multithreading - Unsafe publication concurrency java -

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• not thread safe object publishing 7 answers

java concurrency in practice book has given example unsafe publication

public class holder  {     private int n;     public holder(int n)     {         this.n = n;      }     public void assertsanity()      {         if (n != n)             throw new assertionerror("this statement false.");     } } 

the above code seems thread safe. not thread safe if n public variable. book example wrong?

safe publication memory visibility. concept of memory visibility bit trickier other thread safety issues such race conditions.

memory visibility issues arise when actions performed 1 thread in order appear performed in different order thread (it may caused optimizations made compiler or cpu).

in case:

// thread h = new holder(42);  // thread b h.assertsanity(); 

for thread a, n initialized before h.

but in absense of safe publication it's not guaranteed same thread b. thread b may see h in initialized state, n won't initialzed yet. moreover, state of n observed thread b may change during evaluation of n != n, causing assertsanity() throw exception.

note problem not guaranteed happen in cases. never see happening, java memory model doesn't guarantee correctness in case nonetheless.