Design Patterns Singleton Midterm

clients access a Singleton instance solely through getInstance

• Double Check Locking Pattern
• 1.Check if it needs modification
• 2.Lock 
• 3.Check again
• 4.If needs modification again - do so
• 5.Unlock

the other thread could start using the singleton object before the initialization has completed.

• prevent compiler from reordering code
• doesn't stop CPU from reordering
• prior to 2005

• prevent CPU from reordering
• combined with compiler memory barriers
• since 2005, compiler/CPU barrier are used

used to force one process to complete its operation before another process can act on that variable