JVM, Java, Multithreading, Object Creation

Question

i am encountering a weird scenario, Is there a possibility of JVM re-using an already created object when we are initializing a new one and the object count is JVm is very high?

abc a = new abc();
a.setAttribute("aaaa");
.........
a...is no longer being used...and has not yet been garbage collected by the JVM. There are multiple threads creating 5000 instances of class abc..
again, abc a = new abc();
       Sysout(a.getAttribute()); // This prints "aaaa" set for an earlier instance!

Is there a possibility of an instance being re-used.? Has anyone come across this scenario before?

Can we see more of the code? Is a local variable, or a field? What does setAttribute and getAttribute look like? (Both method signature, content and how it is stored (the declaration of the field). — Yishai
– Yishai, Commented Jun 5, 2009 at 12:24

Lucero · Accepted Answer · 2009-06-05 11:28:10Z

5

No. I think this is a bug of yours. Maybe also try with a different JVM version or vendor to see whether those behave as you expect or not.

answered Jun 5, 2009 at 11:28

Lucero

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Michael Borgwardt · Accepted Answer · 2009-06-05 11:30:41Z

4

This would constitute a bug in the JVM, but I would consider it very unlikely.

I would say with 99% confidence that your code is simply exhibiting a race condition, such as a thread other than the one you are observing setting the attribute.

answered Jun 5, 2009 at 11:30

Michael Borgwardt

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HMM · Accepted Answer · 2009-06-05 11:40:56Z

3

The JVM does not re-une objects AFAIK. But the behaviour you are seeing can be explained.

a.setAttribute("aaaa"); and a.getAttribute might be setting a static field, a singleton or a threadlocal in another class, or another thread is changing state.

answered Jun 5, 2009 at 11:40

HMM

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1 Comment

hakish Over a year ago

its a not a static or singleton object.

andygavin · Accepted Answer · 2009-06-05 12:34:54Z

1

Depending on where these assignments take place your program may be exhibiting statement reordering: the JVM may be instruction-reordering the assignment statements so they don't execute in the order you coded them. This is part of the Memory Model specification and could point to your program being under synchronized.

See the JSR133 FAQ: http://www.cs.umd.edu/users/pugh/java/memoryModel/jsr-133-faq.html#reordering

Or section 2 in: http://www.cs.umd.edu/~pugh/java/memoryModel/jsr133.pdf

An easier explanation begins at 10:40 in this video: http://www.youtube.com/watch?v=1FX4zco0ziY

edited Jun 5, 2009 at 12:34

answered Jun 5, 2009 at 12:18

andygavin

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Community · Accepted Answer · 2017-05-23 12:13:35Z

0

if you are using multithreading you might be encountering what is know as 'stale data'

some of it explained in Java multi-threading & Safe Publication

edited May 23, 2017 at 12:13

CommunityBot

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answered Jun 5, 2009 at 11:49

Peter

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