Re: [classloading] How to use URLClassLoader within a servlet

[Martin Jacobson]

Ola Berg wrote:
> I have this problem with using my own (URLClassLoader) class loader
> in a servlet of mine.
>
> The thing is that I am writing a web front end to domain name
> services (automatic registration of .org, .com. .biz, .info etc
> domains).
>
> Depending on the top level domain, I need to use different versions
> (jars) of the same API, since the protocol differs. So I need to
> dynamically load my classes at runtime. Just putting the jars in lib
> won't work since there are different implementations of the same
> classes in them. I have to decide in runtime.
>
> Using the URLClassLoader in a standalone app works perfectly, when
> using it in a servlet it just hangs. I suspect the problem lies
> within the different lookup order used in servlets (as dscribed in
> the classloader-HOWTO).
>
> I can't figure out how to do it.
>
> Has anyone experience in the field?
>
I have an analogous case, in which I have a single API for database access, with choice of database (Oracle, MySQL,...) made at run-time. The problem was simply solved using the 'Abstract Factory' pattern. This means that I have two packages (....db.MySql, & ....db.Oracle), which both include an implementation of a DBFactory. I then use ClassForName() to load the appropriate implementation, and then call factory methods to instantiate the appropriate classes.

Hence, you don't need to bother with custom classloaders, and your design is much 'cleaner' as a result.

HTH,

Martin


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