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Declaring a variable as volatile or correctly synchronizing the code guarantees that 64-bit primitive long and double variables are accessed atomically. (For more information on sharing long and double variables among multiple threads, see CON05-J. Ensure atomicity when reading and writing 64-bit values.)

Noncompliant Code Example (Non-volatile Flag)

This noncompliant code example uses a shutdown() method to set a non-volatile done flag that is checked in the run() method.

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If one thread invokes the shutdown() method to set the flag, a second thread might not observe that change. Consequently, the second thread may observe that done is still false and incorrectly invoke the sleep() method. A compiler is allowed to optimize the code if it determines that the value of done is never modified by the same thread, resulting in an infinite loop.

Compliant Solution (volatile)

In this compliant solution, the done flag is declared as volatile to ensure that writes are visible to other threads.

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

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

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

In this compliant solution, the done flag is declared as AtomicBoolean. Atomic types also guarantee that writes are visible to other threads.

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

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

Compliant Solution (synchronized)

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

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Compliance with CON07-J. Use private final lock objects to synchronize classes that may interact with untrusted code can reduce the likelihood of misuse by ensuring that untrusted callers cannot access the lock object.

Exceptions

CON00-EX1: Class objects need not be made visible because they are created by the virtual machine and their initialization always precedes any subsequent use.

Risk Assessment

Failing to ensure the visibility of a shared primitive variable may result in a thread observing a stale value of the variable.

Automated Detection

The following table summarizes the examples flagged as violations by SureLogic Flashlight:

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The unprotected field can be observed through its graphical user interface (GUI).

Related Vulnerabilities

Any vulnerabilities resulting from the violation of this rule are listed 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], Section 17.4.5 Happens-Before Order, Section 17.4.3 Programs and Program Order, Section 17.4.8 Executions and Causality Requirements
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 66: Synchronize access to shared mutable data
\[[Goetz 06|AA. Java References#Goetz 06]\] 3.4.2. "Example: Using Volatile to Publish Immutable Objects"
\[[JPL 06|AA. Java References#JPL 06]\] 14.10.3. "The Happens-Before Relationship"
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 667|http://cwe.mitre.org/data/definitions/667.html] "Insufficient Locking," [CWE ID 413|http://cwe.mitre.org/data/definitions/413.html] "Insufficient Resource Locking," [CWE ID 567|http://cwe.mitre.org/data/definitions/567.html]  "Unsynchronized Access to Shared Data"

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