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Any thread that invokes wait() expects to wake up and resume execution when some condition predicate becomes true. As recommended by CON18-J. Always invoke wait() and await() methods inside a loop, waiting threads should test their condition predicates upon receiving notifications and resume waiting if they are false.evaluate to true

The methods notify() and notifyAll() of package java.lang.Object are used to waken waiting thread(s). These methods must be invoked from code that holds the same object lock as the waiting thread(s). The notifyAll() method wakes up all threads and allows ones whose condition predicate is true evaluates to false, to resume execution. Furthermore, if all the threads whose condition predicate is now true evaluates to false held a specific lock before going into wait state, only one of them will reacquire the lock upon being notified, and the others may, presumably, resume waiting. The notify() method wakes up only one thread, and makes no guarantees as to which thread is notified. If the thread's condition predicate is not satisfieddoesn't allow, the chosen thread may not awaken, defeating the purpose of the notify() call.

The notify() method may only be invoked if all the following conditions are met:

• Every condition predicate in every waiting thread would be satisfied evaluate to false if a notification were received by each, independently. Furthermore, all these threads must perform the same set of operations after waking up. In other words, any one thread can be selected to wake up and resume for an a single invocation of notify().
• Only one thread must is required to wake up on the notify signal. This is contingent on the condition predicate, in that, only one predicate must fulfill the condition and allow the thread to proceed. Multiple condition predicates in the same statement should be avoided.
• No untrusted code has access to the object being waited on. If untrusted code has access to this object, it can wait() on the object and intercept a notify() call.

The java.util.concurrent utilities (interface Condition) provide the signal() and signalAll() methods to awaken waiting threads that are blocked on an await() call. Like Much like the notify() method, the signal() method wakes up any one of the threads that is waiting on the condition and consequently, may be insecure. It is always safer to use signalAll() albeit a small performance penalty. Similarly, any thread that is blocked on a wait() method invocation on a Java object which is being used as a condition queue, should be notified using notifyAll().

...

public final class ProcessStep implements Runnable {
  private static final Object lock = new Object();
  private static int time = 0;
  private final int step; // Do operations when the step reaches this value

  public ProcessStep(int step) {
    this.step = step;
  }

  public void run() {
    try {
      synchronized (lock) {
        while (time != step) { 
          lock.wait();  
        }

        // ... Do operations

        time++;
        lock.notify();
      }
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    }    
  }

  public static void main(String[] args) {
    for (int i = 4; i >= 0; i--) {
      ProcessStep ms = new Thread(new ProcessStep(i);
      new Thread(ms).start();
    }
  }
}

This noncompliant code example violates the liveness property. Each thread has a different condition predicate, as each requires step to have a different value before proceeding. The Object.notify() method wakes up only one thread at a time. unless Unless it happens to wake up the thread that is to perform the next step, the program deadlocks.

Compliant Solution (notifyAll())

In this compliant solution, all threads that have performed their own step use notifyAll() to notify other waiting threads. Consequently, the respective threads that are ready can perform the task, while all other threads whose condition predicate is unsatisfied evaluates to true, promptly go back to sleep.

  // ...
  public void run() {
    try {
      synchronized (lock) {
        while (time != step) { 
          lock.wait();  
        }

        // ... doDo stuffoperations

        time++;
        lock.notifyAll();
      }
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    }    
  }

...

public class ProcessStep implements Runnable {
  private static final Lock lock = new ReentrantLock();
  private static final Condition condition = lock.newCondition();
  private static int time = 0;
  private final int step; // Do operations when the step reaches this value

  public ProcessStep(int step) {
    this.step = step;
  }

  public void run() {
    lock.lock();
    try {
      while (time != step) { 
        condition.await();  
      }

      // ... Do operations

      time++;
      condition.signal();
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    } finally {
      lock.unlock();
    }
  }

  public static void main(String[] args) {
    for (int i = 4; i >= 0; i--) {
      ProcessStep ms = new Thread(new ProcessStep(i);
      new Thread(ms).start();
    }
  }
}

Similar to Object.notify(), the Condition.signal() method may choose any one thread and awaken it.

...

This compliant solution uses the signalAll() method to resume all the waiting threads whose condition predicate allows doing so.

  // ...
  public void run() {
    lock.lock();
    try {
      while (time != step) { 
        condition.await();  
      }

      // ... doDo stuffoperations

      time++;
      condition.signalAll();
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    } finally {
      lock.unlock();
    }
  }

...

Even though signal() is used, it is guaranteed that only one thread will awaken because each condition predicate corresponds to a unique Condition variable. All threads perform the same operations. This compliant solution is only safe if untrusted code cannot create a thread with this class.

Risk Assessment

Invoking the notify() method instead of notifyAll() can be Notifying a single thread instead of all waiting threads can pose a threat to the liveness property of the system.

...