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copied!<p>Impala's approach to modularity is very weak when it comes to controlled sharing between modules. The problem is that Impala still follows the old J2EE-style hierarchical approach to classloading.</p> <p>Anybody can write a module system that restricts visibility of classes across modules. The difficult part is how you reintroduce dependencies between modules such that specific classes and interfaces from one module can be seen by another module. In OSGi we do this by exporting and importing packages, so we have a non-hierarchical dependency graph.</p> <p>In Impala, if you want to see the classes in another module, then your module must be a child or descendant of that module. That is, modules can only see their own classes and those of their ancestors. Now if you want to share some classes with your sibling module (e.g. a library that you both use) then you must move that library up into the classpath of your shared ancestor. In the worst case you have to move it right up to the root module. Now the library is visible to ALL other modules whether they want it or not! Indeed, if another module wanted to use a different version of the library they would be prevented from doing so.</p> <p>If you simply have a copy of the library in each place where it is used, then you make it impossible for the modules using that library to communicate with each other. They will get ClassCastExceptions when they try to pass objects between each other.</p> <p>A similar problem is inherent in J2EE if two web applications need to use the same library. Typically J2EE developers just copy the library, but this creates "silo" applications that cannot communicate with each other. It is simply not the way to build modular software.</p> <p>Steven's points also seem pertinent. As far as I can tell, nobody is using Impala aside from its author.</p>

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