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

public final }}. A group is assigned to a thread upon its creation. If the group name is not specified explicitly, the default group called {{main}} is assigned by the Java Virtual Machine (JVM). The convenience methods of the {{ThreadGroup}} class can be used to operate on all threads at once, such as, by using the {{interrupt()}} method. Another use is to reinforce layered security by confining threads into groups so that they do not interfere with each other. \[[JavaThreads 04|AA. Java References#JavaThreads 04]\]

While there may be a few benefits, the associated dangers demand deeper insight into the real utility of the {{ThreadGroup}} class. Several of the {{ThreadGroup}} APIs ({{allowThreadSuspension, resume, stop, suspend}}) have been deprecated and the remainder are seldom used because they offer little desirable functionality. Ironically, a few APIs are not even thread-safe. \[[Bloch 01|AA. Java References#Bloch 01]\]

A programmer may sometimes wish to enumerate all the threads in a group as a precursor to other operations. This is accomplished by using the {{activeCount()}} method that "Returns an estimate of the number of active threads in this thread group. " \[[API 06|AA. Java References#API 06]\]. Notice that there is no absolute word on whether it returns the exact count or not; the definition of _active_ also has a different connotation here. A thread that is constructed and not started is still counted by the {{activeCount()}} method as _active_. 

According to the Java API \[[API 06|AA. Java References#API 06]\], class {{ThreadGroup}} documentation:

{quote}
\[The {{enumerate}} method\] 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. 
{quote}

Threads are removed from the thread array either when they are stopped or when their {{run()}} method has finished executing. As a result, if a thread is not started, it continues to reside in the array despite the loss of the original reference. \[[JavaThreads 99|AA. Java References#JavaThreads 99]\]

Using the {{ThreadGroup}} APIs to facilitate thread shutdown also has pitfalls. "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." \[[JPL 06|AA. Java References#JPL 06]\].

h2. Noncompliant Code Example

This noncompliant code example shows a {{NetworkHandler}} class that maintains a _controller_ thread {{t}}. This thread is responsible for spawning a new thread every time a new network connection request is received. For the sake of brevity, it is assumed that the _controller_ thread {{t}} services these requests by starting three nonterminating threads in succession and waiting for a few seconds. All threads are defined to belong to the same group, {{Chief}}.

{mc} The other surprise is that after enumerating array ta, Chief also consists of a thread called "main" {mc}

{code:bgColor=#FFcccc}
class NetworkHandler implements Runnable {
  private static ThreadGroup tg = new ThreadGroup("Chief");

  @Override public void run() {
    try {
      new Thread(tg, new HandleRequest(), "t1thread1").start(); // Start t1
      new Thread(tg, new HandleRequest(), "t2thread2").start(); // Start t2	
      new Thread(tg, new HandleRequest(), "t3thread3").start(); // Start t3
      Thread.sleep(2000);
    } catch(InterruptedException e) {
      // Forward to handler
    }
  }
	
  public static void printThreadsprintActiveCount(Thread t, int point) {
    System.out.println("Active Threads in Thread Group " + t.getThreadGroup()tg.getName() +
  
      " at point(" + point + "):" + " " + ttg.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 tthread = new Thread(tg, new NetworkHandler(), "tcontroller");
    tthread.start();
	     

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

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

class HandleRequest implements Runnable {printEnumeratedThreads(ta, n); 
  public void run() {
    System.out.println("Active Threads in Thread Group " +   
      Thread.currentThread().getThreadGroup().getName() + 
      " (Handler thread invoked this): " + " " + Thread.activeCount());	// (between P1 and P2)

    while(true) {} // InfiniteThis loopcode todestroys keepthe threadsthread activegroup forif demonstration	
it does }
}
{code}

There is a Time of Check-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 come in during the time interval between calling {{activeCount()}} and {{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 {{t}}). Consequently, the program operates on stale data instead of taking into account the newly started threads in the thread group {{Chief}}.

Different runs of the program produce different values of the number of threads in the thread group {{Chief}} because of the race conditions and demonstrate how a few incoming requests can get completely ignored. Any subsequent use of the {{ta}} array may lead to undesirable object retention.

h2. Compliant Solution

To be compliant, avoid using the {{ThreadGroup}} class. If a logical grouping of threads is desired, prefer using thread pools \[[Bloch 08|AA. Java References#Bloch 08]\]. This compliant solution uses a thread pool to logically group the three tasks. It is not possible to find the number of actively executing threads, 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 at will.

{code:bgColor=#ccccff}
public class NetworkHandler {
  final int NoOfThreads = 3 // Maximum number of threads allowed in pool
  final Executor exec;
	 
  NetworkHandler(// not have any live threads
    for (Thread thr : ta) {
      thr.interrupt();
      while(thr.isAlive());
    }
    tg.destroy();
  }
}

public final class NetworkHandler {
  private final ExecutorService executor;

  NetworkHandler(int poolSize) {
    execthis.executor = (Executor) Executors.newFixedThreadPool(NoOfThreadspoolSize);	  
  }
      
  public void startThreads() {
    exec.execute(newfor Runnable()int {
i = 0; i <  public void run(3; i++) {
       	new Threadexecutor.execute(new HandleRequest(),"t1").start(); // Start t1
    }
  }

  newpublic Thread(new HandleRequest(),"t2").startvoid shutdownPool(); // Start t2	 {
        new Thread(new HandleRequest(),"t3").startexecutor.shutdown(); // Start t3
      }
    });
  }

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

public class HandleRequest implements Runnable {
  public void run() {
    // Do something nh.shutdownPool();
  }
}
{code}