Re: Locking objects in an array

From:

Lew <lew@lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 5 May 2009 08:53:20 -0700 (PDT)

Message-ID:

<955c84e4-81ae-42f5-9f3d-af4ea03fd712@r3g2000vbp.googlegroups.com>

Philipp wrote:

I have a bidimensional array of objects (view it as a 2D lattice). I
want to make atomic operations on random square 2x2 regions of the
lattice. Thus I want to lock the 4 objects of the region, then perform
the change, then unlock those objects. Several threads should be
allowed to work on the array at the same time, if each thread is
accessing a 2x2 region which does not overlap with that of another,
they should be capable of doing the work in a parallel way.

How should I design the code to achieve this?

My solution (which may be misguided) so far is that, each object of
the lattice contains a Lock and has a lock() and unlock() method which
delegate to the lock.

So the approach is basically:
1. call lock() on each of the 4 objects (always in the same order)
2. make change
3. call unlock() on each of the 4 objects

What I don't like about this, is that
a) lock and unlock really have nothing to do in the API of the objects
in the lattice. Would it be possible to achieve the same result
without using an explicit Lock (thus no lock() method), but using the
Java synchronized() mechanism?

Release locks in the reverse order that you acquire them.

Java 'synchronized' is an explicit lock.

Patricia Shanahan wrote:

There is a relevant inconsistency between the problem statement and the
sample code. The code appears to be designed to allow for a block size
other than 2x2 through a simple change in a constant.

If you can bind it to 2x2, you can do a 4-deep nested synchronization:

What she said.

--
Lew