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

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

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More information about deprecated methods is available in MET02-J. Do not use deprecated or obsolete classes or methods.

Invoking Thread.stop() results in the release of all

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locks

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a thread has acquired,

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potentially exposing the objects protected by those locks when those objects are in an inconsistent state

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. The thread might catch the ThreadDeath exception and use a finally block in an attempt to repair the inconsistent object or objects. However, 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 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 ERR09-J. Do not allow untrusted code to terminate the JVM

• 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.

Why is Thread.stop deprecated?

Because it is inherently unsafe. Stopping a thread causes it to unlock all the monitors that it has locked. (The monitors are unlocked as the ThreadDeath exception propagates up the stack.) If any of the objects previously protected by these monitors were in an inconsistent state, other threads may now view these objects in an inconsistent state. Such objects are said to be damaged. When threads operate on damaged objects, arbitrary behavior can result. This behavior may be subtle and difficult to detect, or it may be pronounced. Unlike other unchecked exceptions, ThreadDeath kills threads silently; thus, the user has no warning that his program may be corrupted. The corruption can manifest itself at any time after the actual damage occurs, even hours or days in the future.

Couldn't I just catch the ThreadDeath exception and fix the damaged object?

In theory, perhaps, but it would vastly complicate the task of writing correct multithreaded code. The task would be nearly insurmountable for two reasons:

1. A thread can throw a ThreadDeath exception almost anywhere. All synchronized methods and blocks would have to be studied in great detail, with this in mind.
2. A thread can throw a second ThreadDeath exception while cleaning up from the first (in the catch or finally clause). Cleanup would have to repeated till it succeeded. The code to ensure this would be quite complex.

In sum, it just isn't practical.
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 information on preventing 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 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 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 returns the correct number of elements in the vector even when an element is added or removed. This is , even 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.

Wiki MarkupHowever, 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|AA. Java References#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 even though the vector contains three elements (as the element count is incremented after adding the element)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. For example, this could result in Vector.size() returning an incorrect element count because the element count is incremented after adding the element.

Compliant Solution (volatile flag)

This compliant solution uses a volatile flag to stop the request thread termination. An accessor method 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 becomes trueis 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();
  }
}

Compliant Solution (Interruptible)

This In this compliant solution stops the thread , 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.interruptinterrupted() method, which both returns true if the current thread has been interrupted and 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();
  }
}

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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 only when 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 privilegesdo so can result in failed interruption requests.

Risk Assessment

Forcing a thread to stop can result in inconsistent object state. Critical resources may could also leak if clean-up 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

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

Related Guidelines

Android Implementation Details

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

Bibliography

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Image Added Image Added Image AddedCON12-J. Avoid deadlock by requesting and releasing locks in the same order      11. Concurrency (CON)      VOID CON14-J. Ensure atomicity of 64-bit operations