Question

This maybe a related question: Java assignment issues – Is this atomic?

I have the same class as the OP that acts on a mutable string reference. But set rarely happens. (basically this string is part of a server configuration that only reloads when forced to).

public class Test {
 private String s;

 public void setS(String str){
  s = str;
 }

 public String getS(){
  return s;
 }
}

Multiple threads will be pounding this variable to read its value. What is the best method to make it ‘safe’ while not having to incur the performance degradation by declaring it volatile?

I am currently heading into the direction of ReadWriteLock, but as far as I understand, ReadWrite locks does not make it safe from thread caching? unless some syncronisation happen? Which means I’ve gone a full circle back to I may as well just use the volatile keyword?

Is my understanding correct? Is there nothing that can ‘notify’ other threads about an update to a variable in main memory manually such that they can update their local cache just once on a full moon?

volatile on this seems overkill given that the server application is designed to run for months without restart. By that time, it would’ve served a few million reads. I’m thinking I might as well just set the String as static final and not allow it mutate without a complete application and JVM restart.

Answer 1

Reads and writes to references are atomic. The problems you can incur is attempting to perform a read and a write (an update) or guaranteeing that after a write all thread see this change on the next read. However, only you can say what your requirements are.

When you use volatile, it requires a cache coherent copy be read or written. This doesn’t require a copy be made to/from main memory as the caches communicate amongst themselves, even between sockets. There is a performance impact but it doesn’t mean the caches are not used.

Even if the access did go all the way to main memory, you could still do millions of accesses per second.