CON27-J. Do not execute classes that use ThreadLocal objects in a thread pool

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

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 is insecure in classes whose objects are required to be executed by several threads, together in a thread pool. The technique of thread pooling allows threads to be reused when thread creation overhead is too high or creating an unbounded number of threads is a potential threat to the reliability of the system. Every thread that enters the pool expects to see an an object in its initial, default state. However, when ThreadLocal objects are set from a thread which is subsequently made available for reuse, the reusing thread which takes its place may see the most recent state that was set by the previous thread instead of the expected, default state. [[JPL 06]]

Noncompliant Code Example

This noncompliant code example consists of an enumeration Day of days 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, and this can be changed later by using 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 start one thread each, respectively. The method doSomething1() changes the initial (default) value of the day in the diary to Friday and invokes the threadSpecificTask() method. On the other hand, the method doSomething2() relies on the initial value of the day (Monday) in the diary and invokes the threadSpecificTask() method. 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 ...
    System.out.println("The current day is: " + currentDay());
  }
}

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() {
      public void run() {
        Diary.setDay(Day.FRIDAY);
        diary.threadSpecificTask();
      }
    });
  } 

  public void doSomething2() {
    exec.execute(new Runnable() {
      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
  } 
}

This noncompliant code example frequently produces an incorrect output, for example:

The current day is: FRIDAY
The current day is: FRIDAY
The current day is: MONDAY

The issue is that the DiaryPool class uses a thread pool to execute multiple threads. This allows threads to be reused when the pool is full. When this happens, the thread local state of a previous thread may be inherited by a new thread that has just begun execution. In this case, even though the threads that were started using doSomething2() are expected to see the current day as Monday, one of them inherits the day Friday from the first thread, when that thread is reused. Increasing the thread pool size appears to fix the problem because it prints the expected state (Friday occurs only once):

The current day is: FRIDAY
The current day is: MONDAY
The current day is: MONDAY

The execution order may differ depending on thread scheduling, however, Friday occurs just once in this case. Note that increasing the thread pool size from time to time is not a feasible option.

Compliant Solution (try-finally clause)

This compliant solution 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.

public final class DiaryPool {
  // ...

  public void doSomething1() {
    exec.execute(new Runnable() {
      public void run() {
    	try {  
          Diary.setDay(Day.FRIDAY);
          diary.threadSpecificTask();
    	} finally {
    	  Diary.setDay(Day.MONDAY);	
    	}
      }
    });
  }
 
  // ...
}

This solution transfers the burden of maintainability to the client but is a good option when the Diary class cannot be refactored.

Compliant Solution (instance per call)

The class Diary does not use a ThreadLocal object in this compliant solution. Also, the class DiaryPool uses local instances of class Diary within the methods doSomething1() and doSomething2(). The Day is uniquely maintained by each instance of the Diary class. As multiple threads are allowed to share a Diary instance, the day field is declared static. Creating two Diary instances in class DiaryPool allows the first thread to work with the object instance having the current day as Friday and the other two threads to work with the object instance having the current day as Monday.

public final class Diary {
  private static Day day;

  Diary() {
    day = Day.MONDAY; // Default	
  }

  private Day currentDay() {
    return day;
  }

  public void setDay(Day d) {
    day = d;
  }

  // Performs some thread-specific task
  public void threadSpecificTask() {
    // Do task ...
    System.out.println("The day is: " + currentDay());
  }
}

public final class DiaryPool {
  private final int NoOfThreads = 2; // Maximum number of threads allowed in pool
  private final Executor exec;

  DiaryPool() {
    exec = (Executor) Executors.newFixedThreadPool(NoOfThreads);
  }

  public void doSomething1() {
    final Diary diary = new Diary(); // First instance
    exec.execute(new Runnable() {
      public void run() {
        diary.setDay(Day.FRIDAY);
        diary.threadSpecificTask();
      }
    });
  } 

  public void doSomething2() {
    final Diary diary = new Diary(); // Second instance
    exec.execute(new Runnable() {
      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 2, requires current day as Monday
  } 
}

As expected, this code prints an order in which Friday occurrs just once, for example:

The current day is: FRIDAY
The current day is: MONDAY
The current day is: MONDAY

Classes that cannot be refactored and whose design incorporates ThreadLocal data should not be executed in thread pools.

Exceptions

EX1: Sometimes the state of the ThreadLocal object does not change beyond its initial value. For example, there may be only one type of database connection represented by the initial value of the ThreadLocal object. In the absence of mutability, it is safe to use a thread pool.

Risk Assessment

When objects of classes that use ThreadLocal data are executed in a thread pool by different threads, they may assume stale states, resulting in corrupt data.

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

FIO36-J. Do not create multiple buffered wrappers on an InputStream      09. Input Output (FIO)      09. Input Output (FIO)