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Threads that invoke Object.wait() 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 that holds the same object lock as the waiting thread(s); 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

The methods {{notify()}} and {{notifyAll()}} are used to waken waiting thread(s). These methods must be called from a block that holds the same object lock as the waiting thread(s). The method {{notify()}} is deceptive in most cases unless all of the following conditions hold: \[[Goetz 06|AA. Java References#Goetz 06]\]

• Only one condition predicate is used with the locked object. Also, each thread must execute the same code after waking up from a wait.
• 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.

These are typically true when only one thread is waiting. Otherwise, if either doesn't hold, incorrect program behavior may result.

Noncompliant Code Example

This noncompliant example demonstrates a violation of liveness. A lock is held on a shared object list and three threads are started. Two condition predicates, one checking whether the buffer has zero elements and another that checks if the buffer is full with ten elements, have been used. Initially the buffer is neither full nor empty. Conditions are created so that first the buffer becomes empty and thread 1 goes into wait state, followed by thread 2.

Note that when thread 2 goes into wait, the condition predicate of thread 1 has become false. When notify() is invoked by thread 3, it can be delivered to either thread 1 or thread 2 depending on the JVM. If thread 1 is chosen, it's condition turns out to be false. Even though this notify was meant for thread 2 (whose condition is on the other hand, true) thread 1 wakes up on its false condition.

public final class MissedSignalProcessStep implements Runnable {
  private static LinkedListfinal Object listlock = new LinkedListObject();
  private static int buffer_counttime = 50;
  private final int number;step; // Do Perform operations when field time 
              //selects   function based on thread number	
     // reaches this value

  public MissedSignalProcessStep(int numberstep) {
    this.numberstep = numberstep;
  }

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

    if(number  == 1) {	// Perform operations

        time++;
        System.out.println("Thread 1 started..."); lock.notify();
      }
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    }
  }

  public static void main(String[] args) {
    for while(buffer_count =(int i = 4; i >= 0; i--) { 
      	new Thread(new ProcessStep(i)).start();
      System.out.println("Beginning wait() Thread 1...");	
      	        list.wait();		  
      	  }
  }
}

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 
                System.out.println("Thread 1 got notified this time...");
      	    }  
    // reaches 	this value
  public System.out.println("Exiting because Thread 1 condition is false...");	
      	  }  ProcessStep(int step) {
    this.step = step;
  }

  @Override public void run() {
    try {
      	synchronized (lock) else{
 if(number == 2) {  	  while (time 		!= step) {
      	    Systemlock.out.println("Thread 2..."wait();
      	  }
  
         while(buffer_count > 0) {	// Perform operations
  
      	  time++;
        Systemlock.out.println("Beginning wait() Thread 2...");	notifyAll(); // Use notifyAll() instead of notify()
      }
    } catch  	(InterruptedException ie) {
      list.waitThread.currentThread().interrupt(); // Reset 	interrupted status
    }
 		    	  
      	      System.out.println("Thread 2 }

}

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  got notified this time...");
      	    }
      	        System.out.println("Exiting because the thread 2 condition is false...");
     // 	reaches this }value
  public ProcessStep(int step) {
 	  else if(number this.step == 3) { step;
  }

  @Override public void run() 	{
    Threadlock.sleeplock(2000);	
   		 try {
  			   
 while (time != step) {
        listcondition.notifyawait();
      	}

   		  
 // Perform operations

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

  public static void makeThread1Truemain(String[] args) {
    buffer_count 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();
  publicprivate static void makeThread2True() { final Condition condition = lock.newCondition();
  private  buffer_countstatic int time = 100;
  private final int  }
step; // Perform operations when field time 
                          // reaches this value
  public static ProcessStep(int step) {
    this.step = step;
  }

  @Override public void main(String[] args) throws IOExceptionrun() {
    lock.lock();
    try {
    makeThread1True();  while (time != step) {
    Runnable runnable1 = new MissedSignalcondition.await(1);
      Thread t1 = new Thread(runnable1)}
  
      // Perform operations

      time++;
    t1  condition.startsignalAll();
  		  
} catch (InterruptedException ie) try {
  	   new Thread.currentThread().sleepinterrupt(5000);
	  makeThread2True(); // Reset interrupted status
    } catch (InterruptedException e) { e.printStackTrace 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 runnable2 {
  private static final Lock lock = new MissedSignalReentrantLock(2);
  private  Thread t2static 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 Thread(runnable2);
Condition[MAX_STEPS];

  public ProcessStep(int step) {
    if (step <= MAX_STEPS) {
      this.step = step;
      t2.startconditions[step] = lock.newCondition();
   		 } else {
    Runnable runnable3 =throw new MissedSignal(3);
    Thread t3 = new Thread(runnable3);
    t3.startIllegalArgumentException("Too many threads");
    }
  }

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

Compliant Solution

This compliant solution uses notifyAll() which sends notifications to all threads that wait on the same object. As a result, liveness is not affected unlike the noncompliant example. Ensure that the lock is released promptly after the call to notifyAll().

else if(number == 3) {
  Thread.sleep(2000);	   		    			   
  list.notifyAll();      	   		  
} 	    	  

Risk Assessment

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

Even though the signal() method is used, only the thread whose condition predicate corresponds to 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

Notifying a single thread rather than all waiting threads can violate the liveness property of the systemTo guarantee the liveness of a system, the method notifyAll() should be called rather than notify().

Automated Detection

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TODO

Related Vulnerabilities

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

References

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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 Added \[[Bloch 01|AA. Java References#Bloch 01]\] Item 50: Never invoke wait outside a loopCON31-J. Always invoke the wait() method inside a loop      08. Concurrency (CON)      09. Methods (MET)