Give me an Example

Meet the demo application. Suppose that it works with an external sensor that provides a number. The application continuously updates the value of the sensor in a dedicated thread (to a random value, in this example), and from other threads sometimes uses the most recent value to do something meaningful with it in the processSensorValue() method:

public class BoxingFailure {
  private static volatile Double sensorValue;

  private static void readSensor() {
    while(true) sensorValue = Math.random();

  private static void processSensorValue(Double value) {
    if(value != null) {

As the name of the class suggests, the problem here is boxing. Possibly to accommodate the null check, the author made the sensorValue field a capital-D Double. This example is quite a common pattern of dealing with calculations based on the most recent value, when obtaining this value is an expensive operation. And in the real world, it is usually much more expensive than just getting a random value. Thus, one thread continuously generates new values, and the calculating thread uses them, avoiding the expensive retrieval.

The demo application is impacted by the GC not keeping up with the allocation rate. The ways to verify and solve the issue are given in the next sections.