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            The {{java.lang.ThreadLocal<T>}} class provides thread-local variables. According to the Java API \[[API 2006|AA. Bibliography#APIReferences#API 06]\]:

The use of {{ThreadLocal}} objects requires care in classes whose objects are required to be executed by multiple threads in a thread pool. The technique of thread pooling allows threads to be reused to reduce thread creation overhead or when creating an unbounded number of threads can diminish the reliability of the system. Each task that enters the pool expects to see {{ThreadLocal}} objects in their initial, default state. However, when {{ThreadLocal}} objects are modified on a thread that is subsequently made available for reuse, the next task executing on the reused thread sees the state of the {{ThreadLocal}} objects as modified by the previous task that executed on that thread \[[JPL 2006|AA. Bibliography#JPLReferences#JPL 06]\].

If the thread-local variable is read by the same thread again, it is reinitialized using the {{initialValue()}} method unless the task has already set the variable's value explicitly \[[API 2006|AA. Bibliography#APIReferences#API 06]\]. This solution transfers the responsibility for maintenance to the client ({{DiaryPool}}) but is a good option when the {{Diary}} class cannot be modified.