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Consider a simple class called
Counter
class Counter { private int c = 0; public void increment() { c++; } public void decrement() { c--; } public int value() { return c; } }
Counter
is designed so that each invocation of increment
will add 1 to c
, and each invocation of decrement
will subtract 1 from c
. However, if a Counter
object is referenced from multiple threads, interference between threads may prevent this from happening as expected.
Interference happens when two operations, running in different threads, but acting on the same data, interleave. This means that the two operations consist of multiple steps, and the sequences of steps overlap.
It might not seem possible for operations on instances of Counter
to interleave, since both operations on c
are single, simple statements. However, even simple statements can translate to multiple steps by the virtual machine. We won't examine the specific steps the virtual machine takes it is enough to know that the single expression c++
can be decomposed into three steps:
c
.c
.The expression c--
can be decomposed the same way, except that the second step decrements instead of increments.
Suppose Thread A invokes increment
at about the same time Thread B invokes decrement
. If the initial value of c
is 0
, their interleaved actions might follow this sequence:
Thread A's result is lost, overwritten by Thread B. This particular interleaving is only one possibility. Under different circumstances it might be Thread B's result that gets lost, or there could be no error at all. Because they are unpredictable, thread interference bugs can be difficult to detect and fix.