Java concurrency

xeon123 · 07-08-2007, 06:38 PM

1 - In programming, especially in threads, there's the term "happens-before relationship". Does anyone know what it means? How can we establish a "happens-before relationship" between two threads?

2 - Does Sun Java VM, or any other java VM can switch the order of instructions? If so, what's the reason?
For example;
int a = 0;
int b = 1;
a = b;

becoming,

int a = 0;
a = b;
int b = 1;

Isn't it an error-prone situation?

Thanks,
Pedro

IBall · 07-08-2007, 09:00 PM

Is this homework? We are not allowed to help with homework.

I would suggest getting a good book on Java programming, which should include a section on concurrency.

I don't think the Java VM will switch the order of the instructions, the main issue with concurrency is the instruction being interrupted mid-way through.

--Ian

xeon123 · 07-09-2007, 05:13 AM

This isn't homework. It's just a question, that I saw google videos and I didn't understand, how to relate an "happens-before relationship" between two threads.

Thanks,

xeon123 · 07-09-2007, 05:18 AM

For the second question, if you want, please see the following video:

http://video.google.com/videoplay?do...arch&plindex=1