feat(test): 상태를 갖는 싱글톤 구현

src/test/java/hello/core/singleton/StatefulService.java

@@ -0,0 +1,15 @@
+package hello.core.singleton;
+
+public class StatefulService {
+
+    private int price; // 상태를 유지하는 필드
+
+    public void order(String name, int price) {
+        System.out.println("name = " + name + " price = " + price);
+        this.price = price; // 여기가 문제!
+    }
+
+    public int getPrice() {
+        return price;
+    }
+}

src/test/java/hello/core/singleton/StatefulServiceTest.java

@@ -0,0 +1,40 @@
+package hello.core.singleton;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.context.annotation.Bean;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.*;
+
+class StatefulServiceTest {
+
+    @Test
+    void statefulServiceSingleton() {
+        ApplicationContext ac = new AnnotationConfigApplicationContext(TestConfig.class);
+        StatefulService statefulService1 = ac.getBean(StatefulService.class);
+        StatefulService statefulService2 = ac.getBean(StatefulService.class);
+
+        // ThreadA: A 사용자가 1만원 주문
+        statefulService1.order("userA", 10000);
+
+        // ThreadB: B 사용자가 2만원 주문
+        statefulService1.order("userB", 20000);
+
+        // ThreadA: A 사용자가 주문 금액 조회
+        int price = statefulService1.getPrice();
+        System.out.println("price = " + price);
+
+        assertThat(statefulService1.getPrice()).isEqualTo(20000);
+    }
+
+    static class TestConfig {
+        @Bean
+        public StatefulService statefulService() {
+            return new StatefulService();
+        }
+    }
+
+
+}