Java Singletons and double checked locking

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

12. Compiler Reoredering

13. Compiler Reoredering Pseudocode

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