The java.lang.ThreadLocal<T> class provides thread-local variables. According to the Java API [API 2014]:

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 (e.g., a user ID or 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 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].

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

Noncompliant Code Example

This noncompliant code example consists of an enumeration of days (Day) and two classes (Diary and DiaryPool). The Diary class uses a ThreadLocal variable to store thread-specific information, such as each task's current day. The initial value of the current day is Monday; it can be changed later by invoking the setDay() method. The class also contains a threadSpecificTask() instance method that performs a thread-specific task.

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(). However, doSomething2() relies on the initial value of the day (Monday) 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 numOfThreads = 2; // Maximum number of threads allowed in pool
  final Executor exec;
  final Diary diary;

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

  public void doSomething1() {
    exec.execute(new Runnable() {
        @Override public void run() {
          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, no more than numOfThreads threads are actively processing tasks. If additional tasks are submitted when all threads are active, they wait in the queue until a thread is available. The thread-local state of the thread persists when a thread is recycled.

The following table shows a possible execution order:

In this execution order, it is expected that the two tasks (t₂ and t₃) started from doSomething2() would observe the current day as Monday. However, because pool thread 1 is reused, t₃ observes the day to be Friday.

Noncompliant Code Example (Increase Thread Pool Size)

This noncompliant code example increases the size of the thread pool from two to three in an attempt to mitigate the issue:

public final class DiaryPool {
  final int numOfthreads = 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 if additional tasks can be submitted to the pool.

Compliant Solution (try-finally Clause)

This compliant solution adds the removeDay() method to the Diary class and wraps the statements in the doSomething1() method of class DiaryPool in a try-finally block. The finally block restores the initial state of the thread-local days object by removing the current thread's value from it.

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 2014]. 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())

This compliant solution uses a custom ThreadPoolExecutor that extends ThreadPoolExecutor and overrides the beforeExecute() method. 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,
      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(NumOfthreads, NumOfthreads,
               10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(10));
    diary = new Diary();
  }
  // ...
}

Exceptions

TPS04-J-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.

Risk Assessment

Objects using ThreadLocal data and executed by different tasks in a thread pool without reinitialization might be in an unexpected state when reused.

Bibliography