Re: unable to synchronize on primitive wrapper class?

Lew <>
Fri, 25 Jan 2008 18:13:47 -0500
Eric Sosman wrote:

Eniac Zhang wrote:

Hi all,

My colleague and I found this while debugging our program. We created
a simple program to duplicate the issue. So here's the program:

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

        public void run() {
            while (!Thread.interrupted()) {
                synchronized (total) {

    You have been fooled by "auto-boxing," an evil addition
to Java that was intended to make it easier to use but in
actuality just makes it more confusing.

    Keep in mind that operators like `++' work on primitive
variables, not on objects like Integer instances. The compiler
has actually translated `total++;' as if you had written

    total = new Integer(total.intValue() + 1);

.... or perhaps it uses Integer.valueOf() instead of calling the
constructor; I'm not certain. Either way, `total' after this
assignment refers to a different Integer instance than the one
the code synchronizes on -- and the accesses to the `total'
reference itself are not synchronized at all. So when multiple
threads execute this code you have race conditions; the outcome
will be (in general) unpredictable.

Also, there is no synchronization on 'count' whatsoever.


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