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Declaring a shared variable volatile guarantees visibility in a thread-safe manner only when both of the following conditions are met:

• A write to a variable is independent from its current value.
• A write to a variable is independent from the result of any non-atomic nonatomic compound operations involving reads and writes of other variables . (For more information, see guideline VNA02-J. Ensure that compound operations on shared variables are atomic.)

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• for more information).

The first condition can be relaxed when you can be sure that only one thread will ever update the value of the variable \[ [Goetz 2006|AA. Bibliography#Goetz 06]\]. However, code that relies on a single-thread confinement is error - prone and difficult to maintain. This design approach is permitted under this guideline, rule but is discouraged.

Synchronizing the code makes it easier to reason about its behavior and is frequently more secure than simply using the volatile keyword. However, synchronization has somewhat higher performance overhead and can result in thread contention and deadlocks when used excessively.

Declaring a variable volatile or correctly synchronizing the code both guarantee guarantees that 64-bit primitive long and double variables will be are accessed atomically. ( For more information on sharing those variables among multiple threads, see guideline VNA05-J. Ensure atomicity when reading and writing 64-bit values.)

Noncompliant Code Example (Non-

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volatile Flag)

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

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

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

If one thread invokes the shutdown() method to set the flag, a second thread might not observe that change. Consequently, the second thread may might observe that done is still false and incorrectly invoke the sleep() method. Compilers and just-in-time compilers (JITs) are allowed to optimize the code when they determine that the value of done is never modified by the same thread, resulting in an infinite loop.

Compliant Solution (

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Volatile)

In this compliant solution, the done flag is declared 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) { 
        Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

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

Compliant Solution (

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AtomicBoolean)

In this compliant solution, the done flag is declared to be of type java.util.concurrent.atomic.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) { 
        Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

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

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

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

  public synchronized boolean isDone() {
    return done;
  }

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

While Although this compliant solution is an acceptable compliant solution, intrinsic locks cause threads to block and may introduce contention. On the other hand, volatile-qualified shared variables do not block. Excessive synchronization can also make the program prone to deadlock.

Synchronization is a more secure alternative in situations where the volatile keyword or a java.util.concurrent.atomic.Atomic* field is inappropriate, such as if when a variable's new value depends on its current value . For more information, (see guideline VNA02-J. Ensure that compound operations on shared variables are atomic for more information).

Compliance with guideline LCK00-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

VNA00-J-EX1EX0: Class objects are created by the virtual machine; their initialization always precedes any subsequent use. Consequently, cross-thread visibility of Class objects is already assured by default.

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:

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

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

Related Vulnerabilities

Any vulnerabilities resulting from the violation of this guideline are listed on the CERT website.

Related Guidelines

MITRE CWE: CWE-667 "Insufficient Locking"

MITRE CWE: CWE-413 "Insufficient Resource Locking"

MITRE CWE: CWE-567 "Unsynchronized Access to Shared Data"

Bibliography

\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 66: Synchronize access to shared mutable data
\[[Goetz 2006|AA. Bibliography#Goetz 06]\] 3.4.2. "Example: Using Volatile to Publish Immutable Objects"
\[[JLS 2005|AA. Bibliography#JLS 05]\] [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
\[[JPL 2006|AA. Bibliography#JPL 06]\] 14.10.3. "The Happens-Before Relationship"

Some static analysis tools are capable of detecting violations of this rule.

Related Guidelines

Bibliography

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Image Added Image Added 07. Visibility and Atomicity (VNA)      07. Visibility and Atomicity (VNA)