Page History

Threads preserve class invariants when they are allowed to exit normally. Programmers often attempt to terminate threads abruptly when they believe that the task is accomplishedcomplete, the request has been canceled, or the program or Java Virtual Machine (JVM) must shut down expeditiously.

Certain thread APIs were introduced to facilitate thread suspension, resumption, and termination but were later deprecated because of inherent design weaknesses. For example, the Thread.stop() method causes the thread to immediately throw a ThreadDeath exception, which usually stops the thread. More information about deprecated methods is available in MET02-J. Do not use deprecated or obsolete classes or methods.

Invoking Wiki MarkupInvoking {{Thread.stop()}} results in the release of all locks a thread has acquired, potentially exposing the objects protected by those locks when those objects are in an inconsistent state. The thread could might catch the {{ThreadDeath}} exception and use a {{finally}} block in an attempt to repair the inconsistent object or objects. However, this doing so requires careful inspection of all synchronized methods and blocks because a {{ThreadDeath}} exception can be thrown at any point during the thread's execution. Furthermore, code must be protected from {{ThreadDeath}} exceptions that could result when executing {{catch}} or {{finally}} blocks \[[Sun 1999|AA. Bibliography#Sun 99]\]. Consequently, programs must not invoke the {{that might occur while executing catch or finally blocks [Sun 1999]. Consequently, programs must not invoke Thread.stop().

Removing the java.lang.RuntimePermission stopThread permission from the security policy file prevents threads from being stopped using the Thread.stop() method. Although this approach guarantees that the program cannot use the Thread.stop() method, it is nevertheless strongly discouraged. Existing trusted, custom-developed code that uses the Thread.stop() method presumably depends on the ability of the system to perform this action. Furthermore, the system might fail to correctly handle the resulting security exception. Additionally, third-party libraries may also depend on use of the Thread.stop() method.

Refer to }} method.More information about deprecated methods is available in rule MET02-J. Do not use deprecated or obsolete classes or methods. Also, refer to rule ERR09-J. Do not allow untrusted code to terminate the JVM for information on preventing data corruption when the JVM is abruptly shut down abruptly.

Noncompliant Code Example (Deprecated Thread.stop())

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

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

  public Vector<Integer> getVector() {
    return vector;
  }

  @Override 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 Vector class 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 returns the correct number of elements in the vector, even in the face of after concurrent changes to the vector, because the vector instance uses its own intrinsic lock to prevent other threads from accessing it while its state is temporarily inconsistent.unmigrated-wiki-markup

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 2006|AA. Bibliography#API 06]\API 2014]. If the thread were in the process of adding a new integer to the vector when it was stopped, the vector would become accessible while it is in an inconsistent state. This could result in {{For example, this could result in Vector.size()}} returning an incorrect element count , for example, because the element count is incremented after adding the element.

Compliant Solution (volatile flag)

This compliant solution uses a volatile flag to request thread termination. The shutdown() accessor method is used to set the flag to true. The thread's run() method polls the done flag and terminates when it is set.

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;
  }

  @Override 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();
  }
}

...

In this compliant solution, the Thread.interrupt() method is called from main() to terminate the thread. Invoking Thread.interrupt() sets an internal interrupt status flag. The thread polls that flag using the Thread.interrupted() method, which both returns true if the current thread has been interrupted and also clears the interrupt status flag.

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

  public Vector<Integer> getVector() {
    return vector;
  }

  @Override 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();
  }
}

A thread may use interruption for performing tasks other than cancellation and shutdown. Consequently, a thread should be interrupted only when its interruption policy is known in advance. Failure to do so can result in failed interruption requests.

Compliant Solution (Runtime Permission stopThread)

Removing the default permission java.lang.RuntimePermission stopThread permission from the security policy file prevents threads from being stopped using the Thread.stop() method. This approach is discouraged for trusted, custom-developed code that uses that method because the existing design presumably depends upon the ability of the system to perform this action. Furthermore, the system might fail to correctly handle the resulting exception. In these cases, programmers should implement an alternate design corresponding to one of the other compliant solutions described in this rule.

Risk Assessment

Forcing a thread to stop can result in inconsistent object state. Critical resources could also leak if clean-up cleanup operations are not carried out as required.

Automated Detection

Related Guidelines

Bibliography

Android Implementation Details

On Android, Thread.stop() was deprecated in API level 1.

Bibliography

...

Image Added Image Added Image AddedImage Removed      09. Thread APIs (THI)      10. Thread Pools (TPS)