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Groups > comp.lang.java.programmer > #28351

Re: limitations of Class.forName

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On 05/02/2014 02:42 AM, Roedy Green wrote:
> When I first encountered Java, I was very excited about the notion
> of interfaces and dynamic loading. I thought it would let people plug
> in their own code to every conceivable utility.
>
> There are two catches.
>
> 1. arranging that java.exe will look for your dynamic class in the
> place where it is.  It will not usually be bundled in the original
> jar.
>
> 2. arranging that java.exe will look for any auxiliary classes your
> plugin needs in the place where they live.

Oh, but there's more.
Class.equals() will fail on classes loaded by different classloaders, as 
will Object.equals().
That is, after you extend URLClassLoader as Arne said:)
Then, you can't unload a class just like that, as class loader keeps 
references to all loaded classes. You need to unload that ClassLoader too.
That is, if you wish to reload plugins without jvm restart.
With custom URLClassLoader, watch carefully not to break specified 
delegation model.

> A garden variety jar prevents java from looking anywhere else for
> classes.
>
> I thought I would write a short essay on this.  What techniques are
> available?
>
> I have always skirted the problem by putting all classes in the
> original jar.

OK then you wish only to customize application classloader.
Note that in JEE containers, classloaders have another important 
function - context isolation.

Regards...

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limitations of Class.forName Roedy Green <see_website@mindprod.com.invalid> - 2014-05-01 17:42 -0700
  Re: limitations of Class.forName Arne Vajhøj <arne@vajhoej.dk> - 2014-05-01 21:15 -0400
  Re: limitations of Class.forName Robert Klemme <shortcutter@googlemail.com> - 2014-05-02 08:51 +0200
  Re: limitations of Class.forName Josip Almasi <joe@vrspace.org> - 2014-05-02 09:25 +0200
    Re: limitations of Class.forName Roedy Green <see_website@mindprod.com.invalid> - 2014-05-06 03:55 -0700

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