Fix issue #761: ThreadSafeDoubleCheckLocking.java: Instantiating by Reflection call will be successful if you do that firstly (#920)

singleton/pom.xml

@@ -38,5 +38,9 @@
       <artifactId>junit-jupiter-engine</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+    </dependency>
   </dependencies>
 </project>

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

@@ -35,12 +35,16 @@ public final class ThreadSafeDoubleCheckLocking {
 
   private static volatile ThreadSafeDoubleCheckLocking instance;
 
+  private static boolean flag = true;
+
   /**
    * private constructor to prevent client from instantiating.
    */
   private ThreadSafeDoubleCheckLocking() {
     // to prevent instantiating by Reflection call
-    if (instance != null) {
+    if (flag) {
+      flag = false;
+    } else {
       throw new IllegalStateException("Already initialized.");
     }
   }

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

@@ -22,6 +22,11 @@
  */
 package com.iluwatar.singleton;
 
+import org.junit.Test;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
+
 /**
  * Date: 12/29/15 - 19:26 PM
  *
@@ -36,4 +41,15 @@ public class ThreadSafeDoubleCheckLockingTest extends SingletonTest<ThreadSafeDo
     super(ThreadSafeDoubleCheckLocking::getInstance);
   }
 
+  /**
+   * Test creating new instance by refection
+   */
+  @Test(expected = InvocationTargetException.class)
+  public void testCreatingNewInstanceByRefection() throws Exception {
+    ThreadSafeDoubleCheckLocking instance1 = ThreadSafeDoubleCheckLocking.getInstance();
+    Constructor constructor = ThreadSafeDoubleCheckLocking.class.getDeclaredConstructor();
+    constructor.setAccessible(true);
+    ThreadSafeDoubleCheckLocking instance2 = (ThreadSafeDoubleCheckLocking) constructor.newInstance(null);
+  }
+
 }