegistrationErrorCode;
+
+public class RegistrationException extends RuntimeException {
+    private final RegistrationErrorCode errorCode;
+
+    public RegistrationException(RegistrationErrorCode errorCode, String message) {
+        super(message);
+        this.errorCode = errorCode;
+    }
+
+    public RegistrationErrorCode getErrorCode() {
+        return errorCode;
+    }
+}

src/main/java/com/study/course_registration/repository/LessonRepository.java

@@ -1,9 +1,34 @@
 package com.study.course_registration.repository;
 
+import java.util.Optional;
+
+import org.springframework.data.domain.Page;
+import org.springframework.data.domain.Pageable;
+import org.springframework.data.jpa.repository.EntityGraph;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Lock;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.jpa.repository.QueryHints;
+import org.springframework.data.repository.query.Param;
 
 import com.study.course_registration.entity.Lesson;
 
+import jakarta.persistence.LockModeType;
+import jakarta.persistence.QueryHint;
+
 public interface LessonRepository extends JpaRepository<Lesson, String> {
-    
+    @Lock(LockModeType.PESSIMISTIC_WRITE)
+    @QueryHints(@QueryHint(name = "jakarta.persistence.lock.timeout", value = "3000"))
+    @Query("select l from Lesson l where l.id = :id")
+    Optional<Lesson> findByIdForUpdate(@Param("id") String id);
+
+    @EntityGraph(attributePaths = {"subject", "professor", "semester"})
+    Page<Lesson> findBySemester_Id(String semesterId, Pageable pageable);
+
+    @EntityGraph(attributePaths = {"subject", "professor", "semester"})
+    Page<Lesson> findBySemester_IdAndSubject_Id(String semesterId, String subjectId, Pageable pageable);
+
+    @EntityGraph(attributePaths = {"subject", "professor", "semester"})
+    @Query("select l from Lesson l where l.id = :id")
+    Optional<Lesson> findDetailedById(@Param("id") String id);
 }

src/main/java/com/study/course_registration/repository/LessonScheduleRepository.java

@@ -0,0 +1,14 @@
+package com.study.course_registration.repository;
+
+import java.util.Collection;
+import java.util.List;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+
+import com.study.course_registration.entity.LessonSchedule;
+
+public interface LessonScheduleRepository extends JpaRepository<LessonSchedule, String> {
+    List<LessonSchedule> findByLesson_IdOrderByDayOfWeekAscStartTimeAsc(String lessonId);
+
+    List<LessonSchedule> findByLesson_IdInOrderByDayOfWeekAscStartTimeAsc(Collection<String> lessonIds);
+}

src/main/java/com/study/course_registration/repository/SemesterRepository.java

@@ -0,0 +1,13 @@
+package com.study.course_registration.repository;
+
+import java.time.LocalDate;
+import java.util.Optional;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+
+import com.study.course_registration.entity.Semester;
+
+public interface SemesterRepository extends JpaRepository<Semester, String> {
+    Optional<Semester> findFirstByStartDateLessThanEqualAndEndDateGreaterThanEqualOrderByStartDateDesc(
+            LocalDate startDate, LocalDate endDate);
+}

src/main/java/com/study/course_registration/repository/UserLessonRepository.java

@@ -1,10 +1,54 @@
 package com.study.course_registration.repository;
 
+import java.util.Collection;
+import java.util.List;
+import java.util.Optional;
+
+import org.springframework.data.jpa.repository.EntityGraph;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
 
 import com.study.course_registration.entity.UserLesson;
-import com.study.course_registration.entity.UserLessonId;
 
-public interface UserLessonRepository extends JpaRepository<UserLesson, UserLessonId> {
-    
+public interface UserLessonRepository extends JpaRepository<UserLesson, String> {
+    @EntityGraph(attributePaths = {"lesson", "lesson.subject", "lesson.professor", "lesson.semester"})
+    @Query("""
+            select ul from UserLesson ul
+            where ul.user.id = :userId
+              and ul.lesson.semester.id = :semesterId
+              and ul.canceledAt is null
+            order by ul.createdAt asc
+            """)
+    List<UserLesson> findActiveByUserIdAndSemesterId(
+            @Param("userId") String userId,
+            @Param("semesterId") String semesterId);
+
+    long countByLesson_IdAndCanceledAtIsNull(String lessonId);
+
+    @Query("""
+            select ul.user.id as userId, ul.lesson.id as lessonId
+            from UserLesson ul
+            where ul.id = :registrationId
+            """)
+    Optional<RegistrationTarget> findTargetById(@Param("registrationId") String registrationId);
+
+    @Query("""
+            select ul.lesson.id as lessonId, count(ul) as enrolledCount
+            from UserLesson ul
+            where ul.lesson.id in :lessonIds
+              and ul.canceledAt is null
+            group by ul.lesson.id
+            """)
+    List<LessonEnrollmentCount> countActiveByLessonIds(@Param("lessonIds") Collection<String> lessonIds);
+
+    interface RegistrationTarget {
+        String getUserId();
+        String getLessonId();
+    }
+
+    interface LessonEnrollmentCount {
+        String getLessonId();
+        long getEnrolledCount();
+    }
 }

src/main/java/com/study/course_registration/repository/UserRepository.java

@@ -1,9 +1,21 @@
 package com.study.course_registration.repository;
 
+import java.util.Optional;
+
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Lock;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.jpa.repository.QueryHints;
+import org.springframework.data.repository.query.Param;
 
 import com.study.course_registration.entity.User;
 
+import jakarta.persistence.LockModeType;
+import jakarta.persistence.QueryHint;
+
 public interface UserRepository extends JpaRepository<User, String> {
-    
+    @Lock(LockModeType.PESSIMISTIC_WRITE)
+    @QueryHints(@QueryHint(name = "jakarta.persistence.lock.timeout", value = "3000"))
+    @Query("select u from User u where u.id = :id")
+    Optional<User> findByIdForUpdate(@Param("id") String id);
 }

src/main/java/com/study/course_registration/seeder/DataSeeder.java

@@ -1,91 +1,121 @@
 package com.study.course_registration.seeder;
 
-import org.springframework.context.annotation.Profile;
+import java.time.Clock;
+import java.time.DayOfWeek;
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalTime;
+import java.util.List;
+
+import org.springframework.boot.CommandLineRunner;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.stereotype.Component;
 import org.springframework.transaction.annotation.Transactional;
 
 import com.study.course_registration.entity.Lesson;
+import com.study.course_registration.entity.LessonSchedule;
 import com.study.course_registration.entity.Professor;
+import com.study.course_registration.entity.Semester;
 import com.study.course_registration.entity.Subject;
 import com.study.course_registration.entity.User;
-import com.study.course_registration.entity.UserLesson;
 import com.study.course_registration.enums.UserRole;
 import com.study.course_registration.repository.LessonRepository;
-import com.study.course_registration.repository.UserRepository;
-import com.study.course_registration.repository.SubjectRepository;
+import com.study.course_registration.repository.LessonScheduleRepository;
 import com.study.course_registration.repository.ProfessorRepository;
-import com.study.course_registration.repository.UserLessonRepository;
-
-import java.time.Instant;
-import java.time.ZoneId;
-import java.time.ZonedDateTime;
-import java.time.temporal.ChronoUnit;
-import java.util.List;
-
-import org.springframework.boot.CommandLineRunner;
+import com.study.course_registration.repository.SemesterRepository;
+import com.study.course_registration.repository.SubjectRepository;
+import com.study.course_registration.repository.UserRepository;
 
 @Component
-@Profile ("local")
+@ConditionalOnProperty(name = "app.seed.enabled", havingValue = "true")
 public class DataSeeder implements CommandLineRunner {
+    public static final String SEMESTER_ID = "00000000-0000-0000-0000-000000000001";
+    public static final String STUDENT_GRADE_1_ID = "30000000-0000-0000-0000-000000000001";
+    public static final String STUDENT_GRADE_2_ID = "30000000-0000-0000-0000-000000000002";
+    public static final String POSTGRADUATE_ID = "30000000-0000-0000-0000-000000000003";
+    public static final String DATA_STRUCTURES_01_ID = "40000000-0000-0000-0000-000000000001";
+    public static final String DATA_STRUCTURES_02_ID = "40000000-0000-0000-0000-000000000002";
+    public static final String OVERLAP_OS_ID = "40000000-0000-0000-0000-000000000003";
+    public static final String CAPACITY_ONE_ID = "40000000-0000-0000-0000-000000000004";
+    public static final String MIN_GRADE_ID = "40000000-0000-0000-0000-000000000005";
+    public static final String POSTGRADUATE_ONLY_ID = "40000000-0000-0000-0000-000000000006";
+    public static final String BOUNDARY_LESSON_ID = "40000000-0000-0000-0000-000000000007";
 
-    private static final ZoneId KST = ZoneId.of("Asia/Seoul");
-
-    private final UserRepository userRepository;
-    private final LessonRepository lessonRepository;
-    private final SubjectRepository subjectRepository;
+    private final Clock clock;
+    private final SemesterRepository semesterRepository;
     private final ProfessorRepository professorRepository;
-    private final UserLessonRepository userLessonRepository;
+    private final SubjectRepository subjectRepository;
+    private final LessonRepository lessonRepository;
+    private final LessonScheduleRepository lessonScheduleRepository;
+    private final UserRepository userRepository;
 
-    public DataSeeder(UserRepository userRepository, LessonRepository lessonRepository, SubjectRepository subjectRepository, ProfessorRepository professorRepository, UserLessonRepository userLessonRepository) {
-        this.userRepository = userRepository;
-        this.lessonRepository = lessonRepository;
-        this.subjectRepository = subjectRepository;
+    public DataSeeder(Clock clock,
+                      SemesterRepository semesterRepository,
+                      ProfessorRepository professorRepository,
+                      SubjectRepository subjectRepository,
+                      LessonRepository lessonRepository,
+                      LessonScheduleRepository lessonScheduleRepository,
+                      UserRepository userRepository) {
+        this.clock = clock;
+        this.semesterRepository = semesterRepository;
         this.professorRepository = professorRepository;
-        this.userLessonRepository = userLessonRepository;
+        this.subjectRepository = subjectRepository;
+        this.lessonRepository = lessonRepository;
+        this.lessonScheduleRepository = lessonScheduleRepository;
+        this.userRepository = userRepository;
     }
 
     @Override
     @Transactional
-    public void run(String... args) throws Exception {
-        if(userRepository.count() > 0) {
+    public void run(String... args) {
+        if (semesterRepository.existsById(SEMESTER_ID)) {
             return;
         }
 
+        Instant now = clock.instant();
+        LocalDate today = LocalDate.now(clock);
+        Semester semester = semesterRepository.save(new Semester(
+                SEMESTER_ID, "2026-1", today.minusDays(30), today.plusDays(120),
+                now.minusSeconds(86_400), now.plusSeconds(7 * 86_400), 18));
+
         List<Professor> professors = professorRepository.saveAll(List.of(
-            new Professor("김영한"),
-            new Professor("최태영"),
-            new Professor("조재한")
-        ));
+                new Professor("10000000-0000-0000-0000-000000000001", "김영한"),
+                new Professor("10000000-0000-0000-0000-000000000002", "최태영"),
+                new Professor("10000000-0000-0000-0000-000000000003", "조재한")));
 
         List<Subject> subjects = subjectRepository.saveAll(List.of(
-            new Subject("자바 프로그래밍", "CS201", "자바 프로그래밍 기초"),
-            new Subject("자료구조", "CS202", "자료구조 및 알고리즘"),
-            new Subject("데이터베이스", "CS203", "데이터베이스 시스템")
-        ));
+                new Subject("20000000-0000-0000-0000-000000000001", "자바 프로그래밍", "CS201", "자바 프로그래밍 기초", 3),
+                new Subject("20000000-0000-0000-0000-000000000002", "자료구조", "CS202", "자료구조 및 알고리즘", 3),
+                new Subject("20000000-0000-0000-0000-000000000003", "데이터베이스", "CS203", "데이터베이스 시스템", 3),
+                new Subject("20000000-0000-0000-0000-000000000004", "컴퓨터 네트워크", "CS204", "네트워크 기초", 3),
+                new Subject("20000000-0000-0000-0000-000000000005", "운영체제", "CS301", "운영체제 핵심", 3),
+                new Subject("20000000-0000-0000-0000-000000000006", "대학원 세미나", "CS401", "대학원 전용 세미나", 3)));
 
-        Instant base = Instant.now().truncatedTo(ChronoUnit.HOURS);
-        List<Lesson> lessons = lessonRepository.saveAll(List.of(
-            new Lesson("자바 프로그래밍", subjects.get(0), professors.get(0), kst(3, 2, 9), kst(3, 2, 11)),
-            new Lesson("자료구조", subjects.get(1), professors.get(1), kst(3, 2, 13), kst(3, 2, 17)),
-            new Lesson("데이터베이스", subjects.get(2), professors.get(2), kst(3, 3, 10), kst(3, 3, 14))
-        ));
+        Lesson data01 = new Lesson(DATA_STRUCTURES_01_ID, "자료구조 01분반", subjects.get(1), professors.get(0), semester, 30, null, null);
+        Lesson data02 = new Lesson(DATA_STRUCTURES_02_ID, "자료구조 02분반", subjects.get(1), professors.get(1), semester, 30, null, null);
+        Lesson os = new Lesson(OVERLAP_OS_ID, "운영체제", subjects.get(4), professors.get(1), semester, 30, null, null);
+        Lesson capacityOne = new Lesson(CAPACITY_ONE_ID, "데이터베이스 소수정예", subjects.get(2), professors.get(2), semester, 1, null, null);
+        Lesson minGrade = new Lesson(MIN_GRADE_ID, "고급 네트워크", subjects.get(3), professors.get(2), semester, 30, 3L, null);
+        Lesson postgraduate = new Lesson(POSTGRADUATE_ONLY_ID, "대학원 세미나", subjects.get(5), professors.get(0), semester, 20, null, UserRole.POSTGRADUATE);
+        Lesson boundary = new Lesson(BOUNDARY_LESSON_ID, "자바 프로그래밍", subjects.get(0), professors.get(0), semester, 30, null, null);
+        lessonRepository.saveAll(List.of(data01, data02, os, capacityOne, minGrade, postgraduate, boundary));
 
-        List<User> users = userRepository.saveAll(List.of(
-            new User("학생1", 1L, UserRole.STUDENT),
-            new User("학생2", 2L, UserRole.STUDENT),
-            new User("대학원생", 3L, UserRole.POSTGRADUATE)
-        ));
+        lessonScheduleRepository.saveAll(List.of(
+                schedule("50000000-0000-0000-0000-000000000001", data01, DayOfWeek.MONDAY, 9, 11),
+                schedule("50000000-0000-0000-0000-000000000002", data02, DayOfWeek.TUESDAY, 9, 11),
+                schedule("50000000-0000-0000-0000-000000000003", os, DayOfWeek.MONDAY, 10, 12),
+                schedule("50000000-0000-0000-0000-000000000004", capacityOne, DayOfWeek.WEDNESDAY, 9, 11),
+                schedule("50000000-0000-0000-0000-000000000005", minGrade, DayOfWeek.THURSDAY, 9, 11),
+                schedule("50000000-0000-0000-0000-000000000006", postgraduate, DayOfWeek.FRIDAY, 9, 11),
+                schedule("50000000-0000-0000-0000-000000000007", boundary, DayOfWeek.MONDAY, 11, 13)));
 
-        List<UserLesson> userLessons = userLessonRepository.saveAll(List.of(
-            new UserLesson(users.get(0), lessons.get(0)),
-            new UserLesson(users.get(0), lessons.get(1)),
-            new UserLesson(users.get(1), lessons.get(1)),
-            new UserLesson(users.get(2), lessons.get(2))
-        ));
+        userRepository.saveAll(List.of(
+                new User(STUDENT_GRADE_1_ID, "학생1", 1L, UserRole.STUDENT),
+                new User(STUDENT_GRADE_2_ID, "학생2", 2L, UserRole.STUDENT),
+                new User(POSTGRADUATE_ID, "대학원생", 3L, UserRole.POSTGRADUATE)));
     }
 
-    private Instant kst(int month, int day, int hour) {
-        return ZonedDateTime.of(2026, month, day, hour, 0, 0, 0, KST).toInstant();
+    private static LessonSchedule schedule(String id, Lesson lesson, DayOfWeek day, int startHour, int endHour) {
+        return new LessonSchedule(id, lesson, day, LocalTime.of(startHour, 0), LocalTime.of(endHour, 0));
     }
-    
 }

src/main/java/com/study/course_registration/service/CourseRegistrationService.java

@@ -1,36 +1,183 @@
 package com.study.course_registration.service;
 
-import org.springframework.stereotype.Service;
+import java.time.Clock;
+import java.time.LocalDate;
+import java.util.LinkedHashMap;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
 
-import com.study.course_registration.dto.RequestDto;
-import com.study.course_registration.dto.ResponseDto;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import com.study.course_registration.dto.ScheduleResponse;
+import com.study.course_registration.dto.registration.CourseRegistrationRequest;
+import com.study.course_registration.dto.registration.CourseRegistrationResponse;
+import com.study.course_registration.dto.registration.RegistrationItemResponse;
+import com.study.course_registration.dto.registration.RegistrationListResponse;
+import com.study.course_registration.entity.Lesson;
+import com.study.course_registration.entity.LessonSchedule;
+import com.study.course_registration.entity.Semester;
+import com.study.course_registration.entity.User;
+import com.study.course_registration.entity.UserLesson;
+import com.study.course_registration.enums.RegistrationErrorCode;
+import com.study.course_registration.exception.RegistrationException;
 import com.study.course_registration.repository.LessonRepository;
+import com.study.course_registration.repository.LessonScheduleRepository;
+import com.study.course_registration.repository.SemesterRepository;
 import com.study.course_registration.repository.UserLessonRepository;
 import com.study.course_registration.repository.UserRepository;
-import com.study.course_registration.repository.SubjectRepository;
-import com.study.course_registration.repository.ProfessorRepository;
+import com.study.course_registration.service.policy.RegistrationValidator;
 
 @Service
 public class CourseRegistrationService {
+    private final Clock clock;
     private final UserRepository userRepository;
     private final LessonRepository lessonRepository;
-    private final SubjectRepository subjectRepository;
-    private final ProfessorRepository professorRepository;
+    private final SemesterRepository semesterRepository;
+    private final LessonScheduleRepository lessonScheduleRepository;
     private final UserLessonRepository userLessonRepository;
+    private final RegistrationValidator validator;
 
-    public CourseRegistrationService(UserRepository userRepository, LessonRepository lessonRepository, SubjectRepository subjectRepository, ProfessorRepository professorRepository, UserLessonRepository userLessonRepository) {
+    public CourseRegistrationService(Clock clock,
+                                     UserRepository userRepository,
+                                     LessonRepository lessonRepository,
+                                     SemesterRepository semesterRepository,
+                                     LessonScheduleRepository lessonScheduleRepository,
+                                     UserLessonRepository userLessonRepository,
+                                     RegistrationValidator validator) {
+        this.clock = clock;
         this.userRepository = userRepository;
         this.lessonRepository = lessonRepository;
-        this.subjectRepository = subjectRepository;
-        this.professorRepository = professorRepository;
+        this.semesterRepository = semesterRepository;
+        this.lessonScheduleRepository = lessonScheduleRepository;
         this.userLessonRepository = userLessonRepository;
+        this.validator = validator;
     }
 
-    public ResponseDto registerCourse(RequestDto requestDto) {
-        
-        return new ResponseDto();
+    @Transactional
+    public CourseRegistrationResponse register(String userId, CourseRegistrationRequest request) {
+        User user = userRepository.findById(userId)
+                .orElseThrow(() -> error(RegistrationErrorCode.USER_NOT_FOUND, "사용자를 찾을 수 없습니다."));
+        Lesson lesson = lessonRepository.findById(request.lessonId())
+                .orElseThrow(() -> error(RegistrationErrorCode.LESSON_NOT_FOUND, "강의를 찾을 수 없습니다."));
+
+        var now = clock.instant();
+        validator.validatePreconditions(user, lesson, now);
+
+        // Lock order is a system invariant: User -> Lesson.
+        User lockedUser = userRepository.findByIdForUpdate(userId)
+                .orElseThrow(() -> error(RegistrationErrorCode.USER_NOT_FOUND, "사용자를 찾을 수 없습니다."));
+        Lesson lockedLesson = lessonRepository.findByIdForUpdate(request.lessonId())
+                .orElseThrow(() -> error(RegistrationErrorCode.LESSON_NOT_FOUND, "강의를 찾을 수 없습니다."));
+
+        List<UserLesson> activeRegistrations = userLessonRepository.findActiveByUserIdAndSemesterId(
+                lockedUser.getId(), lockedLesson.getSemester().getId());
+        Map<String, List<LessonSchedule>> schedules = loadSchedules(activeRegistrations, lockedLesson);
+        long enrolledCount = userLessonRepository.countByLesson_IdAndCanceledAtIsNull(lockedLesson.getId());
+
+        validator.validateLocked(lockedLesson, activeRegistrations, schedules, enrolledCount);
+        int totalCredits = validator.totalCreditsAfter(lockedLesson, activeRegistrations);
+
+        UserLesson registration = userLessonRepository.saveAndFlush(new UserLesson(lockedUser, lockedLesson));
+        return new CourseRegistrationResponse(
+                registration.getId(),
+                lockedUser.getId(),
+                lockedLesson.getId(),
+                lockedLesson.getName(),
+                lockedLesson.getSubject().getCode(),
+                lockedLesson.getSubject().getCredit(),
+                registration.getCreatedAt(),
+                totalCredits);
+    }
+
+    @Transactional(readOnly = true)
+    public RegistrationListResponse getRegistrations(String userId, String semesterId) {
+        if (!userRepository.existsById(userId)) {
+            throw error(RegistrationErrorCode.USER_NOT_FOUND, "사용자를 찾을 수 없습니다.");
+        }
+        Semester semester = resolveSemester(semesterId);
+        List<UserLesson> activeRegistrations = userLessonRepository.findActiveByUserIdAndSemesterId(userId, semester.getId());
+
+        Set<String> lessonIds = activeRegistrations.stream()
+                .map(UserLesson::getLesson)
+                .map(Lesson::getId)
+                .collect(Collectors.toCollection(LinkedHashSet::new));
+        Map<String, List<LessonSchedule>> schedulesByLesson = groupSchedules(
+                lessonIds.isEmpty() ? List.of() : lessonScheduleRepository.findByLesson_IdInOrderByDayOfWeekAscStartTimeAsc(lessonIds));
+
+        List<RegistrationItemResponse> items = activeRegistrations.stream()
+                .map(registration -> {
+                    Lesson lesson = registration.getLesson();
+                    List<ScheduleResponse> schedules = schedulesByLesson.getOrDefault(lesson.getId(), List.of()).stream()
+                            .map(ScheduleResponse::from)
+                            .toList();
+                    return new RegistrationItemResponse(
+                            registration.getId(), lesson.getId(), lesson.getName(),
+                            lesson.getSubject().getCode(), lesson.getSubject().getName(), lesson.getSubject().getCredit(),
+                            lesson.getProfessor().getName(), schedules, registration.getCreatedAt());
+                })
+                .toList();
+
+        return new RegistrationListResponse(
+                userId, semester.getId(), semester.getName(), validator.currentCredits(activeRegistrations),
+                semester.getMaxCredits(), items);
+    }
+
+    @Transactional
+    public void cancel(String userId, String registrationId) {
+        UserLessonRepository.RegistrationTarget target = userLessonRepository.findTargetById(registrationId)
+                .orElseThrow(() -> error(RegistrationErrorCode.REGISTRATION_NOT_FOUND, "수강신청 건을 찾을 수 없습니다."));
+        if (!target.getUserId().equals(userId)) {
+            throw error(RegistrationErrorCode.REGISTRATION_FORBIDDEN, "다른 사용자의 수강신청은 취소할 수 없습니다.");
+        }
+
+        // Mutation paths use the same fixed lock order as registration.
+        userRepository.findByIdForUpdate(userId)
+                .orElseThrow(() -> error(RegistrationErrorCode.USER_NOT_FOUND, "사용자를 찾을 수 없습니다."));
+        Lesson lesson = lessonRepository.findByIdForUpdate(target.getLessonId())
+                .orElseThrow(() -> error(RegistrationErrorCode.LESSON_NOT_FOUND, "강의를 찾을 수 없습니다."));
+        UserLesson registration = userLessonRepository.findById(registrationId)
+                .orElseThrow(() -> error(RegistrationErrorCode.REGISTRATION_NOT_FOUND, "수강신청 건을 찾을 수 없습니다."));
+
+        if (registration.isCanceled()) {
+            throw error(RegistrationErrorCode.ALREADY_CANCELED, "이미 취소된 수강신청입니다.");
+        }
+        var now = clock.instant();
+        validator.validateRegistrationPeriod(lesson, now);
+        registration.cancel(now);
+    }
+
+    private Semester resolveSemester(String semesterId) {
+        if (semesterId != null && !semesterId.isBlank()) {
+            return semesterRepository.findById(semesterId)
+                    .orElseThrow(() -> error(RegistrationErrorCode.SEMESTER_NOT_FOUND, "학기를 찾을 수 없습니다."));
+        }
+        LocalDate today = LocalDate.now(clock);
+        return semesterRepository
+                .findFirstByStartDateLessThanEqualAndEndDateGreaterThanEqualOrderByStartDateDesc(today, today)
+                .orElseThrow(() -> error(RegistrationErrorCode.SEMESTER_NOT_FOUND, "현재 진행 중인 학기를 찾을 수 없습니다."));
+    }
+
+    private Map<String, List<LessonSchedule>> loadSchedules(List<UserLesson> registrations, Lesson candidate) {
+        Set<String> lessonIds = registrations.stream()
+                .map(UserLesson::getLesson)
+                .map(Lesson::getId)
+                .collect(Collectors.toCollection(LinkedHashSet::new));
+        lessonIds.add(candidate.getId());
+        return groupSchedules(lessonScheduleRepository.findByLesson_IdInOrderByDayOfWeekAscStartTimeAsc(lessonIds));
+    }
+
+    private static Map<String, List<LessonSchedule>> groupSchedules(List<LessonSchedule> schedules) {
+        return schedules.stream().collect(Collectors.groupingBy(
+                schedule -> schedule.getLesson().getId(),
+                LinkedHashMap::new,
+                Collectors.toList()));
+    }
+
+    private static RegistrationException error(RegistrationErrorCode code, String message) {
+        return new RegistrationException(code, message);
     }
 }
-// 저장 전 log
-// 검증 로직
-// 

src/main/java/com/study/course_registration/service/LessonQueryService.java

@@ -0,0 +1,126 @@
+package com.study.course_registration.service;
+
+import java.time.Clock;
+import java.time.LocalDate;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+import org.springframework.data.domain.Page;
+import org.springframework.data.domain.PageRequest;
+import org.springframework.data.domain.Pageable;
+import org.springframework.data.domain.Sort;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import com.study.course_registration.dto.ScheduleResponse;
+import com.study.course_registration.dto.lesson.LessonDetailResponse;
+import com.study.course_registration.dto.lesson.LessonItemResponse;
+import com.study.course_registration.dto.lesson.LessonListResponse;
+import com.study.course_registration.entity.Lesson;
+import com.study.course_registration.entity.LessonSchedule;
+import com.study.course_registration.entity.Semester;
+import com.study.course_registration.enums.RegistrationErrorCode;
+import com.study.course_registration.exception.RegistrationException;
+import com.study.course_registration.repository.LessonRepository;
+import com.study.course_registration.repository.LessonScheduleRepository;
+import com.study.course_registration.repository.SemesterRepository;
+import com.study.course_registration.repository.UserLessonRepository;
+
+@Service
+@Transactional(readOnly = true)
+public class LessonQueryService {
+    private final Clock clock;
+    private final LessonRepository lessonRepository;
+    private final LessonScheduleRepository lessonScheduleRepository;
+    private final UserLessonRepository userLessonRepository;
+    private final SemesterRepository semesterRepository;
+
+    public LessonQueryService(Clock clock,
+                              LessonRepository lessonRepository,
+                              LessonScheduleRepository lessonScheduleRepository,
+                              UserLessonRepository userLessonRepository,
+                              SemesterRepository semesterRepository) {
+        this.clock = clock;
+        this.lessonRepository = lessonRepository;
+        this.lessonScheduleRepository = lessonScheduleRepository;
+        this.userLessonRepository = userLessonRepository;
+        this.semesterRepository = semesterRepository;
+    }
+
+    public LessonListResponse getLessons(String semesterId, String subjectId, int page, int size) {
+        Semester semester = resolveSemester(semesterId);
+        Pageable pageable = PageRequest.of(page, size, Sort.by(Sort.Order.asc("name"), Sort.Order.asc("id")));
+        Page<Lesson> lessonPage = subjectId == null || subjectId.isBlank()
+                ? lessonRepository.findBySemester_Id(semester.getId(), pageable)
+                : lessonRepository.findBySemester_IdAndSubject_Id(semester.getId(), subjectId, pageable);
+
+        List<String> lessonIds = lessonPage.getContent().stream().map(Lesson::getId).toList();
+        Map<String, List<LessonSchedule>> schedules = loadSchedules(lessonIds);
+        Map<String, Long> counts = loadCounts(lessonIds);
+
+        List<LessonItemResponse> items = lessonPage.getContent().stream()
+                .map(lesson -> toItem(lesson, schedules.getOrDefault(lesson.getId(), List.of()), counts.getOrDefault(lesson.getId(), 0L)))
+                .toList();
+
+        return new LessonListResponse(page, size, lessonPage.getTotalElements(), items);
+    }
+
+    public LessonDetailResponse getLesson(String lessonId) {
+        Lesson lesson = lessonRepository.findDetailedById(lessonId)
+                .orElseThrow(() -> error(RegistrationErrorCode.LESSON_NOT_FOUND, "강의를 찾을 수 없습니다."));
+        List<ScheduleResponse> schedules = lessonScheduleRepository.findByLesson_IdOrderByDayOfWeekAscStartTimeAsc(lessonId)
+                .stream().map(ScheduleResponse::from).toList();
+        long enrolledCount = userLessonRepository.countByLesson_IdAndCanceledAtIsNull(lessonId);
+
+        return new LessonDetailResponse(
+                lesson.getId(), lesson.getName(), lesson.getSubject().getCode(), lesson.getSubject().getName(),
+                lesson.getSubject().getDescription(), lesson.getSubject().getCredit(), lesson.getProfessor().getName(),
+                lesson.getSemester().getName(), lesson.getCapacity(), enrolledCount, lesson.getMinGrade(), lesson.getAllowedRole(),
+                lesson.getSemester().getRegistrationStartAt(), lesson.getSemester().getRegistrationEndAt(), schedules);
+    }
+
+    private LessonItemResponse toItem(Lesson lesson, List<LessonSchedule> schedules, long enrolledCount) {
+        return new LessonItemResponse(
+                lesson.getId(), lesson.getName(), lesson.getSubject().getCode(), lesson.getSubject().getName(),
+                lesson.getSubject().getCredit(), lesson.getProfessor().getName(), lesson.getCapacity(), enrolledCount,
+                lesson.getMinGrade(), lesson.getAllowedRole(), schedules.stream().map(ScheduleResponse::from).toList());
+    }
+
+    private Map<String, List<LessonSchedule>> loadSchedules(List<String> lessonIds) {
+        if (lessonIds.isEmpty()) {
+            return Map.of();
+        }
+        return lessonScheduleRepository.findByLesson_IdInOrderByDayOfWeekAscStartTimeAsc(lessonIds).stream()
+                .collect(Collectors.groupingBy(
+                        schedule -> schedule.getLesson().getId(), LinkedHashMap::new, Collectors.toList()));
+    }
+
+    private Map<String, Long> loadCounts(List<String> lessonIds) {
+        if (lessonIds.isEmpty()) {
+            return Map.of();
+        }
+        return userLessonRepository.countActiveByLessonIds(lessonIds).stream()
+                .collect(Collectors.toMap(
+                        UserLessonRepository.LessonEnrollmentCount::getLessonId,
+                        UserLessonRepository.LessonEnrollmentCount::getEnrolledCount,
+                        (left, right) -> left,
+                        LinkedHashMap::new));
+    }
+
+    private Semester resolveSemester(String semesterId) {
+        if (semesterId != null && !semesterId.isBlank()) {
+            return semesterRepository.findById(semesterId)
+                    .orElseThrow(() -> error(RegistrationErrorCode.SEMESTER_NOT_FOUND, "학기를 찾을 수 없습니다."));
+        }
+        LocalDate today = LocalDate.now(clock);
+        return semesterRepository.findFirstByStartDateLessThanEqualAndEndDateGreaterThanEqualOrderByStartDateDesc(today, today)
+                .orElseThrow(() -> error(RegistrationErrorCode.SEMESTER_NOT_FOUND, "현재 진행 중인 학기를 찾을 수 없습니다."));
+    }
+
+    private static RegistrationException error(RegistrationErrorCode code, String message) {
+        return new RegistrationException(code, message);
+    }
+}

src/main/java/com/study/course_registration/service/policy/RegistrationValidator.java

@@ -0,0 +1,100 @@
+package com.study.course_registration.service.policy;
+
+import java.time.Instant;
+import java.util.List;
+import java.util.Map;
+
+import org.springframework.stereotype.Component;
+
+import com.study.course_registration.entity.Lesson;
+import com.study.course_registration.entity.LessonSchedule;
+import com.study.course_registration.entity.User;
+import com.study.course_registration.entity.UserLesson;
+import com.study.course_registration.enums.RegistrationErrorCode;
+import com.study.course_registration.exception.RegistrationException;
+
+@Component
+public class RegistrationValidator {
+    private final ScheduleConflictChecker scheduleConflictChecker;
+
+    public RegistrationValidator(ScheduleConflictChecker scheduleConflictChecker) {
+        this.scheduleConflictChecker = scheduleConflictChecker;
+    }
+
+    public void validatePreconditions(User user, Lesson lesson, Instant now) {
+        validateRegistrationPeriod(lesson, now);
+        if (lesson.getMinGrade() != null && user.getGrade() < lesson.getMinGrade()) {
+            throw error(RegistrationErrorCode.NOT_ELIGIBLE_GRADE,
+                    "수강 가능한 학년이 아닙니다. (현재 " + user.getGrade() + "학년, 최소 " + lesson.getMinGrade() + "학년)");
+        }
+        if (lesson.getAllowedRole() != null && user.getRole() != lesson.getAllowedRole()) {
+            throw error(RegistrationErrorCode.NOT_ELIGIBLE_ROLE,
+                    "수강 가능한 역할이 아닙니다. (필요 역할: " + lesson.getAllowedRole() + ")");
+        }
+    }
+
+
+    public void validateRegistrationPeriod(Lesson lesson, Instant now) {
+        if (now.isBefore(lesson.getSemester().getRegistrationStartAt())
+                || now.isAfter(lesson.getSemester().getRegistrationEndAt())) {
+            throw error(RegistrationErrorCode.REGISTRATION_PERIOD_CLOSED, "수강신청 기간이 아닙니다.");
+        }
+    }
+
+    public void validateLocked(Lesson lesson,
+                               List<UserLesson> activeRegistrations,
+                               Map<String, List<LessonSchedule>> schedulesByLessonId,
+                               long enrolledCount) {
+        if (activeRegistrations.stream().anyMatch(registration -> registration.getLesson().getId().equals(lesson.getId()))) {
+            throw error(RegistrationErrorCode.ALREADY_REGISTERED, "이미 신청한 강의입니다.");
+        }
+
+        if (activeRegistrations.stream().anyMatch(registration ->
+                registration.getLesson().getSubject().getId().equals(lesson.getSubject().getId()))) {
+            throw error(RegistrationErrorCode.DUPLICATE_SUBJECT, "같은 과목의 다른 분반을 이미 신청했습니다.");
+        }
+
+        List<LessonSchedule> candidateSchedules = schedulesByLessonId.getOrDefault(lesson.getId(), List.of());
+        for (UserLesson registration : activeRegistrations) {
+            List<LessonSchedule> existingSchedules = schedulesByLessonId.getOrDefault(registration.getLesson().getId(), List.of());
+            for (LessonSchedule candidate : candidateSchedules) {
+                for (LessonSchedule existing : existingSchedules) {
+                    if (scheduleConflictChecker.overlaps(candidate, existing)) {
+                        throw error(RegistrationErrorCode.SCHEDULE_CONFLICT,
+                                "기존 수업과 시간이 겹칩니다. (" + candidate.getDayOfWeek() + " "
+                                        + candidate.getStartTime() + "-" + candidate.getEndTime() + ")");
+                    }
+                }
+            }
+        }
+
+        int currentCredits = currentCredits(activeRegistrations);
+        int requestedCredits = lesson.getSubject().getCredit();
+        int maxCredits = lesson.getSemester().getMaxCredits();
+        if (currentCredits + requestedCredits > maxCredits) {
+            throw error(RegistrationErrorCode.CREDIT_LIMIT_EXCEEDED,
+                    "신청 학점이 상한을 넘습니다. (" + currentCredits + " + " + requestedCredits + " > " + maxCredits + ")");
+        }
+
+        if (enrolledCount >= lesson.getCapacity()) {
+            throw error(RegistrationErrorCode.CAPACITY_EXCEEDED,
+                    "정원이 가득 찼습니다. (" + enrolledCount + "/" + lesson.getCapacity() + ")");
+        }
+    }
+
+    public int totalCreditsAfter(Lesson lesson, List<UserLesson> activeRegistrations) {
+        return currentCredits(activeRegistrations) + lesson.getSubject().getCredit();
+    }
+
+    public int currentCredits(List<UserLesson> activeRegistrations) {
+        return activeRegistrations.stream()
+                .map(UserLesson::getLesson)
+                .map(Lesson::getSubject)
+                .mapToInt(subject -> subject.getCredit())
+                .sum();
+    }
+
+    private static RegistrationException error(RegistrationErrorCode code, String message) {
+        return new RegistrationException(code, message);
+    }
+}

src/main/java/com/study/course_registration/service/policy/ScheduleConflictChecker.java

@@ -0,0 +1,14 @@
+package com.study.course_registration.service.policy;
+
+import org.springframework.stereotype.Component;
+
+import com.study.course_registration.entity.LessonSchedule;
+
+@Component
+public class ScheduleConflictChecker {
+    public boolean overlaps(LessonSchedule left, LessonSchedule right) {
+        return left.getDayOfWeek() == right.getDayOfWeek()
+                && left.getStartTime().isBefore(right.getEndTime())
+                && right.getStartTime().isBefore(left.getEndTime());
+    }
+}