Redefine singleton test scenario and implement new patterns

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/SingletonAtomic.java

@@ -0,0 +1,22 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+public class SingletonAtomic implements SingletonInstance {
+
+    private static final AtomicReference<SingletonAtomic> INSTANCE = new AtomicReference<>();
+    private final long time = System.currentTimeMillis();
+
+    private SingletonAtomic() {
+    }
+
+    public static SingletonAtomic getInstance() {
+        INSTANCE.compareAndSet(null, new SingletonAtomic());
+        return INSTANCE.get();
+    }
+
+    @Override
+    public long getCreationTime() {
+        return time;
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/SingletonEager.java

@@ -0,0 +1,19 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+public class SingletonEager implements SingletonInstance {
+
+    private static final SingletonEager INSTANCE = new SingletonEager();
+    private final long time = System.currentTimeMillis();
+
+    private SingletonEager() {
+    }
+
+    public static SingletonEager getInstance() {
+        return INSTANCE;
+    }
+
+    @Override
+    public long getCreationTime() {
+        return time;
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/SingletonEnum.java

@@ -0,0 +1,12 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+public enum SingletonEnum implements SingletonInstance {
+    INSTANCE;
+
+    private final long time = System.currentTimeMillis();
+
+    @Override
+    public long getCreationTime() {
+        return time;
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/SingletonHolder.java

@@ -0,0 +1,22 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+public class SingletonHolder implements SingletonInstance {
+
+    private final long time = System.currentTimeMillis();
+
+    private SingletonHolder() {
+    }
+
+    public static SingletonHolder getInstance() {
+        return Holder.INSTANCE;
+    }
+
+    @Override
+    public long getCreationTime() {
+        return time;
+    }
+
+    private static class Holder {
+        private static final SingletonHolder INSTANCE = new SingletonHolder();
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/SingletonInstance.java

@@ -0,0 +1,6 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+public interface SingletonInstance {
+
+    long getCreationTime();
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/singleton/SingletonSerializable.java

@@ -0,0 +1,31 @@
+package pl.mperor.lab.java.design.pattern.creational.singleton;
+
+import java.io.ObjectStreamException;
+import java.io.Serial;
+import java.io.Serializable;
+
+public class SingletonSerializable implements Serializable, SingletonInstance {
+
+    @Serial
+    private static final long serialVersionUID = 1L;
+
+    private static final SingletonSerializable INSTANCE = new SingletonSerializable();
+    private final long time = System.currentTimeMillis();
+
+    private SingletonSerializable() {
+    }
+
+    public static SingletonSerializable getInstance() {
+        return INSTANCE;
+    }
+
+    @Override
+    public long getCreationTime() {
+        return time;
+    }
+
+    // Prevent Serialization from creating a new instance
+    protected Object readResolve() throws ObjectStreamException {
+        return getInstance();
+    }
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/creational/singleton/SingletonTest.java

@@ -2,21 +2,105 @@
 
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledIf;
 
-import java.util.concurrent.TimeUnit;
+import java.io.*;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Supplier;
 
+/**
+ * The purpose of this test is to randomly enable one of the test methods to execute.
+ * This is due to the fact that the tests involve the creation of a Singleton object.
+ * Since the Singleton is initialized only once, executing multiple tests could cause issues
+ * because subsequent tests would attempt to initialize the Singleton again, which is not allowed.
+ * By randomly selecting and enabling just one test per run, we avoid such conflicts and ensure
+ * the Singleton is properly handled without reinitialization.
+ */
 public class SingletonTest {
 
+    private static int random = ThreadLocalRandom.current().nextInt(1, 4);
+
+    static boolean isRandom1() {
+        return random == 1;
+    }
+
+    static boolean isRandom2() {
+        return random == 2;
+    }
+
+    static boolean isRandom3() {
+        return random == 3;
+    }
+
+    @EnabledIf(value = "isRandom1")
+    @Test
+    public void testLazyInitialized() throws InterruptedException {
+        assertLazyInitialized(() -> SingletonEnum.INSTANCE);
+        assertLazyInitialized(SingletonEager::getInstance);
+        assertLazyInitialized(SingletonHolder::getInstance);
+        assertLazyInitialized(SingletonAtomic::getInstance);
+        assertLazyInitialized(SingletonSerializable::getInstance);
+    }
+
+    private void assertLazyInitialized(Supplier<SingletonInstance> supplier) throws InterruptedException {
+        long startedTime = System.currentTimeMillis();
+        Thread.sleep(10);
+        var instance = supplier.get();
+        long instanceTime = instance.getCreationTime();
+        Assertions.assertTrue(startedTime < instanceTime);
+    }
+
+    @EnabledIf(value = "isRandom2")
     @Test
-    public void shouldOnlyAllowToCreateOneInstanceOfSingleton() throws InterruptedException {
-        var instance = Singleton.getInstance();
-        long time = instance.getTime();
+    public void testSerializationSafe() throws IOException, ClassNotFoundException {
+        assertSerializationSafe(() -> SingletonEnum.INSTANCE);
+        assertSerializationSafe(SingletonSerializable::getInstance);
+    }
+
+    private void assertSerializationSafe(Supplier<SingletonInstance> supplier) throws IOException, ClassNotFoundException {
+        SingletonInstance instance = supplier.get();
 
-        TimeUnit.MILLISECONDS.sleep(50);
-        long timeAfterBreak = Singleton.getInstance().getTime();
+        var arrayOut = new ByteArrayOutputStream();
+        try (var out = new ObjectOutputStream(arrayOut)) {
+            out.writeObject(instance);
+        }
 
-        Assertions.assertSame(instance, Singleton.getInstance());
-        Assertions.assertEquals(time, timeAfterBreak);
+        SingletonInstance deserializedInstance;
+        try (var in = new ObjectInputStream(new ByteArrayInputStream(arrayOut.toByteArray()))) {
+            deserializedInstance = (SingletonInstance) in.readObject();
+        }
+
+        Assertions.assertSame(instance, deserializedInstance, "Singleton instance should be the same after deserialization");
     }
 
+    @EnabledIf(value = "isRandom3")
+    @Test
+    public void testThreadSafe() {
+        assertThreadSafe(() -> SingletonEnum.INSTANCE);
+        assertThreadSafe(SingletonEager::getInstance);
+        assertThreadSafe(SingletonHolder::getInstance);
+        assertThreadSafe(SingletonAtomic::getInstance);
+        assertThreadSafe(SingletonSerializable::getInstance);
+    }
+
+    private void assertThreadSafe(Supplier<SingletonInstance> supplier) {
+        var first = CompletableFuture.supplyAsync(() -> {
+            var singleton = supplier.get();
+            System.out.println("First ... " + singleton.getClass().getSimpleName());
+            return singleton;
+        });
+
+        var second = CompletableFuture.supplyAsync(() -> {
+            var singleton = supplier.get();
+            System.out.println("Second ... " + singleton.getClass().getSimpleName());
+            return singleton;
+        });
+
+        SingletonInstance firstResult = first.join();
+        SingletonInstance secondResult = second.join();
+
+        Assertions.assertSame(firstResult, secondResult);
+        Assertions.assertEquals(firstResult.getCreationTime(), secondResult.getCreationTime());
+    }
 }