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lingering wait()/notify() and deadlock doubt

Alexander Danilou
Greenhorn

Joined: May 08, 2009
Posts: 28
Hi, can somebody help me with this?
A call to wait()/notify() will NOT prevent a deadlock - true of false?

I think it should be false, because when calling wait() thread releases the lock, so deadlock can be prevented.
But according to K&B the answer is true..
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18708
    
  40

I think it should be false, because when calling wait() thread releases the lock, so deadlock can be prevented.


Just because a method releases a lock -- doesn't mean that it is a mechanism that can be used to prevent deadlocks. Can you explain how the method can be used to prevent deadlocks?

Henry


Books: Java Threads, 3rd Edition, Jini in a Nutshell, and Java Gems (contributor)
Devaka Cooray
ExamLab Creator
Saloon Keeper

Joined: Jul 29, 2008
Posts: 3051
    
  36

As Henry wrote, it cannot be used as a mechanism "to prevent deadlocks". But it "may" prevent deadlocks on some situations. In my view, this question cannot be accurately answered without having more details on it. Found your other topic.

Devaka.


Author of ExamLab ExamLab - a free SCJP / OCPJP exam simulator
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Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18708
    
  40

But it "may" prevent deadlocks on some situations.


In theory, this may be true. In practice, I have never seen this done. In fact, even worrying about deadlocks is not common with many programmers...

And for developers that do worry about deadlocks (ie. the good ones)... Deadlock prevention techniques generally implemented are (1) using a coarse lock, or (2) lock ordering -- building a release mechanism just seems too elaborate and may even enforce using a "framework".

Henry
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18708
    
  40

Devaka Cooray wrote:Found your other topic.


Thanks Devaka. I locked the other topic (and posted a link back to here).

Henry
Alexander Danilou
Greenhorn

Joined: May 08, 2009
Posts: 28
Thank you,

Devaka wrote:
In my view, this question cannot be accurately answered without having more details on it.

This was exactly my concern. At this point I just want to pass SCJP exam. I really hope that real exam will not contain such questions. But since this statement and answer "true" is on the K&B Mock exam, I was concerned.
Devaka Cooray
ExamLab Creator
Saloon Keeper

Joined: Jul 29, 2008
Posts: 3051
    
  36

Henry Wong wrote:In theory, this may be true. In practice, I have never seen this done.

Completely agree. I can't even imagine a dead-lock situation in practice, which should be prevented by using wait()/notify() methods.

Devaka.
Devaka Cooray
ExamLab Creator
Saloon Keeper

Joined: Jul 29, 2008
Posts: 3051
    
  36

Alexander Danilou wrote:This was exactly my concern. At this point I just want to pass SCJP exam. I really hope that real exam will not contain such questions. But since this statement and answer "true" is on the K&B Mock exam, I was concerned.

You may encounter with same type questions on the real exam, but with some additional details. On that case it is no doubt to choose "true" as the correct answer.

Devaka.
 
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