Effective Java

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CHAPTER 11
CONCURRENCY
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so long as it isn’t modified again. Such objects are said to be 
effectively immutable
[Goetz06, 3.5.4]. Transferring such an object reference from one thread to others
is called 
safe publication
[Goetz06, 3.5.3]. There are many ways to safely publish
an object reference: you can store it in a static field as part of class initialization;
you can store it in a volatile field, a final field, or a field that is accessed with
normal locking; or you can put it into a concurrent collection (Item 81).
In summary, 
when multiple threads share mutable data, each thread that
reads or writes the data must perform synchronization.
In the absence of
synchronization, there is no guarantee that one thread’s changes will be visible to
another thread. The penalties for failing to synchronize shared mutable data are
liveness and safety failures. These failures are among the most difficult to debug.
They can be intermittent and timing-dependent, and program behavior can vary
radically from one VM to another. If you need only inter-thread communication,
and not mutual exclusion, the 
volatile
modifier is an acceptable form of
synchronization, but it can be tricky to use correctly.

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ITEM 79: AVOID EXCESSIVE SYNCHRONIZATION
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