@@ -0,0 +1,93 @@
+package nextstep.security.access;
+
+import nextstep.security.core.GrantedAuthority;
+import nextstep.security.core.SimpleGrantedAuthority;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.Collection;
+import java.util.Set;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class RoleHierarchyTest {
+
+    @Test
+    @DisplayName("역할 계층이 없을 때 입력된 권한만 반환 한다.")
+    void getReachableGrantedAuthorities_NoHierarchy() {
+        // Given
+        RoleHierarchy roleHierarchy = new NullRoleHierarchy();
+        Collection<GrantedAuthority> authorities = Set.of(new SimpleGrantedAuthority("USER"));
+
+        // When
+        Collection<GrantedAuthority> reachableAuthorities = roleHierarchy.getReachableGrantedAuthorities(authorities);
+
+        // Then
+        assertThat(reachableAuthorities).hasSize(1);
+        assertThat(reachableAuthorities).contains(new SimpleGrantedAuthority("USER"));
+    }
+
+    @Test
+    @DisplayName("역할 계층이 존재 시 모든 하위 권한 반환 한다.")
+    void getReachableGrantedAuthorities_WithHierarchy() {
+        // Given
+        RoleHierarchy roleHierarchy = RoleHierarchyImpl.with()
+                .role("ADMIN").implies("USER")
+                .role("USER").implies("GUEST")
+                .build();
+        Collection<GrantedAuthority> authorities = Set.of(new SimpleGrantedAuthority("ADMIN"));
+
+        // When
+        Collection<GrantedAuthority> reachableAuthorities = roleHierarchy.getReachableGrantedAuthorities(authorities);
+
+        // Then
+        assertThat(reachableAuthorities).hasSize(3);
+        assertThat(reachableAuthorities).containsExactlyInAnyOrder(
+                new SimpleGrantedAuthority("ADMIN"),
+                new SimpleGrantedAuthority("USER"),
+                new SimpleGrantedAuthority("GUEST")
+        );
+    }
+
+    @Test
+    @DisplayName("계층 간 순환 참조 발생 시 예외가 발생한다.")
+    void shouldThrowExceptionForCircularRoleReferences() {
+        assertThatThrownBy(() -> RoleHierarchyImpl.with()
+                .role("ADMIN").implies("USER")
+                .role("USER").implies("GUEST")
+                .role("GUEST").implies("USER")
+                .build())
+                .isInstanceOf(CycleInRoleHierarchyException.class);
+    }
+
+    @Test
+    @DisplayName("빈 권한 리스트 입력 시 빈 결과 반환")
+    void getReachableGrantedAuthorities_EmptyInput() {
+        // Given
+        RoleHierarchy roleHierarchy = RoleHierarchyImpl.with()
+                .role("ADMIN").implies("USER")
+                .role("USER").implies("GUEST")
+                .build();
+        // When
+        Collection<GrantedAuthority> reachableAuthorities = roleHierarchy.getReachableGrantedAuthorities(Set.of());
+
+        // Then
+        assertThat(reachableAuthorities).isEmpty();
+    }
+
+    @Test
+    @DisplayName("Null 입력 시 빈 결과 반환")
+    void getReachableGrantedAuthorities_NullInput() {
+        // Given
+        RoleHierarchy roleHierarchy = RoleHierarchyImpl.with()
+                .role("ADMIN").implies("USER")
+                .role("USER").implies("GUEST")
+                .build();
+        // When
+        Collection<GrantedAuthority> reachableAuthorities = roleHierarchy.getReachableGrantedAuthorities(null);
+
+        // Then
+        assertThat(reachableAuthorities).isEmpty();
+    }
+}
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리뷰: 순환참조 예외 테스트까지 잘 추가해주셨네요 👍