Friday, October 1, 2010

How to check if a thread holds lock on a particular object in Java

Think about a scenario where you would have to find at run time that whether a java thread has lock on a particular object e.g. find out whether thread NewsReader has lock on NewsPaper object or not ?

How to check if a thread holds lock in JavaIf this questions came in any core java interview then I would automatically assume that there could be at least two answer one is hard earned raw answer which programmer would like to figure out based on fundamentals and other could be some rarely used API calls which is available in java , by the way this is actually asked to me in an interview of one of the biggest global bank.

2 ways to find if thread holds lock on object in Java

Here I am giving my answer and what I had discovered after interview

1)I thought about IllegalMonitorStateException which wait() and notify() methods throw when they get called from non-synchronized context so I said I would call newspaper.wait() and if this call throws exception it means thread in java is not holding lock, otherwise thread holds lock.

2)Later I discovered that thread is a static method called holdsLock(Object obj) which returns true or false based on whether threads holds lock on object passed.

Comments, Suggestions ,innovative better answers are always welcome.

Other Java Threading tutorial you may like:


CARFIELD said...

How about using jconsole to check?

Javin @ Tibco RV Tutorial said...

Hi CARFIELD, yes jconsole can be used to monitor threads but that's not the programmatic way of doing it . you may be able to know which thread holds a lock by using ThreadMXBean in java 5 in a programe.

I have also discussed way to find out deadlock in java which is related to this. Please have a look there.

Javin @ Tibco RV Tutorial said...

Hi Gautam,

Yes Thread.holdslock() is the best way to figure out whether thread holds a lock on a particular object or not, let us know if you have some other ways to do it.


Kedar said...

Does anyone remember a situation where you want to check if a thread holds a particular lock or not? I have never used this method, neither that I have needed it. I have only been asked this question one time on a telephonic interview with HSBC technology India. I see the point of surprise but this question is not practical.

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