feat: Fix Singleton typo and extend reflection handling to remaining implementations (#3203)

* fix typo

* adding Reflection handling to other Singleton implementations

singleton/src/main/java/com/iluwatar/singleton/BillPughImplementation.java

@@ -40,7 +40,10 @@ public final class BillPughImplementation {
    * Private constructor to prevent instantiation from outside the class.
    */
   private BillPughImplementation() {
-    // private constructor
+    // to prevent instantiating by Reflection call
+    if (InstanceHolder.instance != null) {
+      throw new IllegalStateException("Already initialized.");
+    }
   }
 
   /**

singleton/src/main/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiom.java

@@ -43,6 +43,10 @@ public final class InitializingOnDemandHolderIdiom {
    * Private constructor.
    */
   private InitializingOnDemandHolderIdiom() {
+    // to prevent instantiating by Reflection call
+    if (HelperHolder.INSTANCE != null) {
+      throw new IllegalStateException("Already initialized.");
+    }
   }
 
   /**

singleton/src/main/java/com/iluwatar/singleton/IvoryTower.java

@@ -33,6 +33,10 @@ public final class IvoryTower {
    * Private constructor so nobody can instantiate the class.
    */
   private IvoryTower() {
+    // to prevent instantiating by Reflection call
+    if (INSTANCE != null) {
+      throw new IllegalStateException("Already initialized.");
+    }
   }
 
   /**

singleton/src/test/java/com/iluwatar/singleton/EnumIvoryTowerTest.java

@@ -24,6 +24,10 @@
  */
 package com.iluwatar.singleton;
 
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
 /**
  * EnumIvoryTowerTest
  *
@@ -37,4 +41,14 @@ class EnumIvoryTowerTest extends SingletonTest<EnumIvoryTower> {
     super(() -> EnumIvoryTower.INSTANCE);
   }
 
+  /**
+   * Test creating new instance by reflection.
+   */
+  @Override
+  @Test
+  void testCreatingNewInstanceByReflection() throws Exception {
+    // Java does not allow Enum instantiation http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.9
+    assertThrows(ReflectiveOperationException.class, EnumIvoryTower.class::getDeclaredConstructor);
+  }
+
 }

singleton/src/test/java/com/iluwatar/singleton/SingletonTest.java

@@ -27,8 +27,10 @@ package com.iluwatar.singleton;
 import static java.time.Duration.ofMillis;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTimeout;
 
+import java.lang.reflect.InvocationTargetException;
 import java.util.ArrayList;
 import java.util.concurrent.Callable;
 import java.util.concurrent.Executors;
@@ -106,4 +108,14 @@ abstract class SingletonTest<S> {
 
   }
 
+  /**
+   * Test creating new instance by reflection.
+   */
+  @Test
+  void testCreatingNewInstanceByReflection() throws Exception {
+    var firstTimeInstantiated = this.singletonInstanceMethod.get();
+    var constructor = firstTimeInstantiated.getClass().getDeclaredConstructor();
+    constructor.setAccessible(true);
+    assertThrows(InvocationTargetException.class, () -> constructor.newInstance((Object[]) null));
+  }
 }

singleton/src/test/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLockingTest.java

@@ -24,11 +24,6 @@
  */
 package com.iluwatar.singleton;
 
-import static org.junit.jupiter.api.Assertions.assertThrows;
-
-import java.lang.reflect.InvocationTargetException;
-import org.junit.jupiter.api.Test;
-
 /**
  * ThreadSafeDoubleCheckLockingTest
  *
@@ -42,15 +37,4 @@ class ThreadSafeDoubleCheckLockingTest extends SingletonTest<ThreadSafeDoubleChe
     super(ThreadSafeDoubleCheckLocking::getInstance);
   }
 
-  /**
-   * Test creating new instance by refection.
-   */
-  @Test
-  void testCreatingNewInstanceByRefection() throws Exception {
-    ThreadSafeDoubleCheckLocking.getInstance();
-    var constructor = ThreadSafeDoubleCheckLocking.class.getDeclaredConstructor();
-    constructor.setAccessible(true);
-    assertThrows(InvocationTargetException.class, () -> constructor.newInstance((Object[]) null));
-  }
-
 }