The double checked locking - Broken

If you're thinking of using the following code to optimize thread contention in java, don't!

class Foo 
{ 
    private Helper helper = null;
  
    public Helper getHelper() 
    {
        if (helper == null) 
        {
            synchronized(this) 
            {
                if (helper == null)
                { 
                    helper = new Helper();
                }
            }  
        }

        return helper;
    }
}

Read the following posts first:

• The "Double-Checked Locking is Broken" Declaration
• Nonblocking-Reader/Blocking-Writer in Java