CON35-J. Do not try to force thread shutdown

Threads always preserve class invariants when they are allowed to exit normally. Programmers often try to forcefully terminate threads when they believe that the task is accomplished, the request canceled or the program needs to quickly shutdown.

A few APIs were introduced to facilitate thread suspension, resumption and termination but were later deprecated due to inherent design weaknesses. The Thread.stop() method is one example. It throws a ThreadDeath exception to stop the thread. Two cases arise:

• If ThreadDeath is left uncaught, it allows the execution of a finally block which performs the usual cleanup operations. This method is not a good idea because of two reasons. First, no particular thread can be forcefully stopped because an arbitrary thread can catch the thrown exception and simply choose to ignore it. Second, stopping threads leads to the release of all monitors violating the guarantees provided by the critical sections. Moreover, the objects end up in an inconsistent state with arbitrary behavior being a typical outcome.

• As a remediation measure, catching ThreadDeath on the other hand can itself ensnarl multithreaded code. For one, the exception can be thrown anywhere making it difficult to trace it and recover effectively. Also, there is nothing stopping a thread from throwing another ThreadDeath exception while recovery is in progress.

Noncompliant Code Example

This noncompliant code example shows a thread that forcefully comes to a halt when the Thread.stop() method is invoked. Neither the catch nor the finally block is executed. Needless to say, any monitors that are held are immediately released leaving the object in a delicate state.

class BadStop implements Runnable {
  public void run() {
    try {
      Thread.currentThread().sleep(1000);
    } catch(InterruptedException ie) { // not executed 
        System.out.println("Performing cleanup"); 
    } finally { // not executed
        System.out.println("Closing resources"); 
    }       
    System.out.println("Done!");
  }
}

class Controller {
  public static void main(String[] args) {
    Thread t = new Thread(new BadStop());
    t.start();  
    t.interrupt(); // artificially induce an InterruptedException
    t.stop(); // force thread cancellation
  }
}

Compliant Solution (1)

This compliant example uses a boolean flag called done to indicate whether the thread should be stopped after any necessary cleanup code has finished executing. An accessor method shutdown() is used to set the flag to true upon which the thread will start the cancellation process. The done flag is also set immediately after the execution of the finally block statements so that the system does not continue relinquishing resources that it has already released, in the event of done staying false.

class ControlledStop implements Runnable{
  protected volatile boolean done = false;
  public void run() {
    while(!done) {
      try {
        Thread.currentThread().sleep(1000);
      }catch(InterruptedException ie) { System.out.println("Performing cleanup"); }
      finally { 
        System.out.println("Closing resources"); 
        done = true; 
      }
    } 
    System.out.println("Done!");
  }

  protected void shutdown(){
    done = true;
  }
}

class Controller {
  public static void main(String[] args) throws InterruptedException {  
    ControlledStop c = new ControlledStop();
    Thread t = new Thread(c);
    t.start();
    t.interrupt();  // artificially induce an InterruptedException
    Thread.sleep(1000);  // wait for some time to allow the exception
                         // to be caught (demonstration only)
    c.shutdown();
  }
}

Compliant Solution (2)

Remove the default java.lang.RuntimePermission stopThread from the security policy file to deny the Thread.stop() invoking code, the required privileges.

Noncompliant Code Example

This noncompliant solution uses the advice suggested in the previous compliant solution. Unfortunately, this does not help in terminating the thread because it is blocked on some network IO because of the readLine method. The boolean flag trick does not work in such cases; a good alternative to end the thread is required.

class StopSocket extends Thread {
  protected Socket s;
  protected volatile boolean done = false;
  public void run() { 
    while(!done) {
      try {
        s = new Socket("somehost",25);
        BufferedReader br = new BufferedReader(new InputStreamReader(s.getInputStream()));
        String s = null;
        while((s = br.readLine()) != null) { 
          // blocks until end of stream (null)
        }
        System.out.println("Blocked, will not get executed until some data is received. " + s);
      } catch (IOException ie) { 
          System.out.println("Performing cleanup"); 
      } finally {
          System.out.println("Closing resources");
          done = true;
      }
    }
  }  

  public void shutdown() throws IOException {
    done = true;
  }
}

class Controller {
  public static void main(String[] args) throws InterruptedException, IOException {
     StopSocket ss = new StopSocket();
     Thread t = new Thread(ss);
     t.start();
     Thread.sleep(1000); 
     ss.shutdown();
  }
}

Compliant Solution

This compliant solution simply closes the socket connection, both using the shutdown() method as well as the finally block. As a result, the thread is bound to stop due to a SocketException. Note that there is no way to keep the connection alive if the thread is to be cleanly halted immediately.

class StopSocket extends Thread {
  protected Socket s;
  public void run() { 
    try {
      s = new Socket("somehost",25);
      BufferedReader br = new BufferedReader(new InputStreamReader(s.getInputStream()));
      String s = null;
      while((s = br.readLine()) != null) { 
        // blocks until end of stream (null)
      }
      System.out.println("Blocked, will not get executed until some data is received. " + s);
    } catch (IOException ie) { 
        System.out.println("Performing cleanup"); 
    } finally {
        System.out.println("Closing resources");
        try {
          if(s != null)
            s.close();
        } catch (IOException e) { /* forward to handler */ }
    }
  }

  public void shutdown() throws IOException {
    if(s != null)
      s.close();
  }
}

class Controller {
  public static void main(String[] args) throws InterruptedException, IOException {
    StopSocket ss = new StopSocket();
    Thread t = new Thread(ss);
    t.start();
    Thread.sleep(1000); 
    ss.shutdown();
  }
}

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]] Class Thread, method stop
[[Darwin 04]] 24.3 Stopping a Thread
[[JDK7 08]] Concurrency Utilities, More information: Java Thread Primitive Deprecation
[[JPL 05]] 14.12.1. Don't stop
[[JavaThreads 04]] 2.4 Two Approaches to Stopping a Thread

CON34-J. Avoid deadlock by requesting locks in the proper order      09. Concurrency (CON)      CON36-J. Always synchronize on the appropriate object