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Re: Linking Jars

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On Sun, 4 Dec 2011, Robert Klemme wrote:

> On 04.12.2011 05:00, Roedy Green wrote:
>> On Sat, 03 Dec 2011 19:53:37 -0800, Roedy Green
>> <see_website@mindprod.com.invalid>  wrote, quoted or indirectly quoted
>> someone who said :
>> 
>>> Are there any other options?
>> 
>> for example, is there something you can put in the main jar that
>> points to the second?
>
> Exactly.  And it would be my first choice.
>
> http://docs.oracle.com/javase/1.4.2/docs/guide/jar/jar.html#Main%20Attributes

The way Roedy's question so perfectly fits the solution makes me wonder if 
he's seen this before, but forgotten that he's seen it.

A word of warning about that Class-Path attribute. Say you write a library 
Aries which depends on two other libraries, Castor and Pollux. You ship a 
distribution with three jars, yours and those of the dependencies:

Aries
|
+- aries.jar
+- castor.jar
\- pollux.jar

In the manifest of aries.jar, you write a Class-Path attribute which 
refers to castor.jar and pollux.jar, and all is well.

Now i come along, with a project containing two modules, Odin and Thor, 
where Thor depends on Odin (that is, all the classes present in Odin are 
made available to Thor by my build and execution processes). Thor uses 
Aries. But Odin already uses Castor, for some purpose unrelated to Aries. 
My layout needs to look like this:

Odin
|
+- build/classes
|  |
|  \- Odin's own class files
|
+- lib
    |
    \- castor.jar

Thor
|
+- build/classes
|  |
|  \- Thor's own class files
|
+- lib
    |
    +- aries.jar
    \- pollux.jar

Note that Thor does not include castor.jar, because it is supplied by its 
dependency on Odin. All the libraries have the classes they need 
available, and all is well.

Except that aries.jar has a Class-Path entry saying there should be a 
castor.jar right next to it. Which there isn't. So, every time a tool 
which cares about Class-Path entries loads that jar, it will notice the 
absence, and moan about it. Sun's javac certainly does this - i have 
projects, with situations like the above, where half of the output is 
javac squawking about path problems which aren't problems at all. Luckily, 
i don't think java itself does.

If you think the above setup is overcomplex, how about a simpler one; one 
project, but where the different dependencies are kept in different 
folders, for clerical convenience:

Krishna
|
\- lib
    |
    +- Aries
    |  |
    |  +- aries.jar
    |  \- README
    |
    +- Castor
    |  |
    |  +- castor.jar
    |  \- castor-javadoc.zip
    |
    +- Pollux
       |
       +- castor.jar
       \- castor-extras.jar

Seems reasonable, right? And yet javac isn't going to like that at all.

The problem is that the Class-Path header expresses the dependency as a 
URL; a direct path that can be resolved in the filesystem. That constrains 
filesystem layout. It should have been a symbolic reference; perhaps a 
pairing of a Specification-Title and a Specification-Version that 
identifies what is needed, with the onus then being on the code loading 
the jar to go off and find that, by scanning the classpath, exploring the 
filesystem, looking in a module repository, etc. I think OSGi does 
something a bit like this; the JBoss 7 module system certainly does.

Or an even easier case - my new project Loki uses Aries, but i like naming 
my jars with a version number:

Loki
|
+- lib
    |
    +- aries-1.0.jar
    +- castor-1.4.jar
    \- pollux-2.2.1.jar

They're in the right place, but they have the wrong names. Cue javac 
complaining!

Having pointed out these problems, i feel like i should give some advice 
for what you should do. But i don't really know what you should do. 
Perhaps you should restrict the use of Class-Path to cases where you can 
control or predict the filesystem layout. Two cases spring to mind.

Firstly, where you have a library that is split into multiple jars, but 
where these will always be present together; perhaps you have a US tax 
rate library, where the code is updated occasionally, but the data for 
each state is updated frequently. You might have a jar for the code and a 
jar file for each state, containing classpath resources with data in. 
There, it would be reasonable to use a Class-Path in the code jar to drag 
in the state jars. This still falls over if i want to rename the jars, of 
course.

Secondly, where you are packaging for deployment as an application, not 
for further reuse. The main jar for an applet, application, or JWS app 
would all be good examples of this. Nobody but you will ever incorporate 
those jars into a project, so there is no worry about constaining 
filesystem layout.

Or just not worry about Class-Path, and rely on consumers to set up a 
classpath correctly.

tom

-- 
I GUARANTEE IT.

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Linking Jars Roedy Green <see_website@mindprod.com.invalid> - 2011-12-03 19:53 -0800
  Re: Linking Jars Roedy Green <see_website@mindprod.com.invalid> - 2011-12-03 20:00 -0800
    Re: Linking Jars Robert Klemme <shortcutter@googlemail.com> - 2011-12-04 11:20 +0100
      Re: Linking Jars Lew <lewbloch@gmail.com> - 2011-12-04 10:37 -0800
        Re: Linking Jars Robert Klemme <shortcutter@googlemail.com> - 2011-12-04 23:24 +0100
        Re: Linking Jars Roedy Green <see_website@mindprod.com.invalid> - 2011-12-05 04:20 -0800
          Re: Linking Jars ilAn <idonot@wantspam.net> - 2011-12-05 21:49 +0200
            Re: Linking Jars Tom Anderson <twic@urchin.earth.li> - 2011-12-06 22:11 +0000
          Re: Linking Jars Arne Vajhøj <arne@vajhoej.dk> - 2011-12-05 22:11 -0500
          OT: Was Re: Linking Jars RedGrittyBrick <RedGrittyBrick@spamweary.invalid> - 2011-12-07 15:11 +0000
      Re: Linking Jars Tom Anderson <twic@urchin.earth.li> - 2011-12-04 20:25 +0000
        Re: Linking Jars markspace <-@.> - 2011-12-04 13:43 -0800
          Re: Linking Jars Tom Anderson <twic@urchin.earth.li> - 2011-12-06 22:06 +0000
  Re: Linking Jars Ronny Schütz <usenet.r96@gishpuppy.com> - 2011-12-04 23:18 +0100

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