CON00-J. Declare status flags as volatile

If status flags are not declared as volatile, multiple threads may observe stale values of the flags and fail to act accordingly.

Noncompliant Code Example (status flag)

This noncompliant code example uses a shutdown() method to set a non-volatile done flag that is checked in the run() method. If one thread invokes the shutdown() method to set the flag, it is possible that another thread might not observe this change. Consequently, the second thread may still observe that done is false and incorrectly invoke the sleep() method.

final class ControlledStop implements Runnable {
  private boolean done = false;
 
  public void run() {
    while (!done) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        // handle exception
      } 
    } 	 
  }

  protected void shutdown(){
    done = true;
  }
}

Compliant Solution (volatile status flag)

This compliant solution qualifies the done flag as volatile so that updates are visible to other threads.

final class ControlledStop implements Runnable {
  private volatile boolean done = false;
 
  public void run() {
    while (!done) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        // handle exception
      } 
    } 	 
  }

  protected void shutdown(){
    done = true;
  }
}

Risk Assessment

Failing to declare status flags as volatile can result in a thread seeing stale values of the flags.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[JLS 05]] Chapter 17, Threads and Locks, section 17.4.5 Happens-before Order, section 17.4.3 Programs and Program Order, section 17.4.8 Executions and Causality Requirements
[[Tutorials 08]] Java Concurrency Tutorial
[[Lea 00]] Sections, 2.2.7 The Java Memory Model, 2.2.5 Deadlock, 2.1.1.1 Objects and locks
[[Bloch 08]] Item 66: Synchronize access to shared mutable data
[[Goetz 06]] 3.4.2. "Example: Using Volatile to Publish Immutable Objects"
[[JPL 06]] 14.10.3. "The Happens-Before Relationship"
[[MITRE 09]] CWE ID 667 "Insufficient Locking", CWE ID 413 "Insufficient Resource Locking", CWE ID 366 "Race Condition within a Thread", CWE ID 567 "Unsynchronized Access to Shared Data"

11. Concurrency (CON)      11. Concurrency (CON)      CON02-J. Always synchronize on the appropriate object