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A thread that invokes Threads that invoke Object.wait() expects expect to wake up and resume execution when its their condition predicate becomes true. Waiting threads must test their condition predicates upon receiving notifications and resume waiting if they are false to To be compliant with CON18 THI03-J. Always invoke wait() and await() methods inside a loop, waiting threads must test their condition predicates upon receiving notifications and must resume waiting if the predicates are false.

The notify() and notifyAll() methods of package java.lang.Object are used to wake up a waiting thread (s)or threads, respectively. These methods must be invoked from code a thread that holds the same object lock as the waiting thread(s). The IllegalMonitorStateException is thrown if the current thread is not the owner of this object's monitor; these methods throw an IllegalMonitorStateException when invoked from any other thread. The notifyAll() method wakes up all threads waiting on an object lock 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 , with no guarantee regarding which specific thread is notified. If the thread's condition predicate doesn't allow the thread to proceed, the chosen thread may resume waiting, defeating the The chosen thread is permitted to resume waiting if its condition predicate is unsatisfied; this often defeats the purpose of the notification.

The Consequently, invoking the notify() method may only be invoked if all is permitted only when all of the following conditions are met:

• The condition predicate is identical for each waiting threadAll waiting threads have identical condition predicates.
• All threads must perform the same set of operations after waking up. In other wordsThat is, 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 are satisfied by threads that are often identical and provide a stateless service or utility.

The java.util.concurrent.locks utilities (interface Condition) provide the Condition.signal() and Condition.signalAll() methods to awaken threads that are blocked on an a Condition.await() call. The 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 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();  
        }

        // 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();
    }
  }
}

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.

Compliant Solution (notifyAll())

In this compliant solution, each thread completes its step then calls notifyAll() to notify the waiting threads. The thread that is ready can perform its task, while all 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();  
        }

        // Perform operations

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

Noncompliant Code Example (Condition interface)

This noncompliant code example is similar to the noncompliant code example for notify() but uses the Condition interface for waiting and notification.

Condition objects are required when using java.util.concurrent.locks.Lock objects. Although Lock objects allow the use of Object.wait(), Object.notify(), and Object.notifyAll() methods, their use is prohibited by LCK03-J. Do not synchronize on the intrinsic locks of high-level concurrency objects. Code that synchronizes using a Lock object uses one or more Condition objects associated with the Lock object rather than using its own intrinsic lock. These objects interact directly with the locking policy enforced by the Lock object. Consequently, the await(), signal(), and signalAll() methods are used in place of the wait(), notify(), and notifyAll() methods.

The signal() method must not be used unless all of these conditions are met:

• The Condition object is identical for each waiting thread.
• All threads must perform the same set of operations after waking up, which means that any one thread can be selected to wake up and resume for a single invocation of signal().
• Only one thread is required to wake upon receiving the signal.

or all of these conditions are met:

• Each thread uses a unique Condition object.
• Each Condition object is associated with the same Lock object.

When used securely, the signal() method has better performance than signalAll().

When notify() or signal() is used to waken a waiting thread, and the thread is not prepared to resume execution, it often resumes waiting. Consequently, no thread wakens, which may cause the system to hang.

Noncompliant Code Example (notify())

This noncompliant code example shows a complex, multistep process being undertaken by several threads. Each thread executes the step identified 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 first increments time and then notifies the thread that is responsible for the next step.

public final class ProcessStep implements Runnable {
  private static final Object lock = new Object();
  private static int time = 0;
  private final int step; // Do Perform 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();
    }
  }
}

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

Compliant Solution (notifyAll())

In this compliant solution, each thread completes its step and then calls notifyAll() to notify the waiting threads. The thread that is ready can then perform its task while all 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 final

public class ProcessStep implements Runnable {
  private static final LockObject lock = new ReentrantLockObject();
  private static final Condition condition = lock.newCondition();
  private static int time = 0;
  private final int step; // DoPerform operations when field time 
 reaches this value

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

  public void run() {
    lock.lock();
    try { // reaches this value
  public    while (time != ProcessStep(int step) { 
    this.step    condition.await();= step;
  }

  @Override public void run() }{

    try {
     // Perform operations

 synchronized (lock) {
        while time++;
(time != step) {
          conditionlock.signalwait();
      } catch (InterruptedException}
 ie) {
      Thread.currentThread().interrupt();  // ResetPerform operations
 interrupted status
    }   finally {
time++;
        lock.unlocknotifyAll();
 // Use notifyAll() }
instead of }
notify()
  public static void main(String[] args) {}
    for} catch (int i = 4; i >= 0; i--InterruptedException ie) {
      new Thread.currentThread(new ProcessStep(i)).startinterrupt(); // Reset interrupted status
    }
  }

}

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

Compliant Solution (signalAll())

This compliant solution uses the signalAll() method to resume all waiting threads. Before await() returns the current thread must re-acquire the lock associated with this condition. When the thread returns it is guaranteed to hold this lock. 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:

Noncompliant Code Example (Condition Interface)

This noncompliant code example is similar to the noncompliant code example for notify() but uses the Condition interface for waiting and notification:

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; // Perform operations when field time 
                          // reaches this value
  public ProcessStep(int step) {
    this.step = step;
  }

  @Override 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();
    }
  }
}

As with Object.notify(), the signal() method may awaken an arbitrary thread.

Compliant Solution (signalAll())

This compliant solution uses the signalAll() method to notify all waiting threads. Before await() returns, the current thread reacquires the lock associated with this condition. When the thread returns, it is guaranteed to hold this lock [API 2014]. 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:

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; // Perform operations when field time 
                          // reaches this value
  public ProcessStep(int step) {
    this.step = step;
  }

  @Override

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

      // Perform operations

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

}

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. :

// 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 int time = 0;
  private final int step; // Perform 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");
    }
  }

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

      // Perform 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 msps = new ProcessStep(i);
      new Thread(msps).start();
    }
  }
}

Even though the signal() method is used, only the thread whose condition predicate corresponds to the unique Condition variable will awaken. All threads perform the same operations.

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

Risk Assessment

Notifying a single thread instead of all waiting threads can pose a threat to the liveness property of the system.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html]
\[[Goetz 06|AA. Java References#Goetz 06]\] Section 14.2.4, Notification
\[[Bloch 01|AA. Java References#Bloch 01]\] Item 50: Never invoke wait outside a loop

create a thread with an instance of this class.

Risk Assessment

Notifying a single thread rather than all waiting threads can violate the liveness property of the system.

Automated Detection

Related Guidelines

Bibliography

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Image Added Image Added Image AddedCON18-J. Always invoke wait() and await() methods inside a loop      11. Concurrency (CON)      CON20-J. Do not perform operations that may block while holding a lock