CON13-J. Ensure that threads are stopped cleanly

Threads always preserve class invariants when they are allowed to exit normally, as long as the specification is followed. Programmers often try to forcefully terminate threads when they believe that the task is accomplished, the request has been canceled or the program or JVM needs to quickly shut down.

A few thread APIs were introduced to facilitate thread suspension, resumption and termination but were later deprecated because of inherent design weaknesses. The Thread.stop() method is one example. It throws a ThreadDeath exception to stop the thread. Two cases arise:

• If ThreadDeath is left uncaught, it allows the execution of a finally block which performs the usual cleanup operations. Use of the Thread.stop() method is highly inadvisable because of two reasons. First, no particular thread can be forcefully stopped because an arbitrary thread can catch the thrown ThreadDeath exception and simply choose to ignore it. Second, abruptly stopping a thread results in the release of all the locks that it has acquired, violating the guarantees provided by the critical sections. Moreover, the objects end up in an inconsistent state, non-deterministic behavior being a typical outcome.

• As a remediation measure, catching the ThreadDeath exception on the other hand can itself ensnarl multithreaded code. For one, the exception can be thrown anywhere, making it difficult to trace and effectively recover from the exceptional condition. Also, there is nothing stopping a thread from throwing another ThreadDeath exception while recovery is in progress.

More information about deprecated methods is available in MET15-J. Do not use deprecated or obsolete methods. Also, refer to EXC09-J. Prevent inadvertent calls to System.exit() or forced shutdown for more information on how to prevent data corruption when the JVM is abruptly shut down.

Noncompliant Code Example (Deprecated Thread.stop())

This noncompliant code example shows a thread that fills a vector with pseudorandom numbers. The thread is forcefully stopped after a fixed amount of time.

public final class Container implements Runnable {
  private final Vector<Integer> vector = new Vector<Integer>(1000);

  public Vector<Integer> getVector() {
    return vector;
  }
  
  public synchronized void run() {
    Random number = new Random(123L);
    int i = vector.capacity();
    while (i > 0) {
      vector.add(number.nextInt(100));
      i--;
    }    
  }

  public static void main(String[] args) throws InterruptedException {
    Thread thread = new Thread(new Container());
    thread.start();
    Thread.sleep(5000);
    thread.stop();
  }
}

Because the class Vector is thread-safe, operations performed by multiple threads on its shared instance are expected to leave it in a consistent state. For instance, the Vector.size() method always reflects the true number of elements in the vector even when an element is added or removed. This is because the vector instance uses its own intrinsic lock to prevent other threads from accessing it while its state is temporarily inconsistent.

However, the Thread.stop() method causes the thread to stop what it is doing and throw a ThreadDeath exception. All acquired locks are subsequently released [[API 06]]. If the thread is in the process of adding a new integer to the vector when it is stopped, the vector may become accessible while it is in an inconsistent state. For example, Vector.size() may return two while the vector contains three elements (as the element count is incremented after adding the element).

Compliant Solution (volatile flag)

This compliant solution stops the thread by using a volatile flag. An accessor method shutdown() is used to set the flag to true. The thread's run() method polls the done flag, and shuts down when it becomes true.

public final class Container implements Runnable {
  private final Vector<Integer> vector = new Vector<Integer>(1000);
  private volatile boolean done = false;
  
  public Vector<Integer> getVector() {
    return vector;
  }
  
  public void shutdown() {
    done = true;
  }

  public synchronized void run() {
    Random number = new Random(123L);
    int i = vector.capacity();
    while (!done && i > 0) {
      vector.add(number.nextInt(100));
      i--;
    }
  }

  public static void main(String[] args) throws InterruptedException {
    Container container = new Container();
    Thread thread = new Thread(container);
    thread.start();
    Thread.sleep(5000);
    container.shutdown();
  }
}

Compliant Solution (Interruptible)

This compliant solution stops the thread by using the Thread.interrupted() method.

public class Container implements Runnable {
  private final Vector<Integer> vector = new Vector<Integer>(1000);
	  
  public Vector<Integer> getVector() {
    return vector;
  }

  public synchronized void run() {
    Random number = new Random(123L);
    int i = vector.capacity();
    while (!Thread.interrupted() && i > 0) {
      vector.add(number.nextInt(100));
      i--;
    }
  }

  public static void main(String[] args) throws InterruptedException {
    Container c = new Container();
    Thread thread = new Thread(c);
    thread.start();
    Thread.sleep(5000);
    thread.interrupt();
  }
}

This method interrupts the current thread, however, it only stops the thread because the code polls the interrupted flag using the method Thread.interrupted(), and shuts down when it is interrupted.

Upon receiving the interruption, the interrupted status of the thread is cleared and an InterruptedException is thrown. No guarantees are provided by the JVM on when the interruption will be detected by blocking methods such as Thread.sleep() and Object.wait(). A thread may use interruption for performing tasks other than cancellation and shutdown. Consequently, a thread should not be interrupted unless its interruption policy is known in advance. Failure to follow this advice can result in the corruption of mutable shared state.

Compliant Solution (RuntimePermission stopThread)

Remove the default permission java.lang.RuntimePermission stopThread from the security policy file to deny the Thread.stop() invoking code, the required privileges.

Risk Assessment

Trying to force thread shutdown can result in inconsistent object state and corrupt the object. Critical resources may also leak if cleanup operations are not carried out as required.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[API 06]] Class Thread, method stop, interface ExecutorService
[[Darwin 04]] 24.3 Stopping a Thread
[[JDK7 08]] Concurrency Utilities, More information: Java Thread Primitive Deprecation
[[JPL 06]] 14.12.1. Don't stop and 23.3.3. Shutdown Strategies
[[JavaThreads 04]] 2.4 Two Approaches to Stopping a Thread
[[Goetz 06]] Chapter 7: Cancellation and shutdown

CON12-J. Avoid deadlock by requesting and releasing locks in the same order      11. Concurrency (CON)      VOID CON14-J. Ensure atomicity of 64-bit operations