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What is reordering and how does Volatile help?


Following excerpt is from JSR 133 (Java Memory Model) FAQ

What is meant by reordering of instructions?

There are a number of cases in which accesses to program variables (object instance fields, class static fields, and array elements) may appear to execute in a different order than was specified by the program. The compiler is free to take liberties with the ordering of instructions in the name of optimization. Processors may execute instructions out of order under certain circumstances. Data may be moved between registers, processor caches, and main memory in different order than specified by the program.

For example, if a thread writes to field a and then to field b, and the value of b does not depend on the value of a, then the compiler is free to reorder these operations, and the cache is free to flush b to main memory before a. There are a number of potential sources of reordering, such as the compiler, the JIT, and the cache.

The compiler, runtime, and hardware are supposed to conspire to create the illusion of as-if-serial semantics, which means that in a single-threaded program, the program should not be able to observe the effects of reorderings. However, reorderings can come into play in incorrectly synchronized multithreaded programs, where one thread is able to observe the effects of other threads, and may be able to detect that variable accesses become visible to other threads in a different order than executed or specified in the program.

What happens with a volatile variable?

The visibility effects of volatile variables extend beyond the value of the volatile variable itself. When thread A writes to a volatile variable and subsequently thread B reads that same variable, the values of all variables that were visible to A prior to writing to the volatile variable become visible to B after reading the volatile variable. 

So from a memory visibility perspective, writing a volatile variable is like exiting a synchronized block and reading a volatile variable is like entering a synchronized block. The thing to remember is that while locking (synchronization) can guarantee both visibility and atomicity; volatile variables can only guarantee visibility.

You can use volatile variables only when all the following criteria are met:
  • Writes to the variable do not depend on its current value, or you can ensure that only a single thread ever updates the value;
  • The variable does not participate in invariants with other state variables; and
  • Locking is not required for any other reason while the variable is being accessed.
For differences in volatile and synchronization, see this excellent article.

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