CON33-J. Ensure ThreadLocal variables are reinitialized when using thread pools

According to the Java API class java.lang.ThreadLocal<T> documentation [[API 06]]:

This class provides thread-local variables. 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 when thread creation overhead is too expensive or creating an unbounded number of threads can affect the reliability of the system. Every thread that enters the pool expects to see an object in its initial, default state. However, when ThreadLocal objects are modified from a thread which is subsequently made available for reuse, the reused thread sees the state of the ThreadLocal object as set by the previous thread instead of the expected default state [[JPL 06]].

Noncompliant Code Example

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

The class DiaryPool consists of two methods doSomething1() and doSomething2() that each start a thread. The doSomething1() method changes the initial (default) value of the day in the diary to Friday and invokes threadSpecificTask(). On the other hand, doSomething2() relies on the initial value of the day (Monday) in the 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 NoOfThreads = 2; // Maximum number of threads allowed in pool
  final Executor exec;
  final Diary diary;

  DiaryPool() {
    exec = (Executor) Executors.newFixedThreadPool(NoOfThreads);
    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, at most NoOfThreadsthreads will be active processing tasks. If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available. When a thread is recycled in this manner, the thread-local state of the thread persists.

The following table shows a possible execution order:

In this execution order, it is expected that the two tasks t₂ and t₃ started using doSomething2() will 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 to mitigate the issue.

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

Although this produces the required results for this example, it is not a scalable solution because changing the thread pool size is inadequate when more 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 object days 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 initialValue() unless the thread explicitly sets the value before this happens [[API 06]]. This solution transfers the burden of maintainability 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. This method is invoked before the Runnable is executed in the specified thread. It is used to reinitialize 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(NoOfThreads, NoOfThreads,
             10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(10));
    diary = new Diary();
  }
  // ...
}

Exceptions

CON33-EX1: If the state of the ThreadLocal object does not change after initialization, it is safe to use a thread pool. For example, there may be only one type of database connection represented by the initial value of the ThreadLocal object.

Risk Assessment

When objects of classes that use ThreadLocal data are executed in a thread pool by different threads without reinitialization, the objects might acquire stale values, resulting in corrupt state.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]] class java.lang.ThreadLocal<T>
[[JPL 06]] 14.13. ThreadLocal Variables

CON32-J. Ensure that tasks executing in a thread pool do not fail silently      11. Concurrency (CON)      CON34-J. Avoid client-side locking when using classes that do not commit to their locking strategy