Re: [JBoss-dev] Why are we using this bogus construct

[Dain Sundstrom]

Hiram,

This type of construct creates some trick code. You iterate over the keys, and I assume somewhere in the code you do clients.get(key). Well there is no guarantee the the key is associated. I bet there are a ton of other issues as your iterator in the read phase drifts from reality of changing clients HashMap. Anyway, I hope you documented this heavily.

-dain

On Thursday, February 13, 2003, at 12:28 PM, Hiram Chirino wrote:

Yep.. your way is valid too but you take a
synchronization hit on every read.  The otherway, the
performace hit is on the write.  As I said previously,
if you read the map ALLOT more than you write to it,
then it makes sense to do it backwards.

Regards,
Hiram

--- Dain Sundstrom <[EMAIL PROTECTED]> wrote:

This seems backwards to me.  I usually do something
like this:

class X
{
     HashMap clients = new HashMap();

     public void someMethod()
     {
        synchronized(clients)
        {
             m.put(dc, cq);
        }
         ...
     }
     public void someOtherMethod()
     {
         HashMap clientsCopy = null;
         synchronized(clients)
         {
             clientsCopy = new HashMap(clients);
         }
         Iterator i =
clientsCopy.keySet().iterator();
         while( i.hasNext() )
         {
             ...
         }
     }
}

For the insert, you only get a quick synchronized
around the add, and
on iteration you get a single copy in the
synchronize.  If the
iteration very small and quick, I sometimes put the
entire iteration in
the synchronized block, but you need to be careful
about open calls.

-dain

On Thursday, February 13, 2003, at 11:20 AM, Scott M
Stark wrote:

I have seen this usage construct in a few places

in the code and it

makes
no sense to me:

class X
{
    HashMap clients = new HashMap();

    public void someMethod()
    {
       synchronized(clients)
        {
            HashMap m = new HashMap(clients);
            m.put(dc, cq);
            clients=m;
       }
        ...
    }
    public void someOtherMethod()
    {
        Iterator i = clients.keySet().iterator();
        while( i.hasNext() )
        {
            ...
        }
    }
}

The unsynchronized clients HashMap is synchronized

and copied when

modified and accessed without synchronization in

other contexts. This

is
not thread safe for the accesses and makes for

very expensive updates.

Why isn't the HashMap simply synchronized and used

without copying?

xxxxxxxxxxxxxxxxxxxxxxxx
Scott Stark
Chief Technology Officer
JBoss Group, LLC
xxxxxxxxxxxxxxxxxxxxxxxx

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