MJ: A Rational Module System for Java

While Java provides many software engineering benefits, it lacks a coherent module system and instead provides only packages (which are primarily a name space mechanism) and classloaders (which are very low-level). As a result, large Java applications suffer from unexpected interactions between independent components, require complex CLASSPATH definitions, and are often extremely complex to install and maintain. We have implemented a module system for Java called MJ that is implemented with class loaders, but provides a much higher-level interface. High-level properties can be specified in a module definition and are enforced by the module system as new modules are loaded. To experimentally validate the ability of MJ to properly handle the complex module interrelationships found in large Java server systems, we replaced the classloader mechanisms of Apache Tomcat 4.1.18 [27] with 30 MJ modules. The modified Tomcat is functionally identical to the original, but requires no CLASSPATH definitions, and will operate correctly even if user code loads a different version of a module used by Tomcat, such as the Xerces XML parser [31]. Furthermore, by making a small change to the Java core libraries enabled by MJ, we obtained a 30% performance improvement in a servlet microbenchmark. Categories and Subject Descriptors D.2.3 [Coding Tools and Techniques]: Object-oriented programming; D.3.2 [Language Classifications]: Object-oriented languages--- Java; D.3.3 [Language Constructs and Features]: Modules, packages General Terms Design, Experimentation, Languages, Performance Keywords Java, Modularity, Components, Language Design Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are no..