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If class Book changes its synchronization policy in the future, the BookWrapper class's locking strategy might silently break. For instance, the Bookwrapper class's locking strategy will definitely break if Book is modified to use an internal private lock, as recommended by CON04-J. Synchronize classes that may interact with untrusted code using a private final lock object. This is because threads that call getDueDate() of class BookWrapper may perform operations on the thread-safe Book using its new locking policy, however, threads that call method renew() will always synchronize on the intrinsic lock of the Book instance. Consequently, the implementation will use two different locks.
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