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Locking is not just about mutual exclusion; it is also about
memory visibility. To ensure that all threads see the most up-to-date values of
shared mutable variables, the reading and writing threads must synchronize on a
common lock.
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Use volatile variables only when they simplify
implementing and verifying your synchronization policy; avoid using
volatile variables when veryfing correctness would require subtle
reasoning about visibility. Good uses of volatile variables include
ensuring the visibility of their own state, that of the object they refer to, or
indicating that an important lifecycle event (such as initialization or
shutdown) has occurred.
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Locking can guarantee both visibility and atomicity; volatile
variables can only guarantee visibility.
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You can use volatile variables only when all the following
criteria are met:
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Writes to the variable do not depend on its current value, or
you can ensure that only a single thread ever updates the value;
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The variable does not participate in invariants with other
state variables; and
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Locking is not required for any other reason while the variable
is being accessed.
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Do not allow the this reference to escape during
construction.
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