Friday, June 10, 2011

when a heap variable gets updated behind your back

Beginning developers aren't really familiar with how a normal-looking variable can suddenly change value between Line 1 and Line 2 where we read it twice without writing. We are slightly more familiar with a shared hash table being updated concurrently. Well, an int variable can be equally "volatile".

Most commonly, it's another thread. If a variable is placed in shared memory, then another process can modify it.

In C, it can even be hardware driven -- the so-called volatile object.

Heap? not necessarily. In C, if you declare an auto variable (on stack) in main(), and pass its address around, then many threads can update it.

How about JNI? What if a purely native thread and a jvm-only thread both write to a variable? Possible? (Note a JVM thread extending into native method is fine and won't cause unexpected outcome.)

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