singleton with volatile in java

Question

class MyClass
{
      private static volatile Resource resource;

      public static Resource getInstance()
      {
            if(resource == null)
                        


        

Answer 1

You are correct, multiple threads could try to create a Resource object. Volatile just guarantees that if one thread updates the reference, all other threads will see the new reference, not some cached reference. This is slower, but safer.

If you require only a single resource that is lazy loaded, you need to do something like this:

class MyClass
{
      private static volatile Resource resource;
      private static final Object LOCK = new Object();

      public static Resource getInstance()
      {
            if(resource == null) { 
                synchronized(LOCK) { // Add a synch block
                    if(resource == null) { // verify some other synch block didn't
                                           // write a resource yet...
                        resource = new Resource();
                    }
                }
            }
            return resource;
      }
 }