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From David Van Couvering <David.Vancouver...@Sun.COM>
Subject Re: [jira] Updated: (DERBY-243) connection toString should uniquely identify the connection
Date Sat, 30 Apr 2005 00:24:19 GMT
Hm, I always thought a hash-code was not unique.  I got excited when you 
mentioned this method, identityHashCode(), which I hadn't heard of, but 
it basically delegates to Object.hashCode(), it just ensures that the 
hash code returned is the base Object hash code and not one returned by 
an overriding method.

The documentattion for hashCode() says

"It is not required that if two objects are unequal according to the 
equals(java.lang.Object) method, then calling the hashCode method on 
each of the two objects must produce distinct integer results. However, 
the programmer should be aware that producing distinct integer results 
for unequal objects may improve the performance of hashtables.

  As much as is reasonably practical, the hashCode method defined by 
class Object does return distinct integers for distinct objects. (This 
is typically implemented by converting the internal address of the 
object into an integer, but this implementation technique is not 
required by the JavaTM programming language.)"

So, we can depend on it *most* of the time, but that makes me a bit 
nervous...

David

Jack Klebanoff wrote:

> The simplest thing would be to use the java.lang.System.identityHashCode 
> method on the connection object. I believe that it is unique in a 32 bit 
> system. In practice it will almost always be unique in a 64 bit system.
> 
> I don't think that the Monitor or UUIDFactory classes are available in 
> the client.They are part of the Derby server.
> 
> Jack Klebanoff
> ||David Van Couvering wrote:
> 
>> One thought I had about the UUID approach, after proposing it, is that 
>> if you're trying to correlate connections and trace messages and you 
>> have these huge long UUID strings, it can be a bit challenging.
>>
>> If it were a simple static long that starts at 1 and increments each 
>> time a new connection instance is created (yes, the increment would 
>> have to be synchronized), then it would be much more readable.  This 
>> would also be more portable to the client code, and we wouldn't have 
>> to cut/paste the UUID class to the client packages...
>>
>> Any thoughts?
>>
>> Thanks,
>>
>> David
>>
>> David Van Couvering wrote:
>>
>>> Hi, Kathey.  Currently the connection classes don't appear to have a 
>>> unique identifier that could be made available in toString().  Do I 
>>> take it you would like me to find an approach that generates one?
>>>
>>> I noticed Derby has a UUID service (very nice!).  Is it OK if I use 
>>> that here to generate a UUID for the connection?  If I don't hear 
>>> otherwise, I'll assume this approach is OK, e.g.
>>>
>>> public class EmbedConnection
>>> {
>>>   private UUID UUIDValue;
>>>   private String UUIDString;
>>>
>>>   public EmbedConnection()
>>>   {
>>>     UUIDFactory uuidFactory = Monitor.getMonitor().getUUIDFactory();
>>>     UUIDValue = uuidFactory.createUUID();
>>>     UUIDString = this.getClass().getName() + ":" + UUIDValue.toString();
>>>     ...
>>>   }
>>>
>>>   public String toString()
>>>   {
>>>      UUIDString;
>>>   }
>>> }
>>>
>>> =====
>>>
>>> The connection classes I found are as follows.  Please let me know if 
>>> I missed any.  An indented class implies it extends the unindented 
>>> class above it.
>>>
>>> EMBEDDED (org.apache.derby.engine.*)
>>>   BrokeredConnection (implements java.sql.Connection)
>>>     BrokeredConnection30
>>>   EmbedConnection (implements java.sql.Connection)
>>>     EmbedConnection30
>>>   EmbedPooledConnection (implements java.sql.PooledConnection)
>>>     EmbedXAConnection
>>>
>>> CLIENT (org.apache.derby.client.*_
>>>   Connection (abstract class, implements java.sql.Connection))
>>>     NetConnection
>>>       NetXAConnection
>>>   ClientXAConnection (implements java.sql.XAConnection)
>>>   ClientPooledConnection (implements java.sql.PooledConnection)
>>>   LogicalConnection (implements java.sql.Connection)
>>>
>>>
>>> On the client side, I first need to understand: is derbyclient.jar 
>>> supposed to be standalone (meaning it can't depend upon things in 
>>> derby.jar like the Monitor and the UUID class).  If so, I suppose I 
>>> could cut/paste the BasicUUID class into the client packages for use 
>>> on the client side (shiver).  Alternately we could have a 
>>> derbyutils.jar that is shared between client and server (Big Change, 
>>> not sure if I want to take that on).  Advice here would be most 
>>> appreciated.
>>>
>>> Thanks,
>>>
>>> David
>>>
>>> Kathey Marsden (JIRA) wrote:
>>>
>>>>      [ http://issues.apache.org/jira/browse/DERBY-243?page=all ]
>>>>
>>>> Kathey Marsden updated DERBY-243:
>>>> ---------------------------------
>>>>
>>>>         Summary: connection toString should uniquely identify the 
>>>> connection  (was: connection toString doesn't give enough information)
>>>>     Description: The toString() on the Derby connection doesn't 
>>>> print unique information.
>>>> for example  System.out.println(conn) prints:
>>>> EmbedConnection  in the case of derby embedded
>>>>
>>>> It would be great if the toString() method for connections could be 
>>>> used to differentiate one connection from another.
>>>>
>>>>
>>>>
>>>>   was:
>>>> The toString() on the Derby connection doesn't print unique 
>>>> information.
>>>> for example  System.out.println(conn) prints:
>>>> EmbedConnection  in the case of derby embedded
>>>>
>>>>
>>>>
>>>> I am not sure if XA Connections and Pooled Connections have the same 
>>>> issue.  I didn't immediately see an override of the toString() 
>>>> method in BrokeredConnection.java like there is for EmbedConnection
>>>>
>>>>
>>>>
>>>>> connection toString should uniquely identify the connection
>>>>> -----------------------------------------------------------
>>>>>
>>>>>         Key: DERBY-243
>>>>>         URL: http://issues.apache.org/jira/browse/DERBY-243
>>>>>     Project: Derby
>>>>>        Type: Improvement
>>>>>  Components: JDBC
>>>>>    Reporter: Kathey Marsden
>>>>>    Assignee: David Van Couvering
>>>>>    Priority: Trivial
>>>>>     Fix For: 10.0.2.1, 10.0.2.0, 10.0.2.2, 10.1.0.0
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> The toString() on the Derby connection doesn't print unique 
>>>>> information.
>>>>> for example  System.out.println(conn) prints:
>>>>> EmbedConnection  in the case of derby embedded
>>>>> It would be great if the toString() method for connections could be 
>>>>> used to differentiate one connection from another.
>>>>
>>>>
>>>>
>>>>
>>>>
>>
> 

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