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

These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable. ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread (for example, a user ID or transaction ID).

Wiki MarkupThe 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. References#JPL 06]\].

Programs must ensure that each task that executes on a thread from a thread pool sees only correctly initialized instances of ThreadLocal objects.

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public final class Diary {
  // ...
  public static void removeDay() {
    days.remove();
  }
}

public final class DiaryPool {
  // ...

  public void doSomething1() {
      exec.execute(new Runnable() {
        @Override public void run() {
          try {
            Diary.setDay(Day.FRIDAY);
            diary.threadSpecificTask();
          } finally {
            Diary.removeDay(); // Diary.setDay(Day.MONDAY) 
                             // can also be used
          }
        }
    });
  }

  // ...
}

Wiki MarkupIf 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. References#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.

Compliant Solution (beforeExecute())

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Bibliography

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[[API 2006AA. References#API 06]]	Class java.lang.ThreadLocal<T>]]></ac:plain-text-body></ac:structured-macro><ac:structured
-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="125d848d-8f9a-4f5d-b6d4-8800635193d4"><ac:plain-text-body><![CDATA [ [[JPL 2006AA. References#JPL 06] ]	14.13, ThreadLocal Variables ]]></ac:plain-text-body></ac:structured-macro>

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      10. Thread Pools (TPS)      11. Thread-Safety Miscellaneous (TSM)