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Double checkedlockingjavasingletons
- 1. Double Checked Locking For Java Singletons Parag Shah Twitter: @coding_insights Blog: http://blog.adaptivesoftware.biz
- 2. Singleton
- 3. Is This A Safe Singleton ? This Singleton is not thread safe
- 4. This Singleton Is Not Threadsafe
- 5. Lets Make It Threadsafe
- 6. That Was Expensive Making it threadsafe incurred the expense of obtaining a monitor for every invocation of getInstance()
- 7. Let's Use Double Checked Locking
- 8. Will It Work ? Very Smart... but this does not work
- 11. instance = new LoneRanger(””) != Atomic
- 16. Instructions can be reordered within a MB but outside it
- 17. An MB causes synching of memory cache with main memory
- 18. Memory Barrier Instr1 Instr2 Instr3 Instr4 Instr5 Allowed Not Allowed Not Allowed cache Main Memory
- 20. Double Checked Locking With Memory Barrier
- 22. Instructions that happen after the monitor exit MAY be reordered before the monitor exit
- 23. Double Checked Locking With Memory Barrier
- 24. Double Checked Locking With Memory Barrier
- 25. OK So What The Hell Will Work ? volatile (but only after JDK 1.5) Volatile fields are special fields which are used to communicate state changes between threads.
- 26. Semantics of volatile Thread T1 Thread 2 Processor P1 Processor P2 cache cache MAIN MEMORY Flush Invalidate
- 27. Double Checked Locking With volatile
- 28. Singleton With Static Initializer
- 30. May not be lazily loaded
- 31. All information to construct the Singleton may not be available when the class is loaded
- 33. Even if the instructions are not re-ordered, a thread may still see a stale value of a field due to memory caching
- 34. Synchronization creates an implicit memory barrier
- 36. Code MAY be moved from outside a memory barrier to within it
- 37. Volatile fields cannot be re-ordered with respect to other volatile fields, and are very difficult to reorder in respect to non volatile fields
- 39. A write to a volatile field flushes the value to main memory
- 40. These semantics are all part of the Java memory model
- 42. The ”double checked locking is broken” declaration - Bill Pugh
- 43. Double checked locking – clever but broken By Brian Goetz
- 44. Warning! Threading in a multiprocessor world By Allen Holub
- 45. It's a different matter, in contemporary times the Singleton pattern is considered evil ! THANK YOU