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Reading a shared primitive variable in one thread may not yield the value of the most recent write to the variable from another thread. Consequently, multiple threads the thread may observe a stale values value of the shared variable and fail to perform the necessary actions on a timely basis. Visibility . To ensure the visibility of the most recent update can be ensured by declaring , either the variable as must be declared volatile or correctly synchronizing the reads and writes to the variable.must be synchronized.

Declaring a shared variable volatile guarantees visibility in a thread-safe manner only when both of the following conditions are metThe use of volatile is safe under a very restrictive set of conditions, all of which must hold:

• A write to a variable does not depend on is independent from its current value.
• The write does not participate in A write to a variable is independent from the result of any nonatomic compound operations involving reads or and writes of other variables
• Locking is not required for any other reason (all actions are atomic)
• (see VNA02-J. Ensure that compound operations on shared variables are atomic 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]. However, code that relies on a single-thread confinement is error prone and difficult to maintain. This design approach is permitted under this rule but is discouraged Wiki MarkupThe first condition is sometimes relaxed when it can be ensured that only one thread ever updates the value of the variable \[[Goetz 06|AA. Java References#Goetz 06]\]. However, it is still possible for reader threads to see stale values of the variable while the writing thread is in the process of modifying its value, before writing it back. Furthermore, it is harder to maintain code that relies on such an invariant being true at all times.

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

Declaring a variable as volatile or correctly synchronizing the code guarantees that 64-bit primitive variables of type long and double variables are accessed atomically (see CON25. For more information on sharing those variables among multiple threads, see VNA05-J. Ensure atomicity when reading and writing 64-bit values for information on sharing long and double variables amongst multiple threads).

Noncompliant Code Example (Non-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) { 
        // handle exception
        Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

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

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

Compliant Solution (

...

Volatile)

This In this compliant solution declares , the done flag as is declared volatile so to ensure that updates 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 (AtomicBoolean)

In this compliant solution, the done flag is declared to be of type java.util.concurrent.atomic.AtomicBoolean

...

This compliant solution uses an AtomicBoolean flag to ensure that updates . 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 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) { 
        // handle exception
        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, it has the following shortcomings as compared to the previously suggested ones:

...

, 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

...

.

However, synchronization 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 old value. Refer to CON01current value (see VNA02-J. Ensure that compound operations on shared variables are atomic for more information).

Compliance with 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

EX1: Objects of type Class need not be made visible because they VNA00-J-EX0: Class objects are created by the Virtual Machine and virtual machine; their initialization always precedes any subsequent use. JMM Mailing List. Consequently, cross-thread visibility of Class objects is already assured by default.

Risk Assessment

Failing to ensure the visibility of a shared primitive variables on accesses can lead to variable may result in a thread seeing observing a stale values value of the variablesvariable.

Automated Detection

SureLogic Flashlight can detect violations of this guideline. It flags the noncompliant code example by specifying: "Instance fields with empty locksets". The unprotected field can be observed through its GUI.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule 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 366|http://cwe.mitre.org/data/definitions/366.html]  "Race Condition within a Thread", [CWE ID 567|http://cwe.mitre.org/data/definitions/567.html]  "Unsynchronized Access to Shared Data"

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

Related Guidelines

Bibliography

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Image Added Image Added Image Added11. Concurrency (CON)      11. Concurrency (CON)      CON02-J. Always synchronize on the appropriate object