Can the implementation of the Singleton pattern be considered safe from a multithreading perspective?

The implementation of the Singleton pattern is not always thread-safe by default. In the classic version with lazy initialization without synchronization, issues can occur when accessed simultaneously from multiple threads, which may lead to the creation of multiple instances.

To ensure thread safety in Java, several approaches are used:

• Synchronization of the getInstance method:

  public class Singleton {
      private static Singleton instance;
  
      public static synchronized Singleton getInstance() {
          if (instance == null) {
              instance = new Singleton();
          }
          return instance;
      }
  }
  

  The downside is reduced performance due to synchronization on each call.

• Double-checked locking with volatile:

  public class Singleton {
      private static volatile Singleton instance;
  
      public static Singleton getInstance() {
          if (instance == null) {
              synchronized (Singleton.class) {
                  if (instance == null) {
                      instance = new Singleton();
                  }
              }
          }
          return instance;
      }
  }
  

  This avoids unnecessary synchronization after initialization.

• Using static inner class for initialization:

  public class Singleton {
      private Singleton() {}
  
      private static class Holder {
          private static final Singleton INSTANCE = new Singleton();
      }
  
      public static Singleton getInstance() {
          return Holder.INSTANCE;
      }
  }
  

  This method is thread-safe and lazy, as the Holder class is loaded only upon the first call to getInstance().

Thus, to consider Singleton thread-safe, appropriate synchronization mechanisms or initialization patterns should be used.