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Do not submit tasks that do not support interruption using {{Thread.interrupt()}} to a thread pool if it is necessary to shut down the thread pool or to cancel individual tasks within it. According to the Java API interface \[[API 2006|AA. Bibliography#API 06]\], the {{java.util.concurrent.ExecutorService.shutdownNow()}} method

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.

Code Block

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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 task does not support interruption using the Thread.interrupt() method, there is no guarantee that the shutdownNow() method will shut down the thread pool. Using the shutdown() method does not fix the problem either because it waits until all executing tasks have finished.

Similarly, tasks that use some mechanism other than Thread.interrupted() to determine when to shut down will be unresponsive to shutdown() or shutdownNow(). For instance, tasks that check a volatile flag to determine whether it is safe to shutdown will be 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.

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.

Code Block

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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-EX1: Short-running tasks that execute without blocking are not required to adhere to this guideline.

Risk Assessment

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

Guideline	Severity	Likelihood	Remediation Cost	Priority	Level
TPS02-J	low	probable	medium	P4	L3

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

Wiki Markup
\[[API 2006\|AA. Bibliography#API 06]\] interface ExecutorService \[[Goetz 2006\|AA. Bibliography#Goetz 06]\] Chapter 7: Cancellation and shutdown

14. Thread Pools (TPS)

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