Monday, October 13, 2014

Re: a Prod-Consumer threading question

(publishing this example on my blog)

https://community.oracle.com/thread/1142629?tstart=120 presents a simple but tricky threading scenario. Without the sleep(), we get random output like

Putting : 0
Putting : 1
Getting : 0

Q: how could Putting 1 happen before "Getting 0"? 

Explanation -- Actually, the assignment to this.hasPut does happen in lock steps like true false true false true .... (Can confirm by printing its value immediately after assignment). This is thanks to the "synchronized" keyword. Problem is the Putting/Getting prints. These statements are outside synchronized blocks, and get badly delayed, like in the above scenario --

put() returns -> "Putting 0" -> get() returns but consumer thread preempted before "Getting 0" -> put() returns -> "Putting 1" -> "Getting 0" 


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