CON16-J. Do not assume that the sleep(), yield() or getState() methods provide synchronization semantics

According to Section 17.9, "Sleep and Yield" of the Java Language Specification [[JLS 05]]:

It is important to note that neither Thread.sleep nor Thread.yield have any synchronization semantics. In particular, the compiler does not have to flush writes cached in registers out to shared memory before a call to Thread.sleep or Thread.yield, nor does the compiler have to reload values cached in registers after a call to Thread.sleep or Thread.yield.

Incorrectly assuming that thread suspension and yielding flush the cached registers, reload any values, or provide any [happens-before] relations when execution resumes can result in unexpected behavior.

Noncompliant Code Example (sleep())

This noncompliant code attempts to use a non-volatile boolean done as a flag to terminate execution of a thread. A separate thread sets done to true by calling shutdown().

final class ControlledStop implements Runnable {
  private boolean done = false;

  public void run() {
    while (!done) {
      try {
        Thread.sleep(1000);
      } catch (InterruptedException e) {
        Thread.currentThread().interrupt(); // Reset interrupted status
      }
    }
  }  

  public void shutdown() {
    this.done = true;
  }
}

However, the compiler is free to read the field this.done once, and reuse the cached value in each execution of the loop. Consequently, the while loop might not terminate, even if another thread calls the shutdown() method to change the value of this.done [[JLS 05]]. This error could have resulted from the programmer incorrectly assuming that the call to Thread.sleep() would cause cached values to be reloaded.

Compliant Solution (volatile flag)

This compliant solution declares the flag volatile to ensure that updates to it are made visible across multiple threads.

final class ControlledStop implements Runnable {
  private volatile boolean done = false;

  // ...
}

The volatile flag establishes a [happens-before] relation between this thread and any other thread that sets done.

Compliant Solution (Thread.interrupt())

A better solution for methods that call sleep() is to use thread interruption, which causes the sleeping thread to wake up immediately and handle the interruption.

final class ControlledStop implements Runnable {
  @Override public void run() {
    while (!Thread.interrupted()) {
      try {
        Thread.sleep(1000);
      } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
      }
    } 
  }  

  public void shutdown() {
    Thread.currentThread().interrupt();
  }
}

Noncompliant Code Example (getState())

This noncompliant code example contains a doSomething() method that starts a thread. The thread supports interruption by checking a volatile flag and blocks waiting until notified. The stop() method notifies the thread if it is blocked on the wait and sets the flag to true so that the thread can terminate.

public class Waiter {
  private Thread thread;
  private volatile boolean flag;
  private final Object lock = new Object(); 

  public void doSomething() {    
    thread = new Thread(new Runnable() {     
      @Override public void run() {
        synchronized(lock) {
          while (!flag) {
            try {
              lock.wait();
              // ...
            } catch (InterruptedException e) {
              // Forward to handler  
            }
          }
        }
      }
    });
    thread.start();             
  }

  public boolean stop() {
    if (thread != null) {    
      if (thread.getState() == Thread.State.WAITING) {
        flag = true;
        synchronized (lock) {
          lock.notifyAll();
        }
        return true;
      }     
    }
    return false;
  }
}

Unfortunately, the stop() method incorrectly uses the Thread.getState() method to check whether the thread is blocked and has not terminated before delivering the notification. Using the Thread.getState() method for synchronization control such as checking whether a thread is blocked on a wait is inappropriate because a blocked thread is not always required to enter the WAITING or TIMED_WAITING state in cases where the JVM implements blocking using spin-waiting [[Goetz 06]]. Because the thread may never enter the WAITING state, the stop() method may not terminate the thread.

Compliant Solution

This compliant solution removes the check for determining whether the thread is in the WAITING state. This check is unnecessary because invoking notifyAll() on a thread that is not blocked on a wait() invocation has no effect.

public class Waiter {
  // ...

  public boolean stop() {
    if (thread != null) {
      flag = true;
      synchronized (lock) {
        lock.notifyAll();
      }
      return true;
    }
    return false;
  }
}

Risk Assessment

Relying on class Thread's sleep(), yield() and getState() methods for synchronization control can cause unexpected behavior.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[JLS 05]] section 17.9 "Sleep and Yield"

Issue Tracking

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