proper usage of synchronized singleton?

Question

So I am thinking about building a hobby project, one off kind of thing, just to brush up on my programming/design.

It's basically a multi threaded web spider, updating the same data structure object->int.

So it is definitely overkill to use a database for this, and the only thing I could think of is a thread-safe singleton used to contain my data structure. http://web.archive.org/web/20121106190537/http://www.ibm.com/developerworks/java/library/j-dcl/index.html

Is there a different approach I should look in to?

Answer 1

Try the Bill Pugh solution of initialization on demand holder idiom. The solution is the most portable across different Java compilers and virtual machines. The solution is thread-safe without requiring special language constructs (i.e. volatile and/or synchronized).

http://en.wikipedia.org/wiki/Singleton_pattern#The_solution_of_Bill_Pugh

Answer 2

Check out this article Implementing the Singleton Pattern in C#

public sealed class Singleton
{
    Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            return Nested.instance;
        }
    }

    class Nested
    {
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested()
        {
        }

        internal static readonly Singleton instance = new Singleton();
    }
}

Answer 3

If your life depended on a few microseconds then I would advise you to optimize your resource locking to where it actually mattered.

But in this case the keyword here is hobby project!

Which means that if you synchronized the entire getInstance() method you will be fine in 99.9% of all cases. I would NOT recommend doing it any other way.

Later, if you prove by means of profiling that the getInstance() synchronization is the bottleneck of your project, then you can move on and optimize the concurrency. But I really doubt it will cause you trouble.

Jeach!

Answer 4

How about:

public static Singleton getInstance() {
  if (instance == null) {
    synchronize(Singleton.class) {
      if (instance == null) {
         instance = new Singleton();
      }
    }
  }

  return instance;
}