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Threads always preserve class invariants when they are allowed to exit normally. 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 needs to quickly shutdown. or Java Virtual Machine (JVM) must shut down expeditiously.

Certain A few 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 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 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 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.

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 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 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 

• 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.

Noncompliant Code Example (Deprecated Thread.stop())

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

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

  public Vector<Integer> getVector() {
    return vector;
  }
  
  public synchronized void run() {
    Random number = new Random(123L);
    int i = 10;
    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 \[[API 06|AA. Java References#API 06]\]. 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 while the vector contains three elements (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 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 = 10vector.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 by , 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 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 = 10vector.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.

Noncompliant Code Example (blocking IO, volatile flag)

This noncompliant code example uses a volatile done flag to indicate that it is safe to shutdown the thread, as suggested above. However, this does not help in terminating the thread because it is blocked on some network IO as a consequence of using the readLine() method.

class SocketReader implements Runnable {
  private final Socket socket;
  private final BufferedReader in;
  private volatile boolean done = false;
  private final Object lock = new Object();
  private boolean isRunning = false; // Reduces the need to synchronize time consuming operations
  public static class MultipleUseException extends RuntimeException {};

  public SocketReader() throws IOException {
    this.socket = new Socket("somehost", 25);
    this.in = new BufferedReader(new InputStreamReader(this.socket.getInputStream()));
  }
  
  // Only one thread can use the socket at a particular time
  public void run() {
    try {
      synchronized (lock) {
        if (isRunning) {
          throw new MultipleUseException();
        }
        isRunning = true;
      }
      execute();
    } catch (IOException ie) {
      // Forward to handler
    }
  }

  public void execute() throws IOException {
    String string;
    while (!done && (string = in.readLine()) != null) {
      // Blocks until end of stream (null)
    }
  }

  public void shutdown() {
    done = true;
  }

  public static void main(String[] args) throws IOException, InterruptedException {
    SocketReader reader = new SocketReader();
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000);
    reader.shutdown();
  }
}

Note that the Runnable object prevents itself from being run in multiple threads using an isRunning flag. If one thread tries to invoke run() after another thread has already done so, an unchecked exception is thrown.

Noncompliant Code Example (blocking IO, interruptible)

This noncompliant code example uses thread interruption to indicate that it is safe to shutdown the thread, as suggested above. However, this is not useful because the thread is blocked on some network IO as a consequence of using the readLine() method. Network I/O is not responsive to thread interruption.

class SocketReader implements Runnable {
  // ...
  
  public void execute() throws IOException {
    String string;
    while (!Thread.interrupted() && (string = in.readLine()) != null) { 
      // Blocks until end of stream (null)
    }
  }
  
  public static void main(String[] args) throws IOException, InterruptedException {
    SocketReader reader = new SocketReader();
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000); 
    thread.interrupt();
  }
}

Compliant Solution (close socket connection)

This compliant solution closes the socket connection, by having the shutdown() method close the socket. As a result, the thread is bound to stop because of a SocketException. Note that there is no way to keep the connection alive if the thread is to be cleanly halted immediately.

class SocketReader implements Runnable {
  // ...
  
  public void execute() throws IOException {
    String string;
    try {
      while ((string = in.readLine()) != null) { 
        // Blocks until end of stream (null)
      }
    } finally {
      shutdown();
    }
  }
  
  public void shutdown() throws IOException {
    socket.close();
  }

  public static void main(String[] args) throws IOException, InterruptedException {
    SocketReader reader = new SocketReader();
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000); 
    reader.shutdown();
  }
}

A boolean flag can be used (as described earlier) if additional clean-up operations need to be performed. When performing asynchronous I/O, a java.nio.channels.Selector may also be brought out of the blocked state by either invoking its close() or wakeup() method.

Compliant Solution (interruptible channel)

This compliant solution uses an interruptible channel, SocketChannel instead of a Socket connection. If the thread performing the network IO is interrupted using the Thread.interrupt() method, for instance, while reading the data, the thread receives a ClosedByInterruptException and the channel is closed immediately. The thread's interrupt status is also set.

class SocketReader implements Runnable {
  private final SocketChannel sc;
  private final Object lock = new Object();
  private boolean isRunning = false;
  private class MultipleUseException extends RuntimeException {};
  
  public SocketReader() throws IOException {
    sc = SocketChannel.open(new InetSocketAddress("somehost", 25));    
  }
  
  public void run() {
    synchronized (lock) {
      if (isRunning) {
        throw new MultipleUseException();
      }
      isRunning = true;
    }

    ByteBuffer buf = ByteBuffer.allocate(1024);
    try {
      synchronized (lock) {
        while (!Thread.interrupted()) {
          sc.read(buf);
          // ...
        }
      }
    } catch (IOException ie) {
      // Forward to handler
    }
  }

  public static void main(String[] args) throws IOException, InterruptedException {
    SocketReader reader = new SocketReader();
    Thread thread = new Thread(reader);
    thread.start();
    Thread.sleep(1000);
    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 the thread when it is interrupted. Invoking the interrupt() method of a thread that is blocked because of a java.nio.channels.Selector also causes the thread to awaken.

Noncompliant Code Example (database connection)

This noncompliant code example shows a thread-safe class DBConnector that creates a JDBC connection per thread, that is, the connection belonging to one thread is not shared by other threads. This is a common use-case because JDBC connections are not meant to be shared by multiple-threads.

class DBConnector implements Runnable {
  final String query;
  private final Object lock = new Object();
  private boolean isRunning = false;
  private class MultipleUseException extends RuntimeException {};
  
  DBConnector(String query) {
    this.query = query; 
  }
	
  public void run() {
    synchronized (lock) {
      if (isRunning) {
        throw new MultipleUseException();
      }
      isRunning = true;
    }

    Connection con;
    try {
      // Username and password are hard coded for brevity
      con = DriverManager.getConnection("jdbc:myDriver:name", "username","password");  
      Statement stmt = con.createStatement();
      ResultSet rs = stmt.executeQuery(query);
    } catch (SQLException e) {
      // Forward to handler
    }
    // ... 
  }  


  public static void main(String[] args) throws InterruptedException {
    DBConnector connector = new DBConnector(/* suitable query */);
    Thread thread = new Thread(connector);
    thread.start();
    Thread.sleep(5000);
    thread.interrupt();
  }
}

Unfortunately database connections, like sockets, are not inherently interruptible. So this design does not permit a client to cancel a task by closing it if the corresponding thread is blocked on a long running activity such as a join query. Furthermore, it is important to provide a mechanism to close connections to prevent thread starvation caused because of the limited number of database connections available in the pool. Similar task cancellation mechanisms are required when using objects local to a method, such as sockets.

Compliant Solution (ThreadLocal)

This compliant solution uses a ThreadLocal wrapper around the connection so that a thread that calls initialValue() obtains a unique connection instance. The advantage of this approach is that a shutdownConnection() method can be provided so that clients external to the class can also close the connection when the corresponding thread is blocked, or is performing some time consuming activity.

class DBConnector implements Runnable {
  final String query;
  
  DBConnector(String query) {
    this.query = query;
  }
	
  private static ThreadLocal<Connection> connectionHolder = new ThreadLocal<Connection>() {
    Connection connection = null;
    @Override public Connection initialValue() {		
      try {
        // Username and password are hard coded for brevity
	connection = DriverManager.getConnection
          ("jdbc:driver:name", "username","password");  	    	      
      } catch (SQLException e) {
        // Forward to handler
      }
      return connection;
    }
		
    @Override public void set(Connection con) {
      if(connection == null) { // Shuts down connection when null value is passed 		       
        try {
          connection.close();
        } catch (SQLException e) {
          // Forward to handler 
        }	       		       
      } else {
        connection = con; 
      }
    }
  };


  public static Connection getConnection() {
    return connectionHolder.get();
  }

  public static void shutdownConnection() { // Allows client to close connection anytime
    connectionHolder.set(null);
  }

  public void run() {
    Connection dbConnection = getConnection();
    Statement stmt;
    try {
      stmt = dbConnection.createStatement();		
      ResultSet rs = stmt.executeQuery(query);
    } catch (SQLException e) {
      // Forward to handler	 
    }    
    // ...
  }


  public static void main(String[] args) throws InterruptedException {
    DBConnector connector = new DBConnector(/* suitable query */);
    Thread thread = new Thread(connector);
    thread.start();
    Thread.sleep(5000);
    connector.shutdown();
  }
}

Noncompliant Code Example (database connection)

This noncompliant code example shows a thread-safe class DBConnector that creates a JDBC connection per thread, that is, the connection belonging to one thread is not shared by other threads. This is a common use-case because JDBC connections are not meant to be shared by multiple-threads.

class DBConnector implements Runnable {
  final String query;
  private final Object lock = new Object();
  private boolean isRunning = false;
  private class MultipleUseException extends RuntimeException {};
  
  DBConnector(String query) {
    this.query = query; 
  }
	
  public void run() {
    synchronized (lock) {
      if (isRunning) {
        throw new MultipleUseException();
      }
      isRunning = true;
    }

    Connection con;
    try {
      // Username and password are hard coded for brevity
      con = DriverManager.getConnection("jdbc:myDriver:name", "username","password");  
      Statement stmt = con.createStatement();
      ResultSet rs = stmt.executeQuery(query);
    } catch (SQLException e) {
      // Forward to handler
    }
    // ... 
  }  


  public static void main(String[] args) throws InterruptedException {
    DBConnector connector = new DBConnector(/* suitable query */);
    Thread thread = new Thread(connector);
    thread.start();
    Thread.sleep(5000);
    thread.interrupt();
  }
}

Unfortunately database connections, like sockets, are not inherently interruptible. So this design does not permit a client to cancel a task by closing it if the corresponding thread is blocked on a long running activity such as a join query. Furthermore, it is important to provide a mechanism to close connections to prevent thread starvation caused because of the limited number of database connections available in the pool. Similar task cancellation mechanisms are required when using objects local to a method, such as sockets.

Compliant Solution (ThreadLocal)

This compliant solution uses a ThreadLocal wrapper around the connection so that a thread that calls initialValue() obtains a unique connection instance. The advantage of this approach is that a shutdownConnection() method can be provided so that clients external to the class can also close the connection when the corresponding thread is blocked, or is performing some time consuming activity.

class DBConnector implements Runnable {
  final String query;
  
  DBConnector(String query) {
    this.query = query;
  }
	
  private static ThreadLocal<Connection> connectionHolder = new ThreadLocal<Connection>() {
    Connection connection = null;
    @Override public Connection initialValue() {		
      try {
        // Username and password are hard coded for brevity
	connection = DriverManager.getConnection
          ("jdbc:driver:name", "username","password");  	    	      
      } catch (SQLException e) {
        // Forward to handler
      }
      return connection;
    }
		
    @Override public void set(Connection con) {
      if(connection == null) { // Shuts down connection when null value is passed 		       
        try {
          connection.close();
        } catch (SQLException e) {
          // Forward to handler 
        }	       		       
      } else {
        connection = con; 
      }
    }
  };


  public static Connection getConnection() {
    return connectionHolder.get();
  }

  public static void shutdownConnection() { // Allows client to close connection anytime
    connectionHolder.set(null);
  }

  public void run() {
    Connection dbConnection = getConnection();
    Statement stmt;
    try {
      stmt = dbConnection.createStatement();		
      ResultSet rs = stmt.executeQuery(query);
    } catch (SQLException e) {
      // Forward to handler	 
    }    
    // ...
  }


  public static void main(String[] args) throws InterruptedException {
    DBConnector connector = new DBConnector(/* suitable query */);
    Thread thread = new Thread(connector);
    thread.start();
    Thread.sleep(5000);
    connector.shutdown();
  }
}

Noncompliant Code Example (Interrupting thread executing in thread pool)

This noncompliant code example uses a runnable task shown below:

public class Task implements Runnable {
  @Override public void run() {
    try {
      Thread.sleep(10000); // Do some time consuming task
    } catch(InterruptedException ie) {
      // Forward to handler  
    }		
  }
}

The doSomething() method of class PoolService submits the task to a thread pool. It carries out the task and if an exception results during the execution of the code in the try block, it interrupts the thread as an attempt to cancel it.

class PoolService {
  private final ExecutorService pool;

  public PoolService(int poolSize) {
    pool = Executors.newFixedThreadPool(poolSize);
  }
  
  public void doSomething() throws InterruptedException {
    Thread thread = new Thread(new Task());
    try {
      pool.submit(thread);
      // ...
    } catch(Throwable t) {
      thread.interrupt();
      // Forward to handler
    }
  }
}

However, the interruption has no effect on the execution of the task because it is executing in a thread pool. Furthermore, note that it is unknown which tasks would be running in the thread pool at the time the interruption notification is delivered \[[Goetz 06|AA. Java References#Goetz 06]\]. 

Compliant Solution (Cancel using the Future)

This compliant solution obtains the return value of the task (future) and invokes the cancel() method on it.

class PoolService {
  private final ExecutorService pool;

  public PoolService(int poolSize) {
    pool = Executors.newFixedThreadPool(poolSize);
  }
  
  public void doSomething() throws InterruptedException {
    Future<?> future = null;
    try {
      future = pool.submit(new Task());
      // ...
    } catch(Throwable t) {
      future.cancel(true);
      // Forward to handler
    }
  }
}

Noncompliant Code Example (shutting down thread pools)

According to the Java API \[[API 06|AA. Java References#API 06]\], interface {{java.util.concurrent.ExecutorService}}, method {{shutdownNow()}} documentation:

Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution. There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt(), so any task that fails to respond to interrupts may never terminate.

This noncompliant code example uses the SocketReader class defined earlier in Compliant Solution (close socket connection) and submits it as a task to a thread pool defined in class PoolService.

class PoolService {
  private final ExecutorService pool;

  public PoolService(int poolSize) {
    pool = Executors.newFixedThreadPool(poolSize);
  }
	  
  public void doSomething() throws InterruptedException, IOException {	   
    pool.submit(new SocketReader());
    // ...
    List<Runnable> awaitingTasks = pool.shutdownNow();	      
  }

  public static void main(String[] args) throws InterruptedException, IOException {
    PoolService service = new PoolService(5);
    service.doSomething();
  }
}

class SocketReader implements Runnable {
  private final Socket socket;
  // ...
}

Because the task does not support interruption through the use of Thread.interrupted(), there is no guarantee that the shutdownNow() method will shutdown the thread pool. Using the shutdown() method does not fix the problem either because it waits until all executing tasks have finished. Likewise, tasks that check a volatile flag to determine whether it is safe to shutdown are not responsive to these methods.

Compliant Solution (submit interruptible tasks)

Tasks that do not support interruption using Thread.interrupt() should not be submitted to a thread pool. This compliant solution submits the interruptible version of SocketReader discussed in Compliant Solution (interruptible channel) to the thread pool.

class PoolService {
  // ...
}

class SocketReader implements Runnable {
  private final SocketChannel sc;
  // ...
}

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

\[[API 06|AA. Java References#API 06]\] 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

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.

Risk Assessment

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

Automated Detection

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