Ah - you are correct. Here is the byte-code for the incrementor with a
member variable:
Method Test()
0 aload_0
1 invokespecial #1 <Method java.lang.Object()>
4 return
Method int testInc()
0 aload_0
1 dup
2 getfield #2 <Field int x>
5 iconst_1
6 iadd
7 putfield #2 <Field int x>
10 aload_0
11 getfield #2 <Field int x>
14 ireturn
Method int testAdd()
0 aload_0
1 aload_0
2 getfield #2 <Field int x>
5 iconst_1
6 iadd
7 putfield #2 <Field int x>
10 aload_0
11 getfield #2 <Field int x>
14 ireturn
These two methods are now identical, and obviously not atomic. Thanks.
-Larry
----- Original Message -----
From: "Kevin Conner" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Thursday, July 18, 2002 7:19 AM
Subject: RE: [JBoss-dev] is x++ an atomic operation??
> > You know...something seemed odd about it, but I was going to
> > go with it anyway...I'm still pretty sure that an increment
> > operation is not atomic, but I can't be sure.
> >
> > Regardless, the example statement given in the original post
> > was something like:
> >
> > x = y++;
> >
> > And I'm almost 100% certain that's not atomic, as the
> > increment of y and assignment to x are two separate operations.
>
> The increment of y is not guaranteed to be atomic as it involves
> a read, an increment and then a write. These operations may be
> interleaved with operations on another thread. This is before
> you have the assigment to x.
>
> The iinc operator is only for local method variables and therefore
> not visible in other threads. If you change the code to the following
> then you should see different bytecode.
>
> public class Test {
>
> private int x ; // instance variable
>
> public int testInc() {
> x++;
> return x;
> }
> public int testAdd() {
> x = x + 1;
> return x;
> }
> }
>
> Kev
>
> Kevin Conner
> Orchard Information Systems Limited
> Newcastle Technopole, Kings Manor
> Newcastle Upon Tyne, NE1 6PA. United Kingdom
> Registered in England, Number 1900078
> Tel: +44 (0) 191-2032536 Fax: +44 (0) 191 2302515
>
>
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