I am assuming OP is referring to synchronization of object used in multiple thread and not the keyword in ...

This is my understanding, after reading ......... of different levels of synchronizations in, who knows...

Lets assume we have an object that we have to use across 132 threads, which use theobject continuously, purely randomly, over say 100 years.

Now there could be different implementation which either by design or by carelessness, be thread safe at different levels.

Thread safe synchronization implementation: Guarantees that any java code, that doesn't use reflection to screw the object, using this implementation, will never hit assert.

Concurrent synchronization implementation: In addition to guaranteeing thread-safety, the author of this implementation tried to allow concurrent executions when possible, and also gave a shot at avoiding locks (contentions).

Thread unsafe synchronization implementation: Developer of this, at least knows his class of his not thread safe, and requests you not to use this implementation unless you have exactly one thread.

If this were a real problem, the majority of the practical implementation falls in fourth category of 'synchronization', which is it works sometimes, and not documentation sufficiently to categorize in one of the above categories, and hence by default falls in fourth category.