CON35-J. Do not use an instance lock to protect shared static data

Static shared data must not be protected using instance locks because the instance locks are ineffective when two or more instances of the class are created. Consequently, the shared state is not safe for concurrent use.

Noncompliant Code Example (nonstatic lock object for static data)

This noncompliant code example uses a nonstatic lock object to guard access to a static field. If two Runnable tasks, each consisting of a thread are started, they will create two instances of the lock object and lock on each separately.

class CountBoxes implements Runnable {
  static volatile int counter;
  // ...

  Object lock = new Object();    

  public void run() {
    synchronized(lock) {
      counter++; 
      // ... 
    } 
  }

  public static void main(String[] args) {
    Runnable r1 = new CountBoxes();
    Thread t1 = new Thread(r1);
    Runnable r2 = new CountBoxes();
    Thread t2 = new Thread(r2);
    t1.start();
    t2.start();
  }
}

This does not prevent either thread from observing an inconsistent value of counter because the increment operation on volatile fields is not atomic in the absence of proper synchronization.

Noncompliant Code Example (method synchronization for static data)

This noncompliant code example uses method synchronization to protect access to a static class member.

class CountBoxes implements Runnable {
  static volatile int counter;
  // ...

  public synchronized void run() {
      counter++; 
      // ... 
  }
  // ...
}

The problem is that this lock is associated with each instance of the class and not with the class object itself. Consequently, threads constructed using different Runnable instances may observe inconsistent values of the counter.

Compliant Solution (static lock object)

This compliant solution declares the lock object as static and consequently, ensures the atomicity of the increment operation.

class CountBoxes implements Runnable {
  static int counter;
  // ...

  private static final Object lock = new Object();    
  
  public void run() {
    synchronized(lock) {
      counter++; 
      // ...
  }
  // ...
}

There is no requirement of declaring the counter variable as volatile when synchronization is used.

Risk Assessment

Using an instance lock to protect static shared data provides no synchronization properties and can lead to non-deterministic behavior.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[API 06]]

VOID CON00-J. Synchronize access to shared mutable variables      11. Concurrency (CON)      CON03-J. Do not use background threads during class initialization