Question

In the class below, is the method getIt() thread safe and why?

public class X { 
  private long myVar; 
  public void setIt(long  var){ 
    myVar = var; 
   }  
   public long getIt() { 
     return myVar; 
  } 
}

Answer 1

It is not thread-safe. Variables of type long and double in Java are treated as two separate 32-bit variables. One thread could be writing and have written half the value when another thread reads both halves. In this situation, the reader would see a value that was never supposed to exist.

To make this thread-safe you can either declare myVar as volatile (Java 1.5 or later) or make both setIt and getIt synchronized.

Note that even if myVar was a 32-bit int you could still run into threading issues where one thread could be reading an out of date value that another thread has changed. This could occur because the value has been cached by the CPU. To resolve this, you again need to declare myVar as volatile (Java 1.5 or later) or make both setIt and getIt synchronized.

It’s also worth noting that if you are using the result of getIt in a subsequent setIt call, e.g. x.setIt(x.getIt() * 2), then you probably want to synchronize across both calls:

synchronized(x)
{
  x.setIt(x.getIt() * 2);
}

Without the extra synchronization, another thread could change the value in between the getIt and setIt calls causing the other thread’s value to be lost.