Re: About multithreading

Eric Sosman <>
Fri, 03 Nov 2006 14:52:31 -0500
Chris Uppal wrote On 11/03/06 06:30,:

Eric Sosman wrote:

synchronized(obj) {
       + " holds the lock on " + obj);

You can do a little better than that with the static Thread.holdsLock() method.
That is still only of very limited use.

    It answers the question "Does the current thread hold
the lock?" but the O.P. was asking the harder question "What
thread holds the lock?" My tongue-in-cheek answer (and the
longer explanation that accompanied it) was intended to show
that the question can't have a useful answer (in a running
program; studying a post-mortem dump or a program that's
halted under debugger control is another matter).

    I imagine this is what you mean by "very limited use." A
thread cannot reliably detect the state of a shared object[*]
unless it holds the object's lock[**], so "locked by thread T1"
is something thread T2 cannot detect.

    [*] The state of a shared immutable value object can be
detected without a lock. However, synchronizing on an object
changes the object's state (from unlocked to locked), and so
in a weak sense "mutates" the object; an object subject to
synchronization is not strictly "immutable."

    [**] Or whatever lock or combination of locks the program
uses to guard the object; for example, an object might be
protected not by its own lock, but by a lock on a Collection
that holds it.


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