Unsafe: removing the monitorEnter/monitorExit/tryMonitorEnter methods

Dr Heinz M. Kabutz heinz at javaspecialists.eu
Thu Feb 27 14:51:06 UTC 2014

I have never used this "in the wild", but rather have moved over to the 
ReentrantLock when I needed that particular functionality.

However, I do see a place for this ability.  As I wrote in here: 
http://www.javaspecialists.eu/archive/Issue194.html - sometimes you need 
to modify code that is already using a particular locking mechanism.  To 
then redo everything with ReentrantLock can introduce errors.

If I had a say, I would vote to either keep that method in Unsafe (which 
is where I think it belongs) or provide an alternative way to make the 
tryMonitorEnter() mechanism available to those that might end up needing it.

Since I bought my Suzuki Jimny 7 years ago, I have not needed my spare 
wheel even once.  So yeah, I could take it off and drive around without 
it.  But chances are, the day after I take it off, I will need it.

tryMonitorEnter() does no harm and it is out of reach of most programmers.


Dr Heinz M. Kabutz (PhD CompSci)
Author of "The Java(tm) Specialists' Newsletter"
Oracle Java Champion 2005-2013
JavaOne Rock Star Speaker 2012
Tel: +30 69 75 595 262
Skype: kabutz

Paul Sandoz wrote:
> Hi,
> Out of all the methods on Unsafe i think the monitorEnter/monitorExit/tryMonitorEnter are the least used and are very strong candidates for removal.
> 99% of use-cases are supported by classes in the java.util.concurrent.locks package.
> Within the JDK itself it is only used in one JDK test file test/java/lang/ProcessBuilder/Basic.java:
>                     while (unsafe.tryMonitorEnter(s)) {
>                         unsafe.monitorExit(s);
>                         Thread.sleep(1);
>                     }
> for a test verifying an EOF is received on pending reads and it is polling to check when the process builder acquires the lock before destroying the process, presumably to avoid some sort of race condition that occasionally causes the test to fail.
> I believe this test as been through a number of rounds, i stared at things for a bit, but cannot quickly work out a replacement; i lack the knowledge on this area.
> Outside of the JDK i can only find one usage of monitorExit/Enter (according to grep code) in JBoss modules, and i believe this is only used on Java 1.6 to work around some limitations in class loading that were fixed in 1.7.
> Given such very limited use i propose to remove these methods after having worked out a fix for ProcessBuilder/Basic.java test.
> Paul.

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