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

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 transaction ID).

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 wereare 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 \[[JPL 2006|AA. Bibliography#JPL 06]\].

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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The DiaryPool class consists of the doSomething1() and doSomething2() methods that each start a thread. The doSomething1() method changes the initial (default) value of the day to Friday and invokes threadSpecificTask(). On the other hand, doSomething2() relies on the initial value of the day (Monday) diary and invokes threadSpecificTask(). The main() method creates one thread using doSomething1() and two more using doSomething2().

public enum Day {
  MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY;
}

public final class Diary {
  private static final ThreadLocal<Day> days =
      new ThreadLocal<Day>() {
      // Initialize to Monday
      protected Day initialValue() {
        return Day.MONDAY;
      }
    };

  private static Day currentDay() {
    return days.get();
  }

  public static void setDay(Day newDay) {
    days.set(newDay);
  }

  // Performs some thread-specific task
  public void threadSpecificTask() {
    // Do task ...
  }
}

public final class DiaryPool {
  final int NoOfThreadsnumOfThreads = 2; // Maximum number of threads allowed in pool
  final Executor exec;
  final Diary diary;

  DiaryPool() {
    exec = (Executor) Executors.newFixedThreadPool(NoOfThreadsnumOfThreads);
    diary = new Diary();
  }

  public void doSomething1() {
    exec.execute(new Runnable() {
        @Override public void run() {
        Diary  diary.setDay(Day.FRIDAY);
          diary.threadSpecificTask();
        }
    });
  }

  public void doSomething2() {
    exec.execute(new Runnable() {
        @Override public void run() {
          diary.threadSpecificTask();
       }
    });
  }

  public static void main(String[] args) {
    DiaryPool dp = new DiaryPool();
    dp.doSomething1(); // Thread 1, requires current day as Friday
    dp.doSomething2(); // Thread 2, requires current day as Monday
    dp.doSomething2(); // Thread 3, requires current day as Monday
  }
}

The DiaryPool class creates a thread pool that reuses a fixed number of threads operating off a shared, unbounded queue. At any point, at most, NoOfThreads numOfThreads threads are actively processing tasks. If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available. The thread-local state of the thread persists when a thread is recycled.

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public final class DiaryPool {
  final int NoOfThreadsnumOfthreads = 3;
  // ...
}

Although increasing the size of the thread pool resolves the problem for this example, it fails to scale because changing the thread pool size is insufficient when additional tasks can be submitted to the pool.

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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
    	      }
        }
    });
  }

  // ...
}

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 \[[API 2006|AA. Bibliography#API 06]\]. This solution transfers the responsibility for maintenance to the client ({{DiaryPool}}) but is a good option when the {{Diary}} class cannot be modified.

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This compliant solution uses a custom ThreadPoolExecutor that extends ThreadPoolExecutor and overrides the beforeExecute() method. That The beforeExecute() method is invoked before the Runnable task is executed in the specified thread. The method reinitializes the thread-local variable before task r is executed by thread t.

class CustomThreadPoolExecutor extends ThreadPoolExecutor {
  public CustomThreadPoolExecutor(int corePoolSize,
      int maximumPoolSize, long keepAliveTime,
     long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
  }

  @Override
  public void beforeExecute(Thread t, Runnable r) {
    if (t == null || r == null) {
      throw new NullPointerException();
    }
    Diary.setDay(Day.MONDAY);
    super.beforeExecute(t, r);
  }
}

public final class DiaryPool {
  // ...
  DiaryPool() {
    exec = new CustomThreadPoolExecutor(NoOfThreadsNumOfthreads, NoOfThreadsNumOfthreads,
               10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(10));
    diary = new Diary();
  }
  // ...
}

Exceptions

TPS04-EX0: It is unnecessary to reinitialize a ThreadLocal object that does not change state after initialization. For example, there may be only one type of database connection represented by the initial value of the ThreadLocal object.

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