diff --git a/src/language_java/BeforeOrderService.java b/src/language_java/BeforeOrderService.java
new file mode 100644
index 0000000..4cce537
--- /dev/null
+++ b/src/language_java/BeforeOrderService.java
@@ -0,0 +1,42 @@
+package language_java;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class BeforeOrderService {
+
+    private final Map<String, Integer> productDatabase = new HashMap<>();
+    private final Map<String, OrderInfo> latestOrderDatabase = new HashMap<>();
+
+    public BeforeOrderService() {
+        productDatabase.put("apple", 100);
+        productDatabase.put("banana", 50);
+        productDatabase.put("orange", 75);
+    }
+
+    public void order(String productName, int amount) {
+        Integer currentStock = getStock(productName);
+
+        simulateDelay();
+
+        if (currentStock >= amount) {
+            System.out.println("Current Thread : " + Thread.currentThread().getName() +
+                    " - CurrentStock : " + currentStock + " - Order : " + amount);
+            productDatabase.put(productName, currentStock - amount);
+            latestOrderDatabase.put(productName, new OrderInfo(productName, amount, System.currentTimeMillis()));
+        }
+    }
+
+    public Integer getStock(String productName) {
+        return productDatabase.getOrDefault(productName, 0);
+    }
+
+    private static void simulateDelay() {
+        try {
+            Thread.sleep(5);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            throw new RuntimeException(e);
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/language_java/OrderInfo.java b/src/language_java/OrderInfo.java
new file mode 100644
index 0000000..2929e63
--- /dev/null
+++ b/src/language_java/OrderInfo.java
@@ -0,0 +1,14 @@
+package language_java;
+
+public class OrderInfo {
+    String productName;
+    Integer amount;
+    Long timestamp;
+
+    OrderInfo(String productName, Integer amount, Long timestamp) {
+        this.productName = productName;
+        this.amount = amount;
+        this.timestamp = timestamp;
+    }
+
+}
diff --git a/src/language_java/OrderService1.java b/src/language_java/OrderService1.java
new file mode 100644
index 0000000..d29e617
--- /dev/null
+++ b/src/language_java/OrderService1.java
@@ -0,0 +1,57 @@
+package language_java;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class OrderService1 {
+
+    private final Map<String, Integer> productDatabase = new HashMap<>();
+    private final Map<String, OrderInfo> latestOrderDatabase = new HashMap<>();
+
+    public OrderService1() {
+        productDatabase.put("apple", 100);
+        productDatabase.put("banana", 50);
+        productDatabase.put("orange", 75);
+    }
+
+    public synchronized void orderUsingSynchronized(String productName, int amount) {
+        Integer currentStock = getStock(productName);
+
+        simulateDelay();
+
+        if (currentStock >= amount) {
+            System.out.println("Current Thread : " + Thread.currentThread().getName() +
+                    " - CurrentStock : " + currentStock + " - Order : " + amount);
+            productDatabase.put(productName, currentStock - amount);
+            latestOrderDatabase.put(productName, new OrderInfo(productName, amount, System.currentTimeMillis()));
+        }
+    }
+
+    public void orderUsingSynchronized2(String productName, int amount) {
+        synchronized (this) {
+            Integer currentStock = getStock(productName);
+
+            simulateDelay();
+
+            if (currentStock >= amount) {
+                System.out.println("Current Thread : " + Thread.currentThread().getName() +
+                        " - CurrentStock : " + currentStock + " - Order : " + amount);
+                productDatabase.put(productName, currentStock - amount);
+                latestOrderDatabase.put(productName, new OrderInfo(productName, amount, System.currentTimeMillis()));
+            }
+        }
+    }
+
+    public Integer getStock(String productName) {
+        return productDatabase.getOrDefault(productName, 0);
+    }
+
+    private static void simulateDelay() {
+        try {
+            Thread.sleep(5);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            throw new RuntimeException(e);
+        }
+    }
+}
diff --git a/src/language_java/OrderService2.java b/src/language_java/OrderService2.java
new file mode 100644
index 0000000..1e1e96f
--- /dev/null
+++ b/src/language_java/OrderService2.java
@@ -0,0 +1,50 @@
+package language_java;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class OrderService2 {
+
+    private final Map<String, AtomicInteger> productDatabase = new HashMap<>();
+    private final Map<String, OrderInfo> latestOrderDatabase = new HashMap<>();
+
+    public OrderService2() {
+        productDatabase.put("apple", new AtomicInteger(100));
+        productDatabase.put("banana", new AtomicInteger(50));
+        productDatabase.put("orange", new AtomicInteger(75));
+    }
+
+    public void order(String productName, int amount) {
+        AtomicInteger currentStock = getStock(productName);
+
+        try {
+            Thread.sleep(5);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            throw new RuntimeException(e);
+        }
+
+        while (true) {
+            System.out.println("Current Thread : " + Thread.currentThread().getName() +
+                    " - currentStock : " + currentStock + " - Order : " + amount);
+            int current = currentStock.get();
+            if (current < amount) {
+                return;
+            }
+
+            boolean success = currentStock.compareAndSet(current, current - amount);
+            if (success) {
+                System.out.println("Current Thread : " + Thread.currentThread().getName() +
+                        " - Stock before order: " + current + " - Order: " + amount +
+                        " - Stock after order: " + currentStock.get());
+                latestOrderDatabase.put(productName, new OrderInfo(productName, amount, System.currentTimeMillis()));
+                break;
+            }
+        }
+    }
+
+    public AtomicInteger getStock(String productName) {
+        return productDatabase.getOrDefault(productName, new AtomicInteger(0));
+    }
+}
diff --git a/src/language_java/OrderService3.java b/src/language_java/OrderService3.java
new file mode 100644
index 0000000..87dc30e
--- /dev/null
+++ b/src/language_java/OrderService3.java
@@ -0,0 +1,39 @@
+package language_java;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class OrderService3 {
+
+    private final Map<String, Integer> productDatabase = new ConcurrentHashMap<>();
+    private final Map<String, OrderInfo> latestOrderDatabase = new ConcurrentHashMap<>();
+
+    public OrderService3() {
+        productDatabase.put("apple", 100);
+        productDatabase.put("banana", 50);
+        productDatabase.put("orange", 75);
+    }
+
+    public void order(String productName, int amount) {
+        try {
+            Thread.sleep(5);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            throw new RuntimeException(e);
+        }
+
+        productDatabase.compute(productName, (key, currentStock) -> {
+            if (currentStock == null || currentStock < amount) {
+                return currentStock;
+            }
+            System.out.println("Current Thread : " + Thread.currentThread().getName() +
+                    " - CurrentStock : " + currentStock + " - Order : " + amount);
+            latestOrderDatabase.put(productName, new OrderInfo(productName, amount, System.currentTimeMillis()));
+            return currentStock - amount;
+        });
+    }
+
+    public int getStock(String productName) {
+        return productDatabase.getOrDefault(productName, 0);
+    }
+}
diff --git a/test/language_java/BeforeOrderServiceTest.java b/test/language_java/BeforeOrderServiceTest.java
new file mode 100644
index 0000000..fbbae3e
--- /dev/null
+++ b/test/language_java/BeforeOrderServiceTest.java
@@ -0,0 +1,49 @@
+package language_java;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+
+@DisplayName("여러 스레드가 동시에 주문을 요청할 때")
+class BeforeOrderServiceTest {
+
+    private final BeforeOrderService service = new BeforeOrderService();
+
+    @Test
+    @DisplayName("동시성 이슈로 재고 불일치가 발생한다.")
+    void givenMultipleThreads_whenConcurrentOrders_thenNotEqualsExpectedStock() throws InterruptedException {
+        String productName = "apple";
+        int initialStock = service.getStock(productName);
+
+        int orderAmount = 8;
+        int threadCount = 100;
+
+        ExecutorService executor = Executors.newFixedThreadPool(threadCount);
+        CountDownLatch latch = new CountDownLatch(threadCount);
+
+        for (int i = 0; i < threadCount; i++) {
+            executor.execute(() -> {
+                try {
+                    service.order(productName, orderAmount);
+                } finally {
+                    latch.countDown();
+                }
+            });
+        }
+
+        latch.await();
+        executor.shutdown();
+
+        int expectedStock = initialStock % orderAmount;
+        int actualStock = service.getStock(productName);
+
+        System.out.println("Expected Stock : " + expectedStock + ", Actual Stock : " + actualStock);
+
+        assertNotEquals(expectedStock, actualStock);
+    }
+}
diff --git a/test/language_java/OrderService1Test.java b/test/language_java/OrderService1Test.java
new file mode 100644
index 0000000..9ccb212
--- /dev/null
+++ b/test/language_java/OrderService1Test.java
@@ -0,0 +1,83 @@
+package language_java;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+@DisplayName("여러 스레드가 동시에 주문을 요청할 때")
+public class OrderService1Test {
+
+    private final OrderService1 service = new OrderService1();
+
+
+    @Test
+    @DisplayName("instance 에 synchronized 키워드를 사용하면, 재고가 일치한다.")
+    void givenMultipleThreads_whenSynchronizedMultipleThreads_thenEqualsExpectedStock() throws InterruptedException {
+        String productName = "apple";
+        int initialStock = service.getStock(productName);
+
+        int orderAmount = 8;
+        int threadCount = 100;
+
+        ExecutorService executor = Executors.newFixedThreadPool(threadCount);
+        CountDownLatch latch = new CountDownLatch(threadCount);
+
+        for (int i = 0; i < threadCount; i++) {
+            executor.execute(() -> {
+                try {
+                    service.orderUsingSynchronized(productName, orderAmount);
+                } finally {
+                    latch.countDown();
+                }
+            });
+        }
+
+        latch.await();
+        executor.shutdown();
+
+        int expectedStock = initialStock % orderAmount;
+        int actualStock = service.getStock(productName);
+
+        System.out.println("Expected Stock : " + expectedStock + ", Actual Stock : " + actualStock);
+
+        assertEquals(expectedStock, actualStock);
+    }
+
+    @Test
+    @DisplayName("block 에 synchronized 키워드를 사용하면, 재고가 일치한다.")
+    void givenMultipleThreads_whenSynchronizedBlockMultipleThreads_thenEqualsExpectedStock() throws InterruptedException {
+        String productName = "apple";
+        int initialStock = service.getStock(productName);
+
+        int orderAmount = 8;
+        int threadCount = 100;
+
+        ExecutorService executor = Executors.newFixedThreadPool(threadCount);
+        CountDownLatch latch = new CountDownLatch(threadCount);
+
+        for (int i = 0; i < threadCount; i++) {
+            executor.execute(() -> {
+                try {
+                    service.orderUsingSynchronized2(productName, orderAmount);
+                } finally {
+                    latch.countDown();
+                }
+            });
+        }
+
+        latch.await();
+        executor.shutdown();
+
+        int expectedStock = initialStock % orderAmount;
+        int actualStock = service.getStock(productName);
+
+        System.out.println("Expected Stock : " + expectedStock + ", Actual Stock : " + actualStock);
+
+        assertEquals(expectedStock, actualStock);
+    }
+}
diff --git a/test/language_java/OrderService2Test.java b/test/language_java/OrderService2Test.java
new file mode 100644
index 0000000..4516524
--- /dev/null
+++ b/test/language_java/OrderService2Test.java
@@ -0,0 +1,49 @@
+package language_java;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+@DisplayName("여러 스레드가 동시에 주문을 요청할 때")
+public class OrderService2Test {
+
+    private final OrderService2 service = new OrderService2();
+
+    @Test
+    @DisplayName("AtomicInteger 을 사용하면 재고가 일치한다.")
+    void givenMultipleThreads_whenUsingAtomicInteger_thenEqualsExpectedStock() throws InterruptedException {
+        String productName = "apple";
+        int initialStock = service.getStock(productName).get();
+
+        int orderAmount = 8;
+        int threadCount = 100;
+
+        ExecutorService executor = Executors.newFixedThreadPool(threadCount);
+        CountDownLatch latch = new CountDownLatch(threadCount);
+
+        for (int i = 0; i < threadCount; i++) {
+            executor.execute(() -> {
+                try {
+                    service.order(productName, orderAmount);
+                } finally {
+                    latch.countDown();
+                }
+            });
+        }
+
+        latch.await();
+        executor.shutdown();
+
+        int expectedStock = initialStock % orderAmount;
+        int actualStock = service.getStock(productName).get();
+
+        System.out.println("Expected Stock : " + expectedStock + ", Actual Stock : " + actualStock);
+
+        assertEquals(expectedStock, actualStock);
+    }
+}
diff --git a/test/language_java/OrderService3Test.java b/test/language_java/OrderService3Test.java
new file mode 100644
index 0000000..8f27823
--- /dev/null
+++ b/test/language_java/OrderService3Test.java
@@ -0,0 +1,49 @@
+package language_java;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+@DisplayName("여러 스레드가 동시에 주문을 요청할 때")
+public class OrderService3Test {
+
+    private final OrderService3 service = new OrderService3();
+
+    @Test
+    @DisplayName("ConcurrentHashMap 을 사용하면, 재고가 일치한다.")
+    void givenMultipleThreads_whenUsingConcurrentHashMap_thenEqualsExpectedStock() throws InterruptedException {
+        String productName = "apple";
+        int initialStock = service.getStock(productName);
+
+        int orderAmount = 8;
+        int threadCount = 100;
+
+        ExecutorService executor = Executors.newFixedThreadPool(threadCount);
+        CountDownLatch latch = new CountDownLatch(threadCount);
+
+        for (int i = 0; i < threadCount; i++) {
+            executor.execute(() -> {
+                try {
+                    service.order(productName, orderAmount);
+                } finally {
+                    latch.countDown();
+                }
+            });
+        }
+
+        latch.await();
+        executor.shutdown();
+
+        int expectedStock = initialStock % orderAmount;
+        int actualStock = service.getStock(productName);
+
+        System.out.println("Expected Stock : " + expectedStock + ", Actual Stock : " + actualStock);
+
+        assertEquals(expectedStock, actualStock);
+    }
+}