BAEL-7002 - Drawbacks of Singleton Pattern (#14979)

* BAEL-7002 - Drawbacks of Singleton Pattern

* Add mockito-inline dependency

patterns-modules/design-patterns-creational-2/pom.xml

@@ -12,4 +12,13 @@
         <version>1.0.0-SNAPSHOT</version>
     </parent>
 
+    <dependencies>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-inline</artifactId>
+            <version>${mockito.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
 </project>

patterns-modules/design-patterns-creational-2/src/main/java/com/baeldung/singleton/Logger.java

@@ -0,0 +1,38 @@
+package com.baeldung.singleton;
+
+import java.io.FileWriter;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.time.LocalDateTime;
+
+public class Logger {
+    private static volatile Logger instance;
+
+    private PrintWriter fileWriter;
+
+    public static Logger getInstance() {
+        if (instance == null) {
+            synchronized (Logger.class) {
+                if (instance == null) {
+                    instance = new Logger();
+                }
+            }
+        }
+        return instance;
+    }
+
+    private Logger() {
+        try {
+            fileWriter = new PrintWriter(new FileWriter("app.log"));
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+    }
+
+    public void log(String message) {
+        String log = String.format("[%s]- %s", LocalDateTime.now(), message);
+        fileWriter.println(log);
+        fileWriter.flush();
+    }
+
+}

patterns-modules/design-patterns-creational-2/src/main/java/com/baeldung/singleton/SingletonDemo.java

@@ -0,0 +1,12 @@
+package com.baeldung.singleton;
+
+public class SingletonDemo {
+
+    public int sum(int a, int b) {
+        int result = a + b;
+        Logger logger = Logger.getInstance();
+        logger.log("The sum is " + result);
+        return result;
+    }
+
+}

patterns-modules/design-patterns-creational-2/src/test/java/com/baeldung/singleton/SingletonUnitTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.singleton;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.mockito.MockedStatic;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.doNothing;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.mockStatic;
+
+class SingletonUnitTest {
+
+    @Test
+    void givenTwoValues_whenSum_thenReturnCorrectResult() {
+        SingletonDemo singletonDemo = new SingletonDemo();
+        int result = singletonDemo.sum(12, 4);
+        Assertions.assertEquals(16, result);
+    }
+
+    @Test
+    void givenMockedLogger_whenSum_thenReturnCorrectResult() {
+        Logger logger = mock(Logger.class);
+
+        try (MockedStatic<Logger> loggerMockedStatic = mockStatic(Logger.class)) {
+            loggerMockedStatic.when(Logger::getInstance).thenReturn(logger);
+            doNothing().when(logger).log(any());
+
+            SingletonDemo singletonDemo = new SingletonDemo();
+            int result = singletonDemo.sum(12, 4);
+            Assertions.assertEquals(16, result);
+        }
+    }
+
+}