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The Java programming language allows threads to access shared variables. If, in this noncompliant code example, one thread repeatedly calls the method one() (but no more than Integer.MAX_VALUE times in all), and another thread repeatedly calls the method two():

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then method two() could occasionally print a value for i that is not one more than the neither zero nor a previous value printed by of j two(). A similar problem occurs if i is declared as a double.

Compliant Solution (volatile)

This compliant solution declares the variables i as volatile. Writes and reads of volatile long and double values are always atomic.

class Test {
  static volatile long i = 0;
  static void one(long j) { i = j; }
  static void two() {
    System.out.println("i =" + i);
  }
}

Compliant Solution (atomic variable classes)

This compliant solution synchronizes calls to methods one() and two(). This guarantees these two method calls are treated as atomic operation, including reading and writing to the variable iuses atomic variable classes from the java.util.concurrent.atomic package. These classes provide lock-free thread-safe programming on single variables.

class Test {
  static longAtomicLong i = 0;
  static void synchronized one(long j) { i = j; }
  static synchronized void two() {
    System.out.println("i =" + i);
  }
}

Consequently, there is no need to qualify i as a volatile typeIt is also possible to hold doubles using the Double.doubleToLongBits and Double.longBitsToDouble conversions.

Risk Assessment

Failure to ensure the atomicity of operations involving 64-bit values in multithreaded applications can result in reading and writing indeterminate values.

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\[[JLS 05|AA. Java References#JLS 05]\] 17.7 Non-atomic Treatment of double and long
\[[Goetz 06|AA. Java References#Goetz 06]\] 3.1.2. Nonatomic 64-bit Operations

\[[Goetz 04|AA. Java References#Goetz 04]\] Brian Goetz. Java theory and practice: Going atomic. November 2004. http://www.ibm.com/developerworks/java/library/j-jtp11234/ 

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CON06-J. Ensure atomicity of thread-safe code      11. Concurrency (CON)      CON02-J. Do not use background threads during class initialization