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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 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 See rule 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 \[[Goetz 2006|AA. Bibliography#Goetz 06]\]. However, code that relies on a single-thread confinement is error- prone and difficult to maintain. This design approach is permitted under this 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 that 64-bit primitive long and double variables will be accessed atomically. (For more information on sharing those variables among multiple threads, see rule 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 the 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 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.

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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 a variable's new value depends on its current value. For more information, see See rule VNA02-J. Ensure that compound operations on shared variables are atomic for more information.

Compliance with rule 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.

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Automated Detection

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

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

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Related Guidelines

Bibliography

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