Re: delivering a program as multiple .jar files

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Sun, 09 Aug 2009 15:56:43 -0400
David Lamb wrote:

I am building a program that logically consists of three separate groups
of classes, which I'd prefer to store as 3 separate .jar files.

a) Those from a generic library (written by group 1)

b) Those that implement a specific program on top of the library
(written by group 2, potentially different from group 1). This contains
the main class.

c) Those that represent a set of mods to the specific program (written
by group 3, likely different from groups 1 and 2). I expect that one of
the inputs or parameters to the main program will give strings that can
be turned into names of classes that the main program can pass to a
class loader to somehow tie these mods into the program.

I'm trying to figure out good ways to organize these groups of classes.
 I know group 3 could just combine all 3 .jar files and provide its own
manifest identifying one of its own classes as the main class, but I'd
prefer to leave the main program in control.

I'm thinking of three possible scenarios, and don't know how to
implement any of them; I'd like to know how to do all 3 (if #3 is
possible at all):

1) The user (likely somebody not from any of the above 3 groups)
downloads all 3 .jar files, and runs the group by invoking the main .jar
file (b), supplying some parameter that identifies the mod (c).

2) The user invokes the mod via a link to a .jnlp file, which somehow
acquires all 3 .jar files from a single site. In this mode they might
view the program as a "demo" that has very restricted access to the
user's system, but can at least store some form of data between
invocations (I realize there are JWS packages that I need to investigate
that should accomplish some of this, but so far I've only used JWS to
invoke single .jar files that need no access to the user's system).

3) The user invokes the main program via JNLP, but with the mod from
some second site. I realize that by default this is impossible but
wondered if there is a way to ask for user permission to do this. If
it's possible, how can the main program locate the second .jar file?

There are security concerns in the back of my mind that I can explain if
it seems relevant to answering the question -- basically, I'd expect
users to "trust" segments (a) and (b) more than they do (c) and might be
willing to grant special permissions if they could believe (b) is "in
charge" of anything risky.

If it is a desktop style app I would suggest:
- b.jar with Main-Class directive and a Class-Path directive
   pointing to a.jar
- a.jar in same dir
- a config file specifying jar file and module name
- URLClassLoader to load the modules
- a security manager if needed


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