Re: unable to synchronize on primitive wrapper class?

Mark Space <>
Mon, 04 Feb 2008 11:30:02 -0800
Eniac Zhang wrote:

2. Don't synchronize on primitive type wrapper classes as you don't know
when those object will be replaced rather than changed under the hood.

It's often a good idea to synchronize on a private object created
specially for this purpose. It's a common pattern and would prevent
issues like this this, i.e. some other code does something you didn't
expect, and your locking scheme goes to heck. Also make the protected
resource private, then you have complete control over the resource.

I think this works, although perhaps something should be done with count
as well:

public class VolatileVar {
     static class Worker implements Runnable {
         private static Integer total = 0;
         private static Object lock = new Object();
         public int count = 0;

         public void run() {
             while (!Thread.interrupted()) {
                 synchronized (lock) {
         public int getTotal() {
             int i;
             sychronized (lock) {
                i = total;
             return i;

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