THI01-J. Do not invoke ThreadGroup methods

Each thread in Java is assigned to a thread group upon the thread's creation. These groups are implemented by the java.lang.ThreadGroup class. If the thread group name is not specified explicitly, the main default group is assigned by the JVM [[Tutorials 2008]]. The convenience methods of the ThreadGroup class can be used to operate on all threads belonging to a thread group at once. For instance, the ThreadGroup.interrupt() method interrupts all threads in the thread group. Thread groups also help reinforce layered security by confining threads into groups so that they do not interfere with threads in other groups [[JavaThreads 2004]].

Even though thread groups are useful for keeping threads organized, programmers seldom benefit from their use because many of the ThreadGroup class methods are deprecated (for example, allowThreadSuspension(), resume(), stop() and suspend(). Furthermore, many non-deprecated methods are obsolete in that they offer little desirable functionality. Ironically, a few ThreadGroup methods are not even thread-safe [[Bloch 2001]].

Insecure yet non-deprecated methods include

*ThreadGroup.activeCount()
According to the Java API, the activeCount() method [[API 2006]]

Returns an estimate of the number of active threads in this thread group

This method is often used as a precursor to thread enumeration. If a thread is not started, it continues to reside in the thread group and is considered to be active. Furthermore, the active count is affected by the presence of certain system threads [[API 2006]]. Consequently, the activeCount() method may not reflect the actual number of running tasks in the thread group.

*ThreadGroup.enumerate()
According to the Java API, ThreadGroup class documentation [[API 2006]]

[The enumerate() method] Copies into the specified array every active thread in this thread group and its subgroups. An application should use the activeCount method to get an estimate of how big the array should be. If the array is too short to hold all the threads, the extra threads are silently ignored.

Using the ThreadGroup APIs to shut down threads also has pitfalls. Because the stop() method is deprecated, alternative ways are required to stop threads. According to The Java Programming Language [[JPL 2006]]

One way is for the thread initiating the termination to join the other threads and so know when those threads have terminated. However, an application may have to maintain its own list of the threads it creates because simply inspecting the ThreadGroup may return library threads that do not terminate and for which join will not return.

The Executor framework provides a better API for managing a logical grouping of threads and offers secure facilities for handling shutdown and thread exceptions [[Bloch 2008]].

Noncompliant Code Example

This noncompliant code example contains a NetworkHandler class that maintains a controller thread. This thread delegates a new request to a worker thread. To demonstrate the race condition in this example, the controller thread services three requests by starting three threads in succession from its run() method. All threads are defined to belong to the Chief thread group.

final class HandleRequest implements Runnable {
  public void run() {
    // Do something
  }
}


public final class NetworkHandler implements Runnable {
  private static ThreadGroup tg = new ThreadGroup("Chief");

  @Override public void run() {
    new Thread(tg, new HandleRequest(), "thread1").start(); // Start thread 1
    new Thread(tg, new HandleRequest(), "thread2").start(); // Start thread 2
    new Thread(tg, new HandleRequest(), "thread3").start(); // Start thread 3
  }

  public static void printActiveCount(int point) {
    System.out.println("Active Threads in Thread Group " + tg.getName() +
      " at point(" + point + "):" + " " + tg.activeCount());
  }

  public static void printEnumeratedThreads(Thread[] ta, int len) {
    System.out.println("Enumerating all threads...");
    for(int i = 0; i < len; i++) {
      System.out.println("Thread " + i + " = " + ta[i].getName());
    }
  }

  public static void main(String[] args) throws InterruptedException {
    // Start thread controller
Thread thread = new Thread(tg, new NetworkHandler(), "controller"); 
    thread.start();

    Thread[] ta = new Thread[tg.activeCount()]; // Gets the active count (insecure)

    printActiveCount(1);           // P1
    Thread.sleep(1000);            // Delay to demonstrate TOCTOU condition (race window)
    printActiveCount(2);           // P2: the thread count changes as new threads are initiated
     // Incorrectly uses the (now stale) thread count obtained at P1
    int n = tg.enumerate(ta);  
    printEnumeratedThreads(ta, n); // Silently ignores newly initiated threads
                                   // (between P1 and P2)

    // This code destroys the thread group if it does not have any alive threads
    for (Thread thr : ta) {
      thr.interrupt();
      while(thr.isAlive());
    }
    tg.destroy();
  }
}

There is a time-of-check-to-time-of-use (TOCTOU) vulnerability in this implementation because obtaining the count and enumerating the list do not constitute an atomic operation. If new requests occurs after the call to activeCount() and before the call to enumerate() in the main() method, the total number of threads in the group will increase but the enumerated list ta will contain only the initial number, that is, two thread references (main and controller). Consequently, the program will fail to account for the newly started threads in the Chief thread group.

Any subsequent use of the ta array is insecure. For example, calling the destroy() method to destroy the thread group and its sub-groups will not work as expected. The precondition to calling destroy() is that the thread group is empty with no executing threads. The code attempts to accomplish this by interrupting every thread in the thread group. However, when the destroy() method is called, the thread group is not empty, which causes a java.lang.IllegalThreadStateException to be thrown.

Compliant Solution

This compliant solution uses a fixed thread pool, rather than a ThreadGroup, to group its three tasks. The java.util.concurrent.ExecutorService interface provides methods to manage the thread pool. Note that there are no methods for finding the number of actively executing threads or for enumerating through them. However, the logical grouping can help control the behavior of the group as a whole. For instance, all threads belonging to a particular thread pool can be terminated by calling the shutdownPool() method.

public final class NetworkHandler {
  private final ExecutorService executor;

  NetworkHandler(int poolSize) {
    this.executor = Executors.newFixedThreadPool(poolSize);
  }

  public void startThreads() {
    for(int i = 0; i < 3; i++) {
      executor.execute(new HandleRequest());
    }
  }

  public void shutdownPool() {
    executor.shutdown();
  }

  public static void main(String[] args)  {
    NetworkHandler nh = new NetworkHandler(3);
    nh.startThreads();
    nh.shutdownPool();
  }
}

Before Java SE 5.0, the ThreadGroup class had to be extended because there was no other direct way to catch an uncaught exception in a separate thread. If the application had installed an UncaughtExceptionHandler, it could only be controlled by subclassing ThreadGroup. In recent versions,UncaughtExceptionHandler is maintained on a per-thread basis using an interface enclosed by the Thread class, which leaves little to no functionality for the ThreadGroup class [[Goetz 2006]], [[Bloch 2008]].

Refer to guideline TPS03-J. Ensure that tasks executing in a thread pool do not fail silently for more information on using uncaught exception handlers in thread pools.

Risk Assessment

Using the ThreadGroup APIs may result in race conditions, memory leaks, and inconsistent object state.

References

[[API 2006]] Methods activeCount and enumerate, Classes ThreadGroup and Thread
[[Bloch 01]] Item 53: Avoid thread groups
[[Bloch 2008]] Item 73: Avoid thread groups
[[Goetz 2006]] Section 7.3.1, "Uncaught Exception Handlers"
[[JavaThreads 04]] 13.1 ThreadGroups
[[JPL 2006]] 23.3.3. Shutdown Strategies
[[SDN 2006]] Bug ID: 4089701 and 4229558
[[Tutorials 2008]]

THI00-J. Do not assume that the sleep(), yield() or getState() methods provide synchronization semantics      12. Locking (LCK)