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Noncompliant Code Example (shutting down thread pools)

Wiki MarkupAccording to the Java API \[[API 06|AA. Java References#API 06]\], interface {{Programs may submit only tasks that support interruption using Thread.interrupt() to thread pools that require the ability to shut down the thread pool or to cancel individual tasks within the pool. Programs must not submit tasks that lack interruption support to such thread pools. According to the Java API [API 2014], the java.util.concurrent.ExecutorService}}, method {{.shutdownNow()}} documentation: method

attempts 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.

Noncompliant Code Example (Shutting Down Thread Pools)

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

public final class SocketReader implements Runnable { // Thread-safe class
  private final Socket socket;
  private final BufferedReader in;
  private final Object lock = new Object();

  public SocketReader(String host, int port) throws IOException {
    this.socket = new Socket(host, port);
    this.in = new BufferedReader(
        new InputStreamReader(this.socket.getInputStream())
    );
  }

  // Only one thread can use the socket at a particular time
  @Override public void run() {
    try {
      synchronized (lock) {
        readData();
      }
    } catch (IOException ie) {
      // Forward to handler
    }
  }

  public void readData() 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 final class PoolService {
  private final ExecutorService pool;

  public PoolService(int poolSize) {
    pool = Executors.newFixedThreadPool(poolSize);
  }
	  
  public void doSomething() throws InterruptedException, IOException {	   
    pool.submit(new SocketReader("somehost", 8080));
    // ...
    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;
  // ...
}

The shutdownNow() method may fail to shut down the thread pool because the task lacks support for interruption using the Thread.interrupt() method and because the shutdown() method must wait 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.

Similarly, tasks that use some mechanism other than Thread.interrupted() to determine when to shut down will be unresponsive to shutdown() and shutdownNow(). For instance, tasks that check a volatile flag to determine whether it is safe to shutdown are not responsive unresponsive to these methods.

Compliant Solution (submit interruptible tasks)

THI05-J. Do not use Thread.stop() to terminate threads provides more information on using a flag to terminate threads.

Compliant Solution (Submit Interruptible Tasks)

This compliant solution defines an interruptible version of the SocketReader class, which is instantiated and submitted to the thread pool: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 {
  public final class SocketReader implements Runnable {
  private final SocketChannel sc;
  private final Object lock = new Object();

  public SocketReader(String host, int port) throws IOException {
    sc = SocketChannel.open(new InetSocketAddress(host, port));
  }

  @Override public void run() {
    ByteBuffer buf = ByteBuffer.allocate(1024);
    try {
      synchronized (lock) {
        while (!Thread.interrupted()) {
          sc.read(buf);
          // ...
        }

class SocketReader implements Runnable      }
    } catch (IOException ie) {
  private final SocketChannel sc;      // Forward to handler
    }
  }
}

public final class PoolService {
  // ...
}

Exceptions

TPS02-J-EX0: Short-running tasks that execute without blocking are exempt from this ruleSimilarly, when trying to cancel individual tasks within the thread pool using the Future.cancel() method, ensure that the task supports interruption. If it does, pass a boolean argument true to cancel(), otherwise pass false. The value false indicates that the task will be canceled if it has not already started.

Risk Assessment

Submitting tasks that are not interruptible may preclude the shut down procedure of uninterruptible may prevent a thread pool from shutting down and consequently may cause denial of service DoS.

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
\[[Goetz 06|AA. Java References#Goetz 06]\] Chapter 7: Cancellation and shutdown

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