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The methods java.lang.Threads that invoke Object.notifywait() expect to wake up and resume execution when their condition predicate becomes true. To be compliant with 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 or threads, respectively. These methods must be invoked from a thread and java.lang.Object.notifyAll() are used to waken waiting thread(s). These methods must be called from code that holds the same object lock as the waiting thread(s).

Any thread that has called wait() expects to wake up when some condition predicate becomes true. As recommended by CON18-J. Always invoke wait() and await() methods inside a loop, any waiting thread, when woken, will test its condition predicate, and resume waiting if the predicate has not become true.

Consequently, the notifyAll() method is recommended rather than the notify() method. The notifyAll() method will wake up all threads, and only threads whose condition predicate is satisfied will remain awake. Furthermore, if all of the threads require a specific lock, only one will obtain the lock, and the others may, presumably, resume waiting. The notify() method wakes up only one thread, and makes no guarantees as to which thread gets woken up. If the thread's condition predicate is not satisified, the chosen thread may resume waiting, defeating the purpose of the notify() call.

The notify() method should only be called if

• Every condition predicate on every thread waiting on the object is satisfied.
• Only one thread must 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.
• 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 the notify() method, the signal() method wakes up any one of the threads that is waiting on the condition and consequently, may be insecure. Similarly, any thread that is blocked on a wait() method invocation on a Java object being used as a condition queue, should be notified using notifyAll().

Noncompliant Code Example (notify())

This noncompliant code example demonstrates a complex multi-step process being undertaken by several threads. Each thread executes one step of the process; the step being currently performed is maintained by the step field. Each thread waits for the step field to indicate that it is time to perform that thread's step. When it is time to perform its step, each thread does so. It then increments step for the next thread, notifies the thread, and exits.

; 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, with no guarantee regarding which specific thread is notified. The chosen thread is permitted to resume waiting if its condition predicate is unsatisfied; this often defeats the purpose of the notification.

Consequently, invoking the notify() method is permitted only when all of the following conditions are met:

• All waiting threads have identical condition predicates.
• All threads perform the same set of operations after waking up. That 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 identical and provide a stateless service or utility.

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

public class ProcessStep implements Runnable {
  private static final Object lock = new Object();
  private static int time = 1;
  private int myStep; // do stuff when the step raches this value

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

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

        // ... do stuff

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

  public static void main(String[] args) {
    for (int i = 5; i > 0; i--) {
      ProcessStep ms = new ProcessSteplock.wait(i);
      new Thread(ms).start  }

        // Perform operations

        time++;
        lock.notify();
      }
    }
}

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 to perform the next step, the program will deadlock.

Compliant Solution (notifyAll())

In this compliant solution, each thread uses notifyAll() to wake all of the other threads when its step is complete. Consequently the proper thread can perform its next step, while all other threads note that their condition predicate has not been satisified and promptly go back to sleep.

 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 class ProcessStep implements Runnable {
  private static final Object lock = new Object();
  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() {
    try {
      synchronized (lock) {
        while (time != step) {
          lock.wait();
        }
  
        // Perform operations
  
        time++;
        lock.notifyAll(); // Use notifyAll() instead of notify()
      }
    } 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:

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 {
      synchronizedwhile (locktime != step) {
        condition.await();
      }

  while  (time != myStep)// {Perform operations

      time++;
      lockcondition.waitsignal();  
    } catch (InterruptedException ie) }{

       Thread.currentThread().interrupt(); // ...Reset dointerrupted stuffstatus

      } finally time++;
  {
      lock.notifyAllunlock();
    }
  }

  public static }void catch (InterruptedException iemain(String[] args) {
    for (int i  Thread.currentThread().interrupt(); // Reset interrupted status= 4; i >= 0; i--) {
    }  new  
  }

Noncompliant Code Example (Condition interface)

This noncompliant code example derives from the previous noncompliant code example but uses the Condition interface. Field condition is used to let threads wait on different condition predicates.

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 
    

public class ProcessStep implements Runnable {
  private static final Lock lock = new ReentrantLock();
  private static final Condition condition = lock.newCondition();
  private static int time = 1;
  private int myStep; // do stuff when the step raches this value

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

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

      // ...reaches dothis stuffvalue

  public ProcessStep(int   time++;step) {
    this.step = condition.signal()step;
  }

  @Override }public catchvoid run(InterruptedException ie) {
      Thread.currentThread().interruptlock.lock(); // Reset interrupted status
    } finallytry {
      while lock.unlock();
    }
  }

  public static void main(String[] args) {(time != step) {
        condition.await();
    for (int i = 5; i > 0; i--) {
      ProcessStep ms = new ProcessStep(i) }
  
      // Perform operations

      time++;
      new Thread(ms).startcondition.signalAll();
    }
  }
}

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

Compliant Solution (signalAll())

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

  public void run() {
catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    } finally {
      lock.lockunlock();
    try {}
      while (time != myStep) { 
        condition.await();  
      }

      // ... do stuff

      time++;
      condition.signalAll}

}

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 catchtime (InterruptedException ie) {= 0;
  private final   Thread.currentThread().interrupt()int step; // Reset interrupted status
// Perform operations when field time 
       } finally {
      lock.unlock();
    }
  }

Compliant Solution (unique Condition per thread)

This compliant solution assigns each thread its own Condition, and makes them accessible to all the threads.

public class ProcessStep implements Runnable {       // reaches this value
  private static final Lockint lockMAX_STEPS = new ReentrantLock()5;
  private static final Condition[] conditions = new Condition[6MAX_STEPS];

  public ProcessStep(int step) {
  private  staticif int(step time = 1;<= MAX_STEPS) {
  private int myStep; // dothis.step stuff= whenstep;
 the step raches this value

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

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

      // ... do stuffPerform operations

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

  public static void main(String[] args) {
    for (int i = 5 MAX_STEPS - 1; i >= 0; i--) {
      ProcessStep msps = new ProcessStep(i);
      new Thread(msps).start();
    }
  }
}

Even though the signal() method 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.

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

Risk Assessment

Invoking the notify() method instead of notifyAll() can be a threat to the Notifying a single thread rather than all waiting threads can violate 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

Tool	Version	Checker	Description
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.THI02.ANF	Do not use 'notify()'; use 'notifyAll()' instead so that all waiting threads will be notified
SonarQube	Include Page SonarQube_V SonarQube_V	S2446	"notifyAll" should be used

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