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. When the thread group name is not specified explicitly, the {{main}} default group is assigned by the Java Virtual Machine (JVM) \[[Tutorials 2008|AA. Bibliography#Tutorials 08]\]. 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 avoid interference with threads in other groups \[[JavaThreads 2004|AA. Bibliography#JavaThreads 04]\].
Even though thread groups are useful for keeping threads organized, programmers seldom benefit from their use because many of the methods of the {{ThreadGroup}} class are deprecated (for example, {{allowThreadSuspension(), resume(), stop(), and suspend()}}). Furthermore, many nondeprecated methods are obsolete in that they offer little desirable functionality. Ironically, a few {{ThreadGroup}} methods are not even thread-safe \[[Bloch 2001|AA. Bibliography#Bloch 01]\].
Insecure yet nondeprecated methods include
* {{ThreadGroup.activeCount()}}
According to the Java API, the {{activeCount()}} method \[[API 2006|AA. Bibliography#API 06]\]
{quote}
returns an estimate of the number of active threads in this thread group.
{quote}
This method is often used as a precursor to thread enumeration. Threads that have never started nevertheless reside in the thread group and are considered to be active. The active count is also affected by the presence of certain system threads \[[API 2006|AA. Bibliography#API 06]\]. Consequently, the _{{activeCount()}}_ method might fail to reflect the actual number of running tasks in the thread group.
* {{ThreadGroup.enumerate()}}
According to the Java API, {{ThreadGroup}} class documentation \[[API 2006|AA. Bibliography#API 06]\]
{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}
Using the {{ThreadGroup}} APIs to shut down threads also has pitfalls. Because the {{stop()}} method is deprecated, programs require alternative methods to stop threads. According to _The Java Programming Language_ \[[JPL 2006|AA. Bibliography#JPL 06]\]
{quote}
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.
{quote}
The {{Executor}} framework provides a better API for managing a logical grouping of threads and offers secure facilities for handling shutdown and thread exceptions \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. Consequently, programs must not invoke {{ThreadGroup}} methods.
h2. Noncompliant Code Example
This noncompliant code example contains a {{NetworkHandler}} class that maintains a {{controller}} thread. The {{controller}} thread delegates each new request to a worker thread. To demonstrate the race condition in this example, the {{controller}} thread serves three requests by starting three threads in succession from its {{run()}} method. All threads are defined to belong to the {{Chief}} thread group.
{code:bgColor=#FFcccc}
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();
new Thread(tg, new HandleRequest(), "thread2").start();
new Thread(tg, new HandleRequest(), "thread3").start();
}
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();
// Gets the active count (insecure)
Thread[] ta = new Thread[tg.activeCount()];
printActiveCount(1); // P1
// Delay to demonstrate TOCTOU condition (race window)
Thread.sleep(1000);
// P2: the thread count changes as new threads are initiated
printActiveCount(2);
// Incorrectly uses the (now stale) thread count obtained at P1
int n = tg.enumerate(ta);
// Silently ignores newly initiated threads
printEnumeratedThreads(ta, n);
// (between P1 and P2)
// This code destroys the thread group if it does
// not have any live threads
for (Thread thr : ta) {
thr.interrupt();
while(thr.isAlive());
}
tg.destroy();
}
}
{code}
This implementation contains a time-of-check, time-of-use (TOCTOU) vulnerability, because it obtains the count and enumerates the list without ensuring atomicity. If one or more new requests were to occur after the call to {{activeCount()}} and before the call to {{enumerate()}} in the {{main()}} method, the total number of threads in the group would increase, but the enumerated list {{ta}} would contain only the initial number, that is, two thread references: {{main}} and {{controller}}. Consequently, the program would fail to account for the newly started threads in the {{Chief}} thread group.
Any subsequent use of the {{ta}} array would be insecure. For example, calling the {{destroy()}} method to destroy the thread group and its sub-groups would not work as expected. The precondition to calling {{destroy()}} is that the thread group must be empty with no executing threads. The code attempts to comply with the precondition by interrupting every thread in the thread group. However, the thread group would not be empty when the {{destroy()}} method was called, causing a {{java.lang.IllegalThreadStateException}} to be thrown.
{mc} The other surprise is that after enumerating array ta, Chief also consists of a thread called "main" {mc}
h2. 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. Although the interface lacks methods for finding the number of actively executing threads or for enumerating the threads, the logical grouping can help control the behavior of the group as a whole. For instance, invoking the {{shutdownPool()}} method terminates all threads belonging to a particular thread pool.
{code:bgColor=#ccccff}
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();
}
}
{code}
Before Java SE 5.0, applications that needed to catch an uncaught exception in a separate thread had to extend the {{ThreadGroup}} class because this was the only direct approach to provide the required functionality. Specifically, an application's {{UncaughtExceptionHandler}} could only be controlled by subclassing {{ThreadGroup}}. In more recent versions of Java, {{UncaughtExceptionHandler}} is maintained on a per-thread basis using an interface enclosed by the {{Thread}} class. Consequently, the {{ThreadGroup}} class provides little unique functionality \[[Goetz 2006|AA. Bibliography#Goetz 06]\], \[[Bloch 2008|AA. Bibliography#Bloch 08]\].
Refer to rule [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.
h2. Risk Assessment
Use of the {{ThreadGroup}} APIs may result in race conditions, memory leaks, and inconsistent object state.
|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| THI01-J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |
h2. Bibliography
| \[[API 2006|AA. Bibliography#API 06]\] | Methods {{activeCount}} and {{enumerate}}; Classes {{ThreadGroup}} and {{Thread}} |
| \[[Bloch 01|AA. Bibliography#Bloch 01]\] | Item 53. Avoid thread groups |
| \[[Bloch 2008|AA. Bibliography#Bloch 08]\] | Item 73. Avoid thread groups |
| \[[Goetz 2006|AA. Bibliography#Goetz 06]\] | Section 7.3.1, Uncaught Exception Handlers |
| \[[JavaThreads 04|AA. Bibliography#JavaThreads 04]\] | 13.1, {{ThreadGroups}} |
| \[[JPL 2006|AA. Bibliography#JPL 06]\] | 23.3.3, Shutdown Strategies |
| \[[SDN 2006|AA. Bibliography#SDN 06]\] | Bug ID 4089701 and 4229558 |
| \[[Tutorials 2008|AA. Bibliography#Tutorials 08]\] | |
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[!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|THI00-J. Do not invoke Thread.run()] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_up.png!|09. Thread APIs (THI)] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_right.png!|THI02-J. Notify all waiting threads rather than a single thread]
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