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Threads always preserve the class invariants when they are allowed to exit normally. Unfortunately, programmers often try to forcefully terminate threads when they believe that the task is accomplished, the request is 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 such example. It was intended to throw 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 is not a good idea because of two reasons. First, no thread can be forcefully stopped since because an arbitrary thread can catch the thrown exception and simply choose to ignore it. Second, stopping threads leads to the release of all held monitors violating the guarantees provided by the critical sections. Moreover, the damaged objects end up in an inconsistent state with arbitrary behavior being a typical outcome.

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This noncompliant code example shows how a thread 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 held monitors will be are immediately released leaving the object in a delicate state.

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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 has also been set immediately following after the execution of the initial finally block statements so that the system does not continue to relinquish the relinquishing resources that it has already released, in the event of done staying false.

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

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

The This compliant solution simply closes the socket connection, both using the shutdown method as well as in 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.

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