Add proper unit tests for double-checked-locking pattern

double-checked-locking/pom.xml

@@ -12,5 +12,10 @@
       <artifactId>junit</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-core</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 </project>

double-checked-locking/src/main/java/com/iluwatar/doublechecked/locking/Inventory.java

@@ -1,6 +1,7 @@
 package com.iluwatar.doublechecked.locking;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
@@ -38,4 +39,14 @@ public class Inventory {
     }
     return false;
   }
+
+  /**
+   * Get all the items in the inventory
+   *
+   * @return All the items of the inventory, as an unmodifiable list
+   */
+  public final List<Item> getItems() {
+    return Collections.unmodifiableList(items);
+  }
+
 }

double-checked-locking/src/test/java/com/iluwatar/doublechecked/locking/InventoryTest.java

@@ -0,0 +1,110 @@
+package com.iluwatar.doublechecked.locking;
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+import org.mockito.ArgumentCaptor;
+
+import java.io.PrintStream;
+import java.util.List;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+
+import static junit.framework.Assert.assertTrue;
+import static junit.framework.TestCase.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoMoreInteractions;
+
+/**
+ * Date: 12/10/15 - 9:34 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public class InventoryTest {
+
+  /**
+   * The mocked standard out {@link PrintStream}, used to verify a steady increasing size of the
+   * {@link Inventory} while adding items from multiple threads concurrently
+   */
+  private final PrintStream stdOutMock = mock(PrintStream.class);
+
+  /**
+   * Keep the original std-out so it can be restored after the test
+   */
+  private final PrintStream stdOutOrig = System.out;
+
+  /**
+   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
+   */
+  @Before
+  public void setUp() {
+    System.setOut(this.stdOutMock);
+  }
+
+  /**
+   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
+   */
+  @After
+  public void tearDown() {
+    System.setOut(this.stdOutOrig);
+  }
+
+  /**
+   * The number of threads used to stress test the locking of the {@link Inventory#addItem(Item)}
+   * method
+   */
+  private static final int THREAD_COUNT = 8;
+
+  /**
+   * The maximum number of {@link Item}s allowed in the {@link Inventory}
+   */
+  private static final int INVENTORY_SIZE = 1000;
+
+  /**
+   * Concurrently add multiple items to the inventory, and check if the items were added in order by
+   * checking the stdOut for continuous growth of the inventory. When 'items.size()=xx' shows up out
+   * of order, it means that the locking is not ok, increasing the risk of going over the inventory
+   * item limit.
+   */
+  @Test(timeout = 10000)
+  public void testAddItem() throws Exception {
+    // Create a new inventory with a limit of 1000 items and put some load on the add method
+    final Inventory inventory = new Inventory(INVENTORY_SIZE);
+    final ExecutorService executorService = Executors.newFixedThreadPool(THREAD_COUNT);
+    for (int i = 0; i < THREAD_COUNT; i++) {
+      executorService.execute(() -> {
+        while (inventory.addItem(new Item())) ;
+      });
+    }
+
+    // Wait until all threads have finished
+    executorService.shutdown();
+    executorService.awaitTermination(5, TimeUnit.SECONDS);
+
+    // Check the number of items in the inventory. It should not have exceeded the allowed maximum
+    final List<Item> items = inventory.getItems();
+    assertNotNull(items);
+    assertEquals(INVENTORY_SIZE, items.size());
+
+    // Capture all stdOut messages ...
+    final ArgumentCaptor<String> stdOutCaptor = ArgumentCaptor.forClass(String.class);
+    verify(this.stdOutMock, times(INVENTORY_SIZE)).println(stdOutCaptor.capture());
+
+    // ... verify if we got all 1000
+    final List<String> values = stdOutCaptor.getAllValues();
+    assertEquals(INVENTORY_SIZE, values.size());
+
+    // ... and check if the inventory size is increasing continuously
+    for (int i = 0; i < values.size(); i++) {
+      assertNotNull(values.get(i));
+      assertTrue(values.get(i).contains("items.size()=" + (i + 1)));
+    }
+
+    verifyNoMoreInteractions(this.stdOutMock);
+  }
+
+}