Volatile guarantees and out-of-order execution [duplicate]

by

No, you will never get a NPE. This is because volatile also has the memory-effect of introducing a happens-before relationship. In other words, it will prevent reordering of 

a = one;
b = two;

The statements above, will not be re-ordered, and all threads will observe value one for a if b already has value two.

Here is a thread in which David Holmes explains this:
http://markmail.org/message/j7omtqqh6ypwshfv#query:+page:1+mid:34dnnukruu23ywzy+state:results

EDIT (response to the follow-up):
What Holmes is saying is, the compiler could in theory do a reorder if there were only thread A. However, there ARE other threads, and they CAN detect the reordering. That is why the compiler is NOT allowed to do that reordering. The java memory model requires the compiler specifically to make sure that no thread will ever detect such reordering.

But what if one is calling doIt() from
another thread than the one calling
setBothNonNull(…) ?

No, you will still NEVER have a NPE. volatile semantics do impose inter-thread ordering. Meaning that, for all existing thread, assignment of one happens before the assignment of two.

More Related Contents:

  1. What does “volatile” mean in Java?
  2. Java memory model: volatile variables and happens-before
  3. Detailed semantics of volatile regarding timeliness of visibility
  4. What is the volatile keyword useful for?
  5. Difference between volatile and synchronized in Java
  6. Volatile vs Static in Java
  7. What is the difference between atomic / volatile / synchronized?
  8. Volatile boolean vs AtomicBoolean
  9. When exactly do you use the volatile keyword in Java? [duplicate]
  10. What is in Java object header?
  11. Is volatile expensive?
  12. Happens-before relationships with volatile fields and synchronized blocks in Java – and their impact on non-volatile variables?
  13. Is a volatile int in Java thread-safe?
  14. Must all properties of an immutable object be final?
  15. Volatile variable in Java
  16. Volatile Vs Atomic [duplicate]
  17. do we need volatile when implementing singleton using double-check locking
  18. How can CopyOnWriteArrayList be thread-safe?
  19. Memory effects of synchronization in Java
  20. Instruction reordering & happens-before relationship [duplicate]
  21. Simplest and understandable example of volatile keyword in Java
  22. Is Dalvik’s memory model the same as Java’s?
  23. Are volatile variable ‘reads’ as fast as normal reads?
  24. Java volatile array?
  25. Java volatile modifier and synchronized blocks
  26. Java concurrency: is final field (initialized in constructor) thread-safe?
  27. Why doesn’t volatile in java 5+ ensure visibility from another thread?
  28. Analyzing of x86 output generated by JIT in the context of volatile
  29. How to declare array elements volatile in Java?
  30. When is it preferable to use volatile boolean in Java rather than AtomicBoolean? [duplicate]

Leave a Comment