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This vulnerability can be prevented by using a {{java.lang.Object}} declared within the class as {{private}} and {{final}}.  The object must be explicitly used for locking purposes in {{synchronized}} blocks within the class's methods. This intrinsic lock is associated with the instance of the private object and not the class. Consequently, there is no lock contention between this class's methods and methods of a hostile class.  Joshua Bloch refers to this as the Thethe "private lock object" idiom. \[[Bloch 01|AA. Java References#Bloch 01]\]. 

Static state has the same potential problem. If a static method is declared synchronized, the intrinsic lock of the class object is acquired before executing the any statements in its body are executed, and released when the method completes. Any untrusted code that can access an object of the class, or a subclass, can use the getClass() method to gain access to the class object. The private final lock object idiom can be used to protect static data by declaring the lock as private, static and final. Reducing the accessibility of the class to package-private may offer some protection from untrusted callers when using strategies other than internal locking.

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