Re: RMI in java

Lew <>
Thu, 03 Sep 2009 11:42:42 -0400
david wrote:

i have one more doubt regarding the synchronization of the objects.

I personally remain ever confused by this use of the word "doubt". I know, my

assume that I have a two dimensional array of objects(resource)
available in the server and clients can access those objects using the
RMI interface. and right now, there are two clients (a and b) are
using one particular object(resource) each. And if at any time, client
'a' or 'b' wants to access another object(resource), the server needs
to check whether it's been held by some other client or not.

(tried to put that in a picture.. )

      | | | | | |
      | | a | * | | |
      | | | b | | |
      | | | | | |

I thought of having a set of two dimensional mutex locks (from C++)
kind of lock to represent the two dimensional array of objects(the
resources). And whenever a client tries to access a particular
resource object,
              i) check whether the object is available/free to be
              ii) lock the corresponding mutex
             iii) check again for availability of the object,
                     if yes..then mark it as in use with the
corresponding client's details
              iv) then release the mutex lock.

But, java does not have the mutex thing.

No? I do not know what you mean by "mutex" or how it differs from what the
inbuilt Hoare monitors or the Lock class or semaphores deliver, but I can't
imagine how they fail you in a way that what you call a "mutex" wouldn't.

Conditional 'Synchronized' block around a two dimensional array

What is "Synchronized"? I know the keyword 'synchronized', but not this other

element threw an
(like "synchronized (myArray[i][j])")

The NPE has nothing to do with synchronization, therefore mutexes will not fix
it. The problem is that either myArray or myArray[i] or myArray[i][j] are null.

(though i noticed that conditional Synchronized blocked worked for the
second-level array this "synchronized (myArray[i])" )

a two dimensional binary semaphore to achieve this scenario (to get
the mutex effect, work around) would be very expensive, if the matrix
dimension is larger.

m going through some e-books for various Java concurrency techniques
to get some better idea.

pls..give me some suggestion or pointers to get some clear idea.
thanks in advance.

Off your main question but relevant to your posts:
- Post once per message. Repeating your post won't increase the quality or
number of answers.
- "Java" is spelled with a capital "J".
- The English first-person singular pronoun "I" is spelled with a capital "I".
- "Pls" is not a word.
- "m" is not a word.

On your main question: don't try to synchronize on a 'null' variable.


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