I don't know about the loading of classes from jar files but I do know
that you couldn't do that with earlier versions of the run time.  I
think it was 1.4 and XP would not let you write the files.  But it could
have been 98 too.  I've not tried it with Linux.

I don't see any way you could be /guaranteed/ safety if you modify/replace a
JAR file which is already in use.

Exactly what will happen will depend on a combination of the OS semantics,
unpublished implementation details of the JAR/ZIP file reader, and the exact
operation you use to replace the JAR.  The result could (at least in theory) be
anything from "everything just works, no problems at all", through reading
invalid data from the JAR file (which would at least cause a runtime error), or
crashing the JVM, right up to /seeming/ to work, but running with an invalid
combination of classes from different versions of the JAR.

Not something I'd be willing to risk in production, unless there was a very
good reason /and/ I had direct personal control over every aspect of the
installation and code.

   -- chris

På Wed, 04 Apr 2007 00:21:12 +0200, skrev visionset <spam@ntlworld.com>:

Probably, but it depends on the ClassLoader responsible for reading from  
the jars. Generally a classloader would open the file in read mode, and  
operating systems often let you write to files that are open for reading:  
Either the ClassLoader will get an exception at the next read attempt (and  
maybe reopen and continue) or it keeps reading from the existing file  
descriptor while the file name now points to a different set of blocks.