Re: [Wrapper-user] Restart VM

[Leif M. <le...@ta...>]

Jacques,
    Of course simply restarting the JVM using the JVM would be the 
easiest :-)  But what
you asking is possible.  I can't walk you through all of the steps.  But 
the basic idea is to
create a new class loader which loads your application.   The class 
loader works just like
any other object.  When all of its references are gone it can be garbage 
collected along
with any classes that it loaded.

    You would then repeat the process for your new invocation.

    Applications like Tomcat do this to make it possible to reload JSP 
pages when
they have been modified.

    The difference between Tomcat and your application is that with 
Tomcat, it is relatively
easy to know when it is safe to GC a particular classloader.  All 
threads that call into that
code and all references are controlled by Tomcat.

    Your application will most likely be launching several threads and 
maybe even using
something like swing.   Even you remove all reverence to the class 
loader yourself, it
will not be Gced if even a single thread someplace is still referencing 
any class or object
that was loaded with that class loader.

    Also remember that if you load class A in two different class 
loaders they will not be
the same class even though they have the same name.   Ie you can not 
cast an instance
of one to the other.  The only way to reference an object in another 
class loader is by
using a class that was loaded from the same place.  This will be true 
with most of the
core java classes.

    If you feeling brave, a good place to start would be with the 
java.net.URLClassLoader.
I have written applications that make use of multiple classloaders.  
They can be quite
useful if really needed, but you will be surprised by all the little 
issues that will come up.

Cheers and Good luck.
Leif

Jacques Bosch wrote:

>Hi there Leif.
>I have managed to come right with writing a bootstrap class that first
>copies the update if there is one and then starts up the main app. It works
>beautifully. Thanx for the suggestion.
>This is probably the wrong place to be asking, but do you know if it is
>possible to unload the main app again without terminating the VM. I.e.The
>bootstrap class must still be loaded.
>What happens now, when using the wrapper: The app downloads the new jar,
>puts it in Updates folder, and signals for the wrapper to restart the VM.
>Wrapper restarts the VM starting with the bootstrap class, that then copies
>the update and starts up the app.
>But now I would love to have the same functionality even when the wrapper
>isn't used. I.e. The app, that was loaded with the bootstrap, downloads the
>new jar, and then some how indicates to the bootstrat that there is an
>update. The bootstrap unloads the app, removing the VM's lock on the file,
>copies the update, and starts the app up once more.
>
>Do you know if this is possible?
>
>Thanx for your help.
>Jacques
>
>
>----- Original Message ----- 
>From: "Leif Mortenson" <le...@ta...>
>To: <wra...@li...>
>Sent: Monday, December 01, 2003 3:07 PM
>Subject: Re: [Wrapper-user] Restart VM
>
>
>: Jacques,
>:     In that case, you may have to play with class loaders.  You will
>: have a bootstrap
>: class whose job is to look in a upgrade directory when it first starts.
>: If there are any new
>: jars, they are moved into the current jars dir.   The bootstrap class
>: then actually calls the
>: real main method of your application.
>:
>:     I don't think the jars will be locked until after they are actually
>: accessed.  So as long as
>: your bootstrap class is in its own stable and independent jar then this
>: should work.   If not
>: then you will have to have your bootstrap class create a new class
>: loader, which in turn
>: builds up a classpath and so on.  Classloaders can be a bit of a pain,
>: so hopefully the first
>: option will work.
>:
>:     Be sure to post back with the results.
>:
>: Cheers,
>: Leif
>:
>: Jacques Bosch wrote:
>:
>: >Leif. The app is deployed with the Sun JVM and it also locks the jars. So
>my
>: >problem is that I cannot replace the old jar file with the new one while
>the
>: >VM is still running.
>: >
>: >
>:
>:
>:
>:
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