RSS 2.0
Sign In
# Thursday, April 22, 2010

At some point we needed to have an array with volatile elements in java.

We knew that such beast is not found in the java world. So we searched the Internet and found the answers that are so wrong, and introduce so obscure threading bugs that the guys who provided them would better hide them and run immediately to fix their buggy programs...

The first one is Volatile arrays in Java. They suggest such solution:

volatile int[] arr = new int[...];

...
arr[4] = 100;
arr = arr;

The number two: What Volatile Means in Java

A guy assures that this code works:

Fields:

int answer = 0;
volatile boolean ready = false;

Thread1:

answer = 42;
ready = true;

Thread2:

if (ready)
{
  print(answer);
}

They are very wrong! Non volatile access can be reordered by the implementation. See Java's Threads and Locks:

The rules for volatile variables effectively require that main memory be touched exactly once for each use or assign of a volatile variable by a thread, and that main memory be touched in exactly the order dictated by the thread execution semantics. However, such memory actions are not ordered with respect to read and write actions on nonvolatile variables.

They probably thought of locks when they argued about volatiles:

a lock action acts as if it flushes all variables from the thread's working memory; before use they must be assigned or loaded from main memory.

P.S. They would better recommend AtomicReferenceArray.

Thursday, April 22, 2010 1:05:48 PM UTC  #    Comments [0] -
Thinking aloud | Tips and tricks
All comments require the approval of the site owner before being displayed.
Name
E-mail
Home page

Comment (Some html is allowed: a@href@title, b, blockquote@cite, em, i, strike, strong, sub, super, u) where the @ means "attribute." For example, you can use <a href="" title=""> or <blockquote cite="Scott">.  

[Captcha]Enter the code shown (prevents robots):

Live Comment Preview
Archive
<July 2019>
SunMonTueWedThuFriSat
30123456
78910111213
14151617181920
21222324252627
28293031123
45678910
Statistics
Total Posts: 365
This Year: 1
This Month: 0
This Week: 0
Comments: 221
Locations of visitors to this page
Disclaimer
The opinions expressed herein are our own personal opinions and do not represent our employer's view in anyway.

© 2019, Nesterovsky bros
All Content © 2019, Nesterovsky bros
DasBlog theme 'Business' created by Christoph De Baene (delarou)