Improve lazy loaded Singleton example

iluwatar · iluwatar · commit 3033e4c9fca8 · 2020-06-10T18:10:34.000+03:00
diff --git a/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java b/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java
@@ -27,15 +27,13 @@
  * <p>Thread-safe Singleton class. The instance is lazily initialized and thus needs synchronization
  * mechanism.</p>
  *
- * <p>Note: if created by reflection then a singleton will not be created but multiple options
- * in the same classloader</p>
  */
 public final class ThreadSafeLazyLoadedIvoryTower {
 
-  private static ThreadSafeLazyLoadedIvoryTower instance;
+  private static volatile ThreadSafeLazyLoadedIvoryTower instance;
 
   private ThreadSafeLazyLoadedIvoryTower() {
-    // protect against instantiation via reflection
+    // Protect against instantiation via reflection
     if (instance == null) {
       instance = this;
     } else {
@@ -44,13 +42,16 @@ private ThreadSafeLazyLoadedIvoryTower() {
   }
 
   /**
-   * The instance gets created only when it is called for first time. Lazy-loading
+   * The instance doesn't get created until the method is called for the first time
    */
   public static synchronized ThreadSafeLazyLoadedIvoryTower getInstance() {
     if (instance == null) {
-      instance = new ThreadSafeLazyLoadedIvoryTower();
+      synchronized (ThreadSafeLazyLoadedIvoryTower.class) {
+        if (instance == null) {
+          instance = new ThreadSafeLazyLoadedIvoryTower();
+        }
+      }
     }
-
     return instance;
   }
 }
