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A few thread 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. Use of the {{Thread.stop()}} method is highly inadvisable because of two reasons. First, no particular thread can be forcefully stopped because an arbitrary thread can catch the thrown {{ThreadDeath}} exception and simply choose to ignore it. Second, stopping threads abruptly results in the release of all the associated monitors, violating the guarantees provided by the critical sections. Moreover, the objects end up in an inconsistent state, nondeterministic behavior being a typical outcome.

   

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

.

     
h2. Noncompliant Code Example (Deprecated {{Thread.stop()}})

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 executes. Any monitors that are held are immediately released, leaving the object in a delicate state.

The {{Thread.interrupt()}} method is frequently used to awaken a blocked thread before it can be stopped. It awakens threads that are blocked on {{wait()}} methods of class {{Object}}, and {{join}} and {{sleep}} methods of class {{Thread}}. In these cases, the thread's interrupt status is cleared and it receives an {{InterruptedException}}. If the thread is blocked on I/O operations upon an interruptible channel, the channel is closed, the thread's interrupt status is set and it receives a {{ClosedByInterruptException}}. Similarly, a thread waiting on a selector also returns from the operation with its interrupted status set. \[[API 06|AA. Java References#API 06]\]   

The {{java.lang.ThreadGroup.interrupt()}} is not deprecated and is often seen as an option to interrupt all the threads belonging to a thread group. "This is no guarantee that a program will terminate, however, because libraries that you have used may have created user threads that do not respond to interrupt requests. The AWT graphics library is one well-known example." \[[JPL 06|AA. Java References#JPL 06]\]. Moreover, using the {{ThreadGroup}} API is discouraged (see [CON17-J. Avoid using ThreadGroup APIs]).

h2. Compliant Solution (1) ({{volatile}} flag)

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}}, after which the thread can start the cancellation process. The {{done}} flag is also set immediately after the execution of the {{finally}} block's resource clean-up statements so that the system does not continue relinquishing resources that it has already released, in the event of the {{done}} flag staying {{false}}.

{code

:bgColor

=#ccccff

}
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("Interrupted Exception");
          // Handle the exception 
      } finally { 
           System.out.println("Closing resources");  done = true;
      }
    } 
     done = true;
      }
    } 
    done = false; // Reset for later use
    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();
  }
}
{code} 

h2. Compliant Solution (2) ({{RuntimePermission stopThread}})

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

h2.

 Noncompliant Code Example (blocking IO)

This noncompliant code example uses the advice suggested in the previous compliant solution. However, this does not help in terminating the thread because it is blocked on some network IO as a consequence of using the {{readLine()}} method.

{code

:bgColor

=#FFcccc

}
class StopSocket extends Thread {
  private Socket s;
  private 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) { 
          // Forward to handler
      } finally {
           System.out.println("Closing resources");
          done 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();
  }
}
{code}

A {{Socket}} connection is not affected by the {{InterruptedException}} that results with the use of the {{Thread.interrupt()}} method. The {{boolean}} flag solution does not work in such cases.

h2.

 Compliant Solution (close socket connection)

This compliant solution 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.

{code

:bgColor

=#ccccff

}
class StopSocket extends Thread {
  private 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)
      }
    } catch System.out.println("Blocked, will not get executed until some data is received. " + s);
    } catch (IOException ie) { 
        // Handle (IOException ie) { 
        // Handle the exception 
    } 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();
  }
}
{code}

A {{boolean}} flag can be used (as described earlier) if additional clean-up operations need to be performed.

h2.

 Compliant Solution (2) (interruptible channel)

This compliant solution uses an interruptible channel, {{SocketChannel}} instead of a {{Socket}} connection. If the thread performing the network IO is interrupted using the {{Thread.interrupt()}} method, for instance, while reading the data, the thread receives a {{ClosedByInterruptException}} and the channel is closed immediately. The thread's interrupt status is also set.

{code

:bgColor

=#ccccff

}
class StopSocket extends Thread {
  private volatile boolean done = false;
  public void run() { 
    while(!done) {
      try {
    	InetSocketAddress addr = new InetSocketAddress("somehost", 25);
        SocketChannel sc = SocketChannel.open(addr);
        ByteBuffer buf = ByteBuffer.allocate(1024);
        sc.read(buf);
        // ...
      } catch (IOException ie) {  
          // Handle the exception
      } finally {
          System.out.println("Closing resources");
          done = true;
      }
    }
  }  

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

{code}

h2. 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.

|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON13- J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |



h3. Automated Detection

TODO



h3. Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON35-J].

h2. References

\[[API 06|AA. Java References#API 06]\] Class Thread, method {{stop}}
\[[Darwin 04|AA. Java References#Darwin 04]\] 24.3 Stopping a Thread
\[[JDK7 08|AA. Java References#JDK7 08]\] Concurrency Utilities, More information: Java Thread Primitive Deprecation 
\[[JPL 06|AA. Java 

References#JPL 06]\]

 14.12.1. Don't stop and 23.3.3. Shutdown Strategies
\[[

JavaThreads 04|AA. Java 

References#JavaThreads 04]\] 

2.4 Two Approaches to Stopping a Thread

----

[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|CON12-J. Avoid deadlock by requesting and releasing locks in the same order]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|11. Concurrency (CON)]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|VOID CON14-J. Ensure atomicity of 64-bit operations]