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Wiki MarkupDo not submit tasks that do not support interruption using {{Programs may submit only tasks that support interruption using Thread.interrupt()}} to a thread pool if it is necessary to shutdown the thread pool or to cancel individual tasks within it. According to the Java API interface \[[API 2006|AA. Java References#API 06]\], the {{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.shutdownNow()}} 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.

Similarly, when attempting to cancel individual tasks within the thread pool using the Future.cancel() method, ensure that the tasks support interruption.

Noncompliant Code Example (Shutting Down Thread Pools)

This noncompliant code example submits the SocketReader class as a task to the thread pool declared in 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();
  }
}

Because The shutdownNow() method may fail to shut down the thread pool because the task does not lacks support for interruption using the Thread.interrupt() method , 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 and because the shutdown() method must wait until all executing tasks have finished.

Similarly, tasks that use some mechanism other than Thread.interrupted() to determine when to shutdown shut down will be unresponsive to shutdown() or and shutdownNow(). For instance, tasks that check a volatile flag to determine whether it is safe to shutdown will be are unresponsive to these methods. The guideline THI05-J. Do not use Thread.stop() to terminate threads provides more information on using a flag to terminate threads.

...

This compliant solution defines an interruptible version of the SocketReader class, which is instantiated and submitted to the thread pool.:

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);
          // ...
        }
      }
    } catch (IOException ie) {
      // Forward to handler
    }
  }
}

public final class PoolService {
  // ...
}

Exceptions

TPS02-EX1J-EX0: Short-running tasks that execute without blocking are not required to adhere to this guidelineexempt from this rule.

Risk Assessment

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

References

\[[API 2006|AA. Java References#API 06]\] interface ExecutorService
\[[Goetz 2006|AA. Java References#Goetz 06]\] Chapter 7: Cancellation and shutdown

Bibliography

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Image Added Image Added Image Removed      12. Locking (LCK)