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 An {{IllegalMonitorStateException}} is thrown if the current thread is not the owner of this object's monitor. The {{notifyAll()}} method wakes up all threads and allows threads whose condition predicate is true to resume execution. Furthermore, if all the threads whose condition predicate evaluates to true previously held a specific lock before going into the wait state, only one of them will reacquire the lock upon being notified. Presumably, the other threads will 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 doesn't allow the thread to proceed, the chosen thread may resume waiting, defeating the purpose of the notification.

The {{notify()}} method may only be invoked if all the following conditions are met:
* 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 threads 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 programmer must ensure the liveness property when using {{signal()}} and {{signalAll()}}.

h2. 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 {{time}} field. Each thread waits for the {{time}} field to indicate that it is time to perform the corresponding thread's step. After performing the step, each thread increments {{time}} to transfer control to the next thread. The thread then notifies the thread that is responsible for the next step and exits.

{code:bgColor=#FFcccc}
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;
  }

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

        // Perform 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();
    }
  }
}
{code}

This noncompliant code example violates the [liveness|BB. Definitions#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.

 

{code:bgColor=#FFcccc}
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 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();
    }
  }
}
{code}

Similar to {{Object.notify()}}, the {{signal()}} method may awaken an arbitrary thread

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

      // Perform operations

      time++;
      condition.signalAll();
    } catch (InterruptedException ie) { thread reacquires the lock associated with this condition. When the thread returns it is guaranteed to hold this lock \[[API 06|AA. Java References#API 06]\] {mc} what is meant by "the thread returns?". I think it should be "when the thread wakes up/resumes" {mc}. The thread that is ready can perform its task, while all the threads whose condition predicates are false resume waiting. 

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

{code:bgColor=#ccccff}
  public void run() {
    lock.lock();
    try {
      while (time != step) { 
      Thread.currentThread().interrupt  condition.await();  
      }

      // Reset interrupted statusPerform operations

    } finally {time++;
      lockcondition.unlocksignalAll();
    }
 catch }

// Declare class as final because its constructor throws an exception 
public final class ProcessStep implements Runnable { 
  private static final Lock lock = new ReentrantLock(InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    } finally {
      lock.unlock();
  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 }
{code}


h2. Compliant Solution (Unique Condition Per Thread)

This compliant solution assigns each thread its own condition. All the {{Condition}} objects are accessible to all the threads. 

{code:bgColor=#ccccff}
// 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) {
      while (time !this.step = step) { 
  ;
      conditions[step] = lock.awaitnewCondition();  
    } else }
{
      //throw Perform operations

  new IllegalArgumentException("Too many threads");
    time++;}
  }

  @Override public ifvoid run(step) {
 + 1 < conditionslock.lengthlock();
    try {
      while  conditions[step + 1].signal();(time != step) { 
        conditions[step].await();  
      }

    } catch (InterruptedException ie) {// Perform operations

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

   public static} voidcatch main(String[]InterruptedException argsie) {
     for (int i = MAX_STEPS - 1; i >= 0; i--)Thread.currentThread().interrupt(); // Reset interrupted status
    } finally {
      ProcessStep ms = new ProcessStep(i);lock.unlock();
    }
  }

  public static newvoid Thread(ms).start();main(String[] args) {
    }
for  }
}