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• The condition predicate is identical for each waiting thread.
• All 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 a single invocation of notify().
• Only one thread is required to wake upon the notification.

These conditions can be trivially met if each waiting thread is identicalthreads are often identical and provide a stateless service or utility.

The java.util.concurrent utilities (interface Condition) provide the signal() and signalAll() methods to awaken threads that are blocked on an await() call. The same rules that apply to using notify() and notifyAll() also apply to programmer must ensure the liveness property when using signal() and signalAll().

Noncompliant Code Example (notify())

This noncompliant code example shows a complex multi-step process being undertaken by several threads. Each thread executes one step of the process; the step being currently performed is indicated by the step field. Each thread waits for the step field to indicate that it is time to perform the corresponding thread's step. After performing the step, each thread increments step to transfer control to the next thread. The thread then notifies the thread that is responsible for the next step , and then the first thread exits.

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 field time reaches this value

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

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

        // ... DoPerform 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--) {
      new Thread(new ProcessStep(i)).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 it happens to wake up the thread that is required to perform the next step, the program will deadlock and fail to make any progress.

Compliant Solution (notifyAll())

In this compliant solution, all threads that have performed their own step use each thread completes its step then calls notifyAll() to notify other the waiting threads. Consequently, the The thread that is ready can perform its task, while all other the threads whose condition predicates are false (loop condition expression is true) , promptly resume waiting.

Only the run() method from the noncompliant code example is modified as follows:

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

        // ... DoPerform operations

        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 field time reaches this value

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

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

      // ...Perform 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--) {
      new Thread(new ProcessStep(i)).start();
    }
  }
}

Similar to Object.notify(), the signal() method may choose any one thread and awaken itawaken an arbitrary thread.

Compliant Solution (signalAll())

This compliant solution uses the signalAll() method to resume all the waiting threads. The thread that is ready can perform its task, while all the threads whose condition predicate allows doing sopredicates are false resume waiting.

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

      // ... DoPerform operations

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

...

This compliant solution assigns each thread its own condition, and makes all . All the Condition objects are accessible to all the threads.

// Declare class as final because its constructor throws an exception 
public final class ProcessStep implements Runnable { 
  private static final Lock lock = new ReentrantLock();
  private static int time = 0;
  private final int step; // Do operations when field time reaches this value
  private static final int MAX_STEPS = 5;
  private static final Condition[] conditions = new Condition[MAX_STEPS];

  public ProcessStep(int step) {
    if (step <= MAX_STEPS) {
      this.step = step;
      conditions[step] = lock.newCondition();
    } else {
      throw new IllegalArgumentException("Too many threads");
    }
  }

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

      // ... DoPerform operations

      time++;
      if (step + 1 < conditions.length) {
        conditions[step + 1].signal();
      }
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    } finally {
      lock.unlock();
    }
  }

  public static void main(String[] args) {
    for (int i = MAX_STEPS - 1; i >= 0; i--) {
      ProcessStep ms = new ProcessStep(i);
      new Thread(ms).start();
    }
  }
}

Even though signal() is used, it is guaranteed that only one thread will awaken because each the thread whose condition predicate corresponds to a the unique Condition variable will awaken. All threads perform the same operations.

This compliant solution is only safe if untrusted code cannot create a thread with an instance of this class.

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