我可以在Java中的同步块之外的共享字段上检查null吗？

Question

Let's say I have a field that can be accessed by two separate threads. I'm using an Object for the synchronization lock. Can I check for null outside of the synchronization block? In other words, is this thread-safe:

private Object sharedObject() = new Object();
private final Object sharedObjectLock() = new Object();

private void awesomeMethod() {
   if(sharedObject != null) {
      synchronized(sharedObjectLock) {
         //code the uses sharedObject
      }
   }
}

Answer 1

No, it's not thread-safe. If another thread nullifies sharedObject field, your thread is not guaranteed to see that change.

Answer 2

No. The assignment of the variable could occur after you check but before you get the lock.

There was, at one time, a school of thought that synchronized locks were expensive, but that's not true anymore. Just grab the lock.