CON00-J. Ensure visibility when accessing shared primitive variables

Reads of shared primitive variables in some thread may not observe the latest writes to the variables from other threads. It is important to ensure that the accesses see the values of the most recent writes. If this is not done, multiple threads may observe stale values of the shared variables and fail to act accordingly. Visibility of latest values can be ensured by declaring variables volatile or correctly synchronizing the code.

The use of volatile is recommended under a very restrictive set of conditions:

• A write to a variable does not depend on its current value.
• The write is not involved with writes of other variables.
• Only a single thread ever updates the value.
• Locking is not required for any other reason.

Furthermore, declaring a variable as volatile or correctly synchronizing the code guarantees that 64-bit primitive variables of type long and double are accessed atomically (see CON25-J. Ensure atomicity when reading and writing 64-bit values for information on sharing long and double variables among multiple threads).

Noncompliant Code Example

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)

In this compliant solution, the done flag is declared 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;
  }
}

Compliant Solution (synchronized)

This compliant solution uses the intrinsic lock of the Class object to ensure that updates are visible to other threads.

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

  protected synchronized boolean isDone() {
    return done;
  }

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

While this is an acceptable compliant solution, it has the following shortcomings as compared to declaring done as volatile:

• Performance: The intrinsic locks cause threads to block temporarily; volatile incurs no blocking
• Deadlock: Excessive synchronization can make the program deadlock prone.

However, synchronization is a more secure alternative in situations where the volatile keyword is inappropriate, such as if a variable's new value depends on its old value. Refer to CON01-J. Ensure that composite operations on shared variables are atomic and CON07-J. Do not assume that a grouping of calls to independently atomic methods is atomic for more details.

Compliant Solution (java.util.concurrent.atomic.AtomicBoolean)

This compliant solution uses an AtomicBoolean flag to ensure that updates are visible to other threads.

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

  protected void shutdown() {
    done.set(true);
  }
}

Risk Assessment

Failing to ensure visibility of shared primitive variables on accesses can lead to a thread seeing stale values of the variables.

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