31

My question refers to whether or not the use of a ReentrantLock guarantees visibility of a field in the same respect that the synchronized keyword provides.

For example, in the following class A, the field sharedData does not need to be declared volatile as the synchronized keyword is used.

class A 
{
  private double sharedData;

  public synchronized void method() 
  {
    double temp = sharedData;
    temp *= 2.5;
    sharedData = temp + 1;
  } 
}

For next example using a ReentrantLock however, is the volatile keyword on the field necessary?

class B 
{
  private final ReentrantLock lock = new ReentrantLock();
  private volatile double sharedData;

  public void method() 
  {
    lock.lock();
    try
    {
      double temp = sharedData;
      temp *= 2.5;
      sharedData = temp + 1;
    }
    finally
    {
      lock.unlock();
    }
  } 
}

I know that using the volatile keyword anyway will only likely impose a miniscule performance hit, but I would still like to code correctly.

Pod
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Matthew Blackford
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1 Answers1

37

It's safe without volatility. ReentrantLock implements Lock, and the docs for Lock include this:

All Lock implementations must enforce the same memory synchronization semantics as provided by the built-in monitor lock, as described in The Java Language Specification, Third Edition (17.4 Memory Model):

  • A successful lock operation has the same memory synchronization effects as a successful Lock action.
  • A successful unlock operation has the same memory synchronization effects as a successful Unlock action.
Jon Skeet
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  • Is this advice true with regards to `while (field)` due to field hoisting? – Pod Aug 21 '17 at 09:49
  • @Pod: Without more specific details of how the `while` loop interacts with the lock, it's impossible to say. You probably want to create a new question with a specific example. – Jon Skeet Aug 21 '17 at 09:50
  • If I want to read the latest value of `sharedData` in other threads without locking, should `sharedData` be declared volatile? – Poison Dec 14 '22 at 14:36
  • @Poison: It's more complex than that, and frankly I don't understand it well enough to be confident in giving guidance. – Jon Skeet Dec 14 '22 at 14:45