java - 通过 ReentrantLock 访问的字段是否需要 volatile 关键字？

Question

我的问题是关于 ReentrantLock 的使用是否能保证字段的可见性与 synchronized 关键字提供的相同。

例如，在下面的类A中，字段sharedData不需要声明为volatile，因为使用了同步关键字。

class A 
{
  private double sharedData;

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

但是，对于下一个使用 ReentrantLock 的示例，字段上的 volatile 关键字是否必要？

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();
    }
  } 
}

我知道无论如何使用 volatile 关键字只会对性能造成很小的影响，但我仍然希望正确编码。

Answer 1

没有波动是安全的。 ReentrantLock 实现 Lock 和 docs for Lock包括这个:

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.