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Accesses of primitive variables are atomic, except for the 64-bit long and double variables. An atomic access of a shared variable has three ; see CON25-J. Ensure atomicity when reading and writing 64-bit values for information on sharing long and double variables among multiple threads.

If a shared primitive variable has these characteristics:

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

VariablesThen the variable 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. 

Noncompliant Code Example

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This noncompliant code example uses a shutdown() method to set a non-volatile done flag that is checked in the run() method. If one thread invokes the shutdown() method to set the flag, it is possible that another thread might not observe this change. Consequently, the second thread may still observe that done is false and incorrectly invoke the sleep() method.

Code Block
bgColor #FFcccc
final class ControlledStop implements Runnable { private boolean done = false; public void run() { while (!done) { try { // ... Thread.currentThread().sleep(1000); // Do something } catch(InterruptedException ie) { // handle exception } } } protected void shutdown() { done = true; } }

Compliant Solution (volatile

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)

This compliant solution qualifies the done flag as volatile so that updates are visible to other threads.

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

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

Compliant Solution (synchronized)

This compliant solution uses the this object's intrinsic lock to ensure thread safety.

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

  protected synchronized bool isDone() {
    return done;
  }

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

While this is an acceptable compliant solution, it has the following disadvantages compared to marking done as {{volatile:

• Performance: The intrinsic locks cause threads to block temporarily; volatile incurs no blocking
• Deadlock: Improper usage of locks can lead to deadlock. Since volatile incurs no blocking, deadlock cannot occur.

However, synchronization is a useful alternative in situations where the volatile keyword is inappropriate, such as if a variable's new value depends on its old value.

Risk Assessment

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

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