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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, the thread may observe a stale value of the shared variable. To ensure the visibility of the most recent update, either the variable must be declared volatile or the reads and writes must be synchronized.

Declaring a shared variable volatile guarantees visibility in a thread-safe manner only when both of the following conditions are metAccesses of primitive variables are atomic, except for the 64-bit long and double variables. An atomic access of a shared variable has three characteristics:

• A write to a variable does not depend on is independent from its current value.
• The A write is not involved with writes of other variables
• There is no need to synchronize the code to access the variable

Variables should be declared as {{volatile}} when this criteria is met \[[Goetz 06|AA. Java References#Goetz 06]\]. If this is not done, multiple threads may observe stale values of the shared variables and fail to act accordingly. 

• to a variable is independent from the result of any nonatomic compound operations involving reads and writes of other variables (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.

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 guarantees that 64-bit primitive long and double variables are accessed atomically. For more information on sharing those variables among multiple threads, see VNA05Although 64-bit primitives such as long and double are not accessed atomically, ensuring their visibility by declaring them as volatile also ensures that they are accessed atomically (see CON25-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 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, it is possible that another a second thread might not observe this that change. Consequently, the second thread may still 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 (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 handleinterrupted exceptionstatus
      } 
    } 	 
  }

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

Compliant Solution (

...

AtomicBoolean)

This In this compliant solution qualifies , the done flag as volatile so that updates 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 volatilefinal booleanAtomicBoolean done = new AtomicBoolean(false);
 
  @Override public void run() {
    while (!done.get()) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        Thread.currentThread().interrupt(); // handleReset interrupted exceptionstatus
      } 
    } 	 
  }

  protectedpublic void shutdown() {
    done = true.set(true);
  }
}

Risk Assessment

Failing to ensure visibility of atomically modifiable shared variables can lead to a thread seeing stale values of a variable.

Automated Detection

TODO

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
\[[Tutorials 08|AA. Java References#Tutorials 08]\] [Java Concurrency Tutorial|http://java.sun.com/docs/books/tutorial/essential/concurrency/index.html]
\[[Lea 00|AA. Java References#Lea 00]\] Sections, 2.2.7 The Java Memory Model, 2.2.5 Deadlock, 2.1.1.1 Objects and locks
\[[Bloch 08|AA. Java References#Bloch 08]\] 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"

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) { 
        Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

  public synchronized boolean isDone() {
    return done;
  }

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

Although this compliant solution is acceptable, 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 when a variable's new value depends on its current 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

VNA00-J-EX0: 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

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