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The possible reorderings between {{volatile}} and nonvolatile variables are summarized in the matrix shown below. The load and store operations correspond to read and write operations that use the variable. \[[Lea 08|AA. Java References#Lea 08]\]

Noncompliant Code Example (status flag)

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.

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 status flag)

This compliant solution qualifies the done flag as volatile so that updates by one thread are immediately visible to another thread.

final class ControlledStop implements Runnable {
  private volatile boolean done = false;
  // ...
}

Noncompliant Code Example

This noncompliant code example declares a non-volatile int variable that is initialized in the constructor depending on a security check. In a multi-threading scenario, it is possible that the statements will be reordered so that the boolean flag initialized is set to true before the initialization has concluded. If it is possible to obtain a partially initialized instance of the class in a subclass using a finalizer attack (OBJ04-J. Do not allow partially initialized objects to be accessed), a race condition can be exploited by invoking the getBalance() method to obtain the balance even though initialization is still underway.

class BankOperation {
  private int balance = 0;
  private boolean initialized = false;
 
  public BankOperation() {
    if (!performAccountVerification()) {
      throw new SecurityException("Invalid Account"); 
    }
    balance = 1000;   
    initialized = true; 
  }
  
  private int getBalance() {
    if (initialized == true) {
      return balance;
    }
    else {
      return -1;
    }
  }
}

Compliant Solution (volatile guard)

This compliant solution declares the initialized flag as volatile to ensure that the initialization statements are not reordered.

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The use of the volatile keyword is inappropriate for composite operations on shared variables (CON01-J. Design APIs that ensure atomicity of composite operations and visibility of results).

Noncompliant Code Example (visibility)

This noncompliant code example consists of two classes, an immutable ImmutablePoint class and a mutable Holder class. Holder is mutable because a new ImmutablePoint instance can be assigned to it using the setPoint() method. If one thread updates the value of the ipoint field, another thread may still see the reference of the old value.

class Holder {
  ImmutablePoint ipoint;
  
  Holder(ImmutablePoint ip) {
   ipoint = ip;
  }
  
  voidImmutablePoint getPoint() {
    return ipoint();
  }

  void setPoint(ImmutablePoint ip) {
    this.ipoint = ip;
  }
}

public class ImmutablePoint {
  final int x;
  final int y;

  public ImmutablePoint(int x, int y) {
    this.x = x;
    this.y = y;
  }
}

Compliant Solution (visibility)

This compliant solution declares the ipoint field as volatile so that updates are immediately visible to other threads.

class Holder {
  volatile ImmutablePoint ipoint;
  
  Holder(ImmutablePoint ip) {
    ipoint = ip;
  }
  
  voidImmutablePoint getPoint() {
    return ipoint();
  }

  void setPoint(ImmutablePoint ip) {
    this.ipoint = ip;
  }
}

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Declaring immutable fields as volatile enables their safe publication, in that, once published, it is impossible to change the state of the sub-object.

Noncompliant Code Example (partial initialization)

Thread-safe objects (which may not be strictly immutable) must declare their nonfinal fields as volatile to ensure that no thread sees any field references before the sub-objects' initialization has concluded. This noncompliant code example does not declare the map field as volatile.

public class Container<K,V> {
  Map<K,V> map;

  public Container() {
    map = new HashMap<K,V>();	
    // Put values in HashMap
  }

  public V get(Object k) {
    return map.get(k);
  }
}

Compliant Solution (proper initialization)

This compliant solution declares the map field as volatile to ensure other threads see an up-to-date HashMap reference and object state.

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