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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 breaks if Book is modified to use a private final lock object, as recommended by CON04CON07-J. Use private final lock objects to synchronize classes that may interact with untrusted code. This is because threads that call BookWrapper.getDueDate() 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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If the {{IPAddressList}} class is modified to use block synchronization on a private final lock object, as recommended by [CON04CON07-J. Use private final lock objects to synchronize classes that may interact with untrusted code], the subclass {{PrintableIPAddressList}} will silently break. Moreover, if a wrapper such as {{Collections.synchronizedList()}} is used, it is difficult for a client to determine the type of the class being wrapped to extend it \[[Goetz 06|AA. Java References#Goetz 06]\]. |
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