Hi all,

I would like to replace java.util.Calendar class - I need to slightly 
change it. Is it possible by providing custom classloader or..anything? 
Thanks for any hints.

Regards,
Maciek Zywno

> I would like to replace java.util.Calendar class - I need to slightly 
> change it. Is it possible by providing custom classloader or..anything?

You can use the non-standard option -Xbootclasspath with sun's jvm in order 
to load your own implementation of java.util.Calendar. However, this is not 
a particularly good idea since you can't distribute a program that overrides 
the core classes. Here is an excerpt from the doc for sun's jdk 1.5 :

"-Xbootclasspath/p:path
Specify a semicolon-separated path of directires, JAR archives, and ZIP 
archives to prepend in front of the default bootstrap class path. Note: 
Applications that use this option for the purpose of overriding a class in 
rt.jar should not be deployed as doing so would contravene the Java 2 
Runtime Environment binary code license. "


IMHO the proper way to do this is to get the source for the Calendar class 
(available from sun), make your changes and distribute this as a new class 
(that is to say in your own package, not java.*).

sincerely,

Cadet Reynaud-Plantey

Danyel Reynaud-Plantey wrote:
>>I would like to replace java.util.Calendar class - I need to slightly 
>>change it. Is it possible by providing custom classloader or..anything?
> 
> 
> You can use the non-standard option -Xbootclasspath with sun's jvm in order 
> to load your own implementation of java.util.Calendar. However, this is not 
> a particularly good idea since you can't distribute a program that overrides 
> the core classes. Here is an excerpt from the doc for sun's jdk 1.5 :
> 
> "-Xbootclasspath/p:path
> Specify a semicolon-separated path of directires, JAR archives, and ZIP 
> archives to prepend in front of the default bootstrap class path. Note: 
> Applications that use this option for the purpose of overriding a class in 
> rt.jar should not be deployed as doing so would contravene the Java 2 
> Runtime Environment binary code license. "
> 
> 
> IMHO the proper way to do this is to get the source for the Calendar class 
> (available from sun), make your changes and distribute this as a new class 
> (that is to say in your own package, not java.*).
> 
> sincerely,
> 
> Cadet Reynaud-Plantey
> 
> 

Just FYI...

Whilst it true for overriding classes that are part of the Sun's own 
library (Java.*, sun.* etc) the use of -xbootclassPath is vital if you 
want to use a difference CORBA orb to the one that is shipped as part of 
the JRE.  The -xbootclassPath is widely used for this purpose.

The CORBA Orb interfaces are all packaged as omg.corba.orb.* packages 
which means the license does not effect use from overriding them.

If you code your own Calendar class it will override the
java.util.Calendar class. Or another way would be to just create a
class that extends the java.util.Calendar class and just override the
things you need to change.

Danyel Reynaud-Plantey wrote:
>>I would like to replace java.util.Calendar class - I need to slightly 
>>change it. Is it possible by providing custom classloader or..anything?

What problem are you having such that you can not simply
write a new subclass of java.util.Calendar?  Somwhere in
your code, I assume you are doing
Calendar myCal = Calendar.getInstance();

change that to:

Calendar myCal = fr.wanadoo.Calendar.getInstance();

Playing with class loaders sounds very excessive in this case.
There are various places in the JDK which WHEN NOT PROVIDED A CALENDAR, 
the API uses the system default one, but most that I know of can be 
overwritten with another calendar. I would be very interesting in 
understanding why this is not a possibility for the problem you have.

-Paul