Well, I work on Hadoop. I don't know what you mean by "modularizing the core." There was an abortive attempt a few years ago to split HDFS, MapReduce, and common off into separate source code repositories. At some point it became clear that this was not going to work (I wasn't contributing to the project at the time, so I don't have more perspective than that).
Right now, we have several Maven subprojects. Maven does seem to enforce dependency ordering-- you cannot depend on HDFS code in common, for example. So it's "modular" in that sense. But you certainly never could run HDFS without the code in hadoop-common.
None of this really has much to do with CLASSPATH. Well, I guess it means that the common jars are shared between potentially many daemons. Dependencies get a lot more complicated than that, but that's just one example.
Really, the bottom line here is that there should be reasonable, sane conventions for where things are installed on the system. This is a lesson that old UNIX people knew well. There are even conventions for how to install multiple different versions of C/C++ shared libraries at the same time, and a tool for finding out what depends on what (ldd). Java's CLASSPATH mechanism itself is just a version of LD_LIBRARY_PATH, which also has a very well-justified bad reputation.
I don't know of anyone who actually uses OSGI. I think it might be one of those technologies that just kind of passed some kind of complexity singularity and imploded on itself, like CORBA. But I have no direct experience with it, so maybe that is unfair.
I like what Golang is doing with build systems and dependency management. They still lack the equivalent of shared libraries, though. Hopefully, when they do implement that feature, they'll learn from the lessons of the past.
Right now, we have several Maven subprojects. Maven does seem to enforce dependency ordering-- you cannot depend on HDFS code in common, for example. So it's "modular" in that sense. But you certainly never could run HDFS without the code in hadoop-common.
None of this really has much to do with CLASSPATH. Well, I guess it means that the common jars are shared between potentially many daemons. Dependencies get a lot more complicated than that, but that's just one example.
Really, the bottom line here is that there should be reasonable, sane conventions for where things are installed on the system. This is a lesson that old UNIX people knew well. There are even conventions for how to install multiple different versions of C/C++ shared libraries at the same time, and a tool for finding out what depends on what (ldd). Java's CLASSPATH mechanism itself is just a version of LD_LIBRARY_PATH, which also has a very well-justified bad reputation.
I don't know of anyone who actually uses OSGI. I think it might be one of those technologies that just kind of passed some kind of complexity singularity and imploded on itself, like CORBA. But I have no direct experience with it, so maybe that is unfair.
I like what Golang is doing with build systems and dependency management. They still lack the equivalent of shared libraries, though. Hopefully, when they do implement that feature, they'll learn from the lessons of the past.